Carolinacetus gingerichi, GEISLER & SANDERS & LUO, 2005

GEISLER, JONATHAN H., SANDERS, ALBERT E. & LUO, ZHE-XI, 2005, A New Protocetid Whale (Cetacea: Archaeoceti) from the Late Middle Eocene of South Carolina, American Museum Novitates 3480 (1), pp. 1-66 : 6-45

Carolinacetus gingerichi , new species

DIAGNOSIS: Distinguished from other known protocetids by the presence of two autapomorphies: (1) presence of a posterodorsal tongue of the petrosal that is exposed between the exoccipital and the squamosal with the skull in posterior view ( fig. 9 ); (2) mandible with a steep ascending process and a deeply descending ventral margin posteriorly, depth of mandible at the mandibular fossa greater than 2.5 times the depth of the mandible at p4 ( fig. 11 ). It differs from Georgiacetus vogtlensis in having the posterior margin of the external nares above the canine; palatal process of the premaxilla terminating between the canine and P1; ascending process of the premaxilla terminating dorsal to P1; and parietal ridge rounded, not acute. Carolinacetus differs fron Natchitochia jonesi in having anterior thoracic vertebrae that are 10–15% smaller, and no further comparisons can be made because the holotype of the latter taxon includes only vertebrae and fragmentary ribs.

HOLOTYPE: ChM PV5401 ( figs. 3–24 ): incomplete cranium with right petrosal in place, partial tympanic bullae, and detached anterior portion of rostrum; anterior and posterior portions of both mandibles; 11 identifiable teeth; atlas, axis, and 3rd, 4th, 6th, and 7th cervical vertebrae; 1st, 2nd, and 4th–8th thoracic vertebrae; 15 ribs. Collected by A.E. Sanders, J.H. Geisler, C.B. Way, S. Davis, Z.­X. Luo, and J.L. Hanlon, April 1994.

TYPE LOCALITY: South Carolina, Berkeley County; Martin Marietta Berkeley (‘‘Cross’’) Quarry , east side of County Road 59, 2.4 km (1.49 mi) south of South Carolina Routes 6 and 45; USGS Chicora 159 quadrangle, 338219 N, 80813.49 W .

FORMATION AND AGE: Cross Member, Tupelo Bay Formation, late middle Eocene, middle Bartonian, zone NP 17, ca. 40 Ma.

ETYMOLOGY: The specific name is a patronym honoring Philip D. Gingerich for his many contributions to the present knowledge of the origin of Cetacea and of the diversity of protocetids.

REFERRED SPECIMEN: ChM PV6088 , probable sixth thoracic vertebra missing right transverse process, dorsal portion of left transverse process, and tip of spinous process. South Carolina, Berkeley County; Martin Marietta Berkeley Quarry; collected by Billy Palmer, Sr., 9 March 1999. Cross Member , Tupelo Bay Formation , late Middle Eocene .

DESCRIPTION

SKULL

The preserved portions of the holotype skull consist of the anterior end of the rostrum with the right canine in place ( figs. 3 , 4 ), detached anterior part of the left nasal, detached portions of the right and left jugals ( fig. 3 ), partial cranium with the right petrosal in place ( figs. 5–8 ), and detached right occipital condyle. Measurements of the cranial elements are given in table 1.

The preserved rostral portion extends from the tip of the rostrum to a point just posterior to the alveoli for P1. It is missing the dorsal portions of the premaxillae that enclose the roots of I1, the lateral side of the right maxilla, and the left nasal. In comparison with the maxillae, the premaxillae appear ventrally deflected, probably the result of distortion at the maxillary/premaxillary suture. In addition, there is a gap between the dorsal portions of the right and left premaxillae that can be attributed to distortion. In dorsal view, the rostrum gradually tapers anteriorly, with the external nares being located above the canines, unlike many other described protocetids, in which the nasal opening is situated above P1.

The cranial portion is preserved from the dorsal margin of the foramen magnum to the anterior margins of the orbits but is missing

TABLE 1

Measurements (in mm) of Holotype Skull and Auditory Bones of Carolinacetus gingerichi , new genus, new species (ChM PV5401)

Condylobasal length, as preserved (235) a

Greatest length, plane of margins of nuchal crest to tip of rostrum (290)

Anteroposterior length of parietals along sagittal crest 235

Transverse diameter of supraorbital processes at posterior processes 256

Shortest transverse diameter at parietal ridge 33.1

Greatest outside dimensions across nuchal crest 116

Greatest inside dimensions across nuchal crest 95.2

Vertical diameter of occiput (138)

Greatest diameter at outside margins of exoccipitals (284)

Length of external auditory meatus 68.7

Anteroposterior length, detached partial left jugal, as preserved 199.9

Depth at midpoint of detached partial left jugal, as preserved 21

Anteroposterior length of right premaxilla (285)

Depth of right premaxilla at posterior margin of 12 66.2

Least depth of premaxilla between I3 and C1 46.5

Width of rostrum at P1 (87)

Width of rostrum at I3 64.7

Width of rostum behind I1 40.5

Width of palate at I3 25.8

Length of alveolus, right I2 35

Length of alveolus, left I3 23.3

Length of alveolus, left C1 (38)

Length of alveolus, right P1 18

Width of alveolus, left I3 12.2

Width of alveolus, right P1 10.4

Length of diastema between left I3 and C1 43.3

Length of diastema between left C1 and P1 62

Anteroposterior length of right bulla 68.2

Greatest length of posterior process of right bulla 71.2

Greatest width of posterior process of right bulla 28.5

Anteroposterior diameter of internal acoustic meatus of petrosal 14.4

Length of promontorium of petrosal 16.8

a Measurements in parentheses are estimates.

the vertex, the left occipital condyle, the entire ventral region, all of the left squamosal, and the zygomatic process of the right squamosal. In dorsal aspect the cranium is elongate with a prominent sagittal crest, is broadest at the outermost margins of the exoccipital, and is narrowest at a point approximately 75 mm posterior to the posterior margins of the supraorbital processes of the frontals. The temporal fossa is quite extensive, occupying 65% of the anteroposterior length of the cranial region. The nuchal crests of the supraoccipital are horseshoe­shaped in dorsal view, and the entire occiput is projected posterodorsally.

PREMAXILLA: As in other archaeocetes, the premaxilla is elongate and articulates with the maxilla via ascending and palatal processes ( fig. 4 ). The ascending process rises over the dorsal surface of the maxilla and is sutured to it, terminating ca. 21 mm behind the posterior margin of P1. Ventrally, the premaxilla puts forth a palatal process that extends posteriorly for approximately 75 mm behind the anterior tip of the maxilla and terminates at a point adjacent to the diastema between the alveoli for C and P1. Together, the two palatal processes form a wedge between the maxillae. Each of these processes is sutured to the maxilla laterally and articulates medially with the vomer at its posterior end. Just anterior to the anterior edge of the nasal, the dorsal margin of the premaxilla descends rather sharply into a gentle depression anterior to C, begins to rise dorsally above I3, and above I2 achieves the same elevation as the ascending process. The upper edges of the premaxillae flare dorsally and laterally to form the nasal opening directly above the canine. The nasal opening is confluent anteriorly with a mesorostral gutter that extends anteriorly as far as the diastema between I1 and I2.

The alveoli for the incisors within the premaxilla are aligned longitudinally in two parallel rows, not in a transverse arc as in mesonychids and archaic ungulates ( Prothero, 1988; Thewissen, 1994). The alveoli for the right and left I2 are slightly asymmetrical, with the anterior margin of the alveolus for the left I2 being approximately 4 mm anterior to its counterpart on the right side. The first and third alveoli are roughly equal in size, but the alveolus for I2 is considerably deeper and longer anteroposteriorly. The alveoli for I1 are at the extreme anterior end of the rostrum and were at least partially exposed anteriorly. A diastema of 28 mm separates I1 from I2, and I3 is directly behind I2, separated from it by a diastema of only 12 mm. On the ventral surface immediately posterior to the alveolus for I1 there is a small pit that is vascularized and is divided by a sulcus leading into an anterior and a posterior foramen. On the lateral surface posterior to the alveolus for I3 there is a deep, ovate embrasure pit for the lower canine.

MAXILLA: The maxilla is sutured to the premaxilla anteriorly, anteromedially, and dorsally; to the nasals dorsally posterior to the premaxilla; and to the vomer medially. Ventrally, the preserved palatal portion contains the alveoli for C1 and P1 ( fig. 4 ). Between the alveoli for C1 and I3 there is a diastema of ca. 40 mm that spans the maxillary/premaxillary suture and another of ca. 58 mm between C1 and P1. The alveolus for the canine is large, angled posterodorsally, and seated completely on the maxilla. A long, deep, porous embrasure pit occurs in the diastema between C1 and P1 and probably received p1. The anteroposterior length of the alveolus for P1 (13 mm) is small relative to that of the canine and is strongly bilobed. Ventrally, the maxillae form a central trough that deepens and widens posteriorly. The roof of the trough is ca. 29 mm wide and 9 mm deep immediately posterior to the alveolus for P1. In the preserved portion, the walls of the trough reach their greatest height medial to P1. On each side of the trough medial to P1 there are sulci that lead posteriorly to the major palatine foramina 25 mm posterior to P1. On the right side this foramen is only 11 mm beyond P1. The major palatine foramen is the ventral opening for the palatine canal that houses the greater palatine nerve, a ramification of the maxillary branch of the trigeminal nerve ( Sisson, 1921). Three well­defined foramina and evidence of a fourth occur on the lateral side of the maxilla in the region of P1.

VOMER: The anterior end of the vomer articulates with the palatal process of the premaxilla laterally, and the remaining posterior portion is sutured to the maxilla laterally and ventrally. A portion of the vomer is exposed ventrally on the midline of the palate between the alveoli for C and P1 ( fig. 4 ). The total exposure is 40 mm in length beginning immediately posterior to the palatal processes of the premaxillae and terminating posteriorly at the level of the anterior edge of the alveolus for P1. In dorsal view, the vomer is a U­shaped trough on the floor of the mesorostral cavity. The morphology of the vomer is correlated with the presence of the ethmoidal plate or a septal cartilage that separates the left from the right nasal passage. The preserved portion of the vomer in Carolinacetus is low and U­shaped, as in extant cetaceans; however, it differs from Recent taxa in that it does not extend laterally and dorsally to completely floor the mesorostral cavity. Although the preserved portion in the holotype is restricted in length to the region between the alveoli for C1 and P1, the vomer undoubtedly extended for a considerable distance anteriorly and posteriorly, but almost certainly not beyond the anterior end of the maxilla.

NASAL: The nasal bone articulates laterally with the premaxilla and is sutured to the posterior point of its ascending process. Posterior to the ascending process of the premaxilla, the nasal is sutured ventrally and laterally to the maxilla. The nasal forms the dorsal quarter of the lateral surface of the rostrum and forms half of the roof of the mesorostral trough posterior to the nasal opening. The anterior tip of the nasal marks the posterior edge of the nasal opening and is level with the posterior edge of the alveolus for C ( fig. 4 ). This position for the nasal opening is comparable to those in Indocetus ramani and Rodhocetus kasrani (Gingerich et al., 1993, 1994). The ventral internal edge of the nasal is a thin, ventrally expanded lamina. On the inner surface of the nasal, 55 mm posterior to its anterior margin, the turbinate crest begins and broadens vertically and transversely in the posterior direction. Posterior to the level of P1, the turbinate crest thickens to a vertical diameter of 23.7 mm at the broken edge of the specimen. Apparently, the dorsal turbinate (naso­turbinate) was fused to the posterior portions of the turbinate crest as in extant ungulates ( Sisson, 1921).

FRONTAL: In dorsal view, the frontals form laterally broad and anteroposteriorly elongate supraorbital processes ( fig. 5 ). The dorsal surface of the supraorbital process is gently convex and slopes ventrally anteriorly and laterally. Approximately half the distance from the midline to the lateral edge of the supraorbital process are two supraorbital foramina that have lost their original roofing and are exposed as open grooves. Curving posteromedially and opening onto the exterior of the cranium laterally, these foramina enclosed supraorbital frontal veins that drained the facial region and connected to the superior sagittal sinus ( Sisson, 1921).

The posterior edge of the supraorbital process forms the anterior edge of the temporal fossa and the postorbital process of the frontal. The posterior surface of the postorbital process is oriented vertically and is continuous posteriorly with the laterally facing portion of the temporal fossa. In lateral view, the frontoparietal suture originates along the posterior edge of the postorbital process and is angled posteroventrally. The postorbital process extends laterally beyond the anterior portion of the supraorbital process, but breakage in the skull has obscured the degree of that extension. In parasagittal section, the postorbital process is triangular and is point­ ed ventrally. The anterior side of the postorbital process is concave and forms the posterior border of the orbit. The ventral edge of this process forms a sharp ridge that curves posteromedially and also forms the posterior edge of a broad trough containing two major orbital foramina missing their original flooring ( fig. 6 ). The central and deepest portion of the trough probably conveyed the ophthalmic branch of the trigeminal nerve and the optic nerve. In the anterolateral corner of the trough there is a small foramen of undetermined utility. The ophthalmic branch of the trigeminal nerve is separated from the ethmoid canal and foramen by a prominent ridge of bone ca. 3–4 mm thick. The ethmoid canal proceeds in a straight line posteromedially and then abruptly turns anteriorly. Lateral to the anterior ridge of bone separating the ophthalmic trough from the ethmoid canal there is an anteroposteriorly oriented row of four minute foramina. In lateral aspect, the supraorbital process is greatly arched dorsally to form the orbit. The frontal of this specimen has been broken anteriorly, revealing numerous laterally elongate frontal sinus cavities.

PARIETAL: The parietal is sutured to the supraoccipital posteriorly, to the squamosal laterally, to the alisphenoid anteroventrally, and to the frontal anteriorly and ventrally. The posterior portion of the parietal is a thin lamina that extends posterodorsally to the edge of the lambdoidal crest. The ventral portion of the parietal anterior to the squamosal/parietal suture is sutured to the alisphenoid anteromedially. The parietal/alisphenoid suture traverses the lateral wall of the braincase steeply and anterodorsally ( fig. 5 ). In dorsal view, the anterior end of the parietal diverges from the midline and curves laterally along the postorbital process to a point ca. 63 mm anterolaterally from the midline. Breakage on the ventral side of the skull reveals a longitudinal frontal/parietal suture on the ventral surface of the parietals anterior to the alisphenoid ( fig. 6 ). This sutural surface is angled posteroventrally and bifurcates at its posterior end ca. 108 mm from the foramen for the optic nerve. The posterior edge of this suture delineates the point at which the frontal forms its part of the roof of the cranial cavity.

The parietals converge along the midline to form a pronounced sagittal crest ( fig. 5 ). The anterior half of the parietals are narrow and form a distinct longitudinal intertemporal constriction. The dorsolateral surface of the parietal is the origin of the anterior half of the temporalis muscle. A low ridge, continuous posteriorly with the squamosal ridge, sweeps anterodorsally to a point ca. 32 mm ventral to the sagittal crest, then anteriorly parallel to the sagittal crest, and terminates on the medial end of the postorbital process. Ventral to this ridge, the parietal is concave and faces laterally; above the ridge, the parietal is flat to concave and faces dorsolaterally. A line of rugosities occurs on the posteroventral corner of the parietal and runs parallel to the parietal/alisphenoid suture. Endocranially, the parietals form the roof and lateral walls of the cranial cavity ( fig. 6 ). The anterior end of their endocranial exposure has a deep groove for the superior sagittal sinus, situated between the posterior bifurcations of the parietal/frontal sutural surface. The groove extends posteriorly into a highly irregular, convex region that includes a median tentorial projection. Around and posterior to that projection the endocranial surface of the parietal is heavily pitted. The heavily pitted surface probably housed a vascular rete situated dorsal to the cerebellum, as has been suggested for basilosaurids (Breathnach, 1955) and for the protocetid Indocetus (Bajpai et al., 1996) . The cranial cavity is transversely constricted anteriorly but widens posteriorly at the level of the foramen ‘‘pseudovale’’ (see discussion of this term below). Posterolateral to the groove for the dorsal (superior) sagittal sinus is a shallow fossa for the right cerebral hemisphere. The posterior edge of this fossa forms the division between the middle and posterior endocranial fossae, thus marking the division between the cerebrum and cerebellum. Although not well developed in Carolinacetus , this ridge forms a tentorium osseum in Basilosaurus cetoides (Kellogg, 1936: fig. 6 ).

Ventrolateral to the cerebral fossa there is a sharp anteroposterior bulge corresponding to the rhinal sulcus of the brain, which separates a ventrolateral fossa for the piriform lobe from the rest of the cerebral hemisphere. Within the parietal and lateral to the medial bulge that defines the rhinal sulcus, there is a large vascular canal for the cranio­orbital sinus that continues anteriorly toward the orbital region and opens posteriorly into a deep sulcus. That sulcus rapidly widens posteroventrally and forms a pit­like depression on the squamosal/parietal suture anterodorsal to the anterior process of the petrosal. From that pit a small canal continues posteroventrally within the petrosal/squamosal suture and connects with the temporal canal.

SQUAMOSAL: The squamosal forms the posterior half of the temporal fossa and part of the lateral side of the braincase. Its contribution to the endocranial wall is relatively small, consisting only of a narrow strip on the ventral quarter of the lateral side of the braincase. The squamosal is sutured to the parietal dorsally and dorsomedially, to the alisphenoid anteriorly and medially, to the exoccipital and mastoid process of the petrosal posteriorly, and a small portion to the pterygoid medially. Externally, the squamosal/parietal suture traverses the posterior portion of the temporal fossa ( fig. 5 ). As in other archaeocetes (Kellogg, 1936), the squamosal/ parietal suture begins at the posterior margin of the temporal fossa and arches dorsally to a point anterior to the nuchal crest. From there it extends anteriorly parallel to the sagittal crest for a distance of ca. 31 mm and then slopes steeply anteroventrally, terminating at the anteromedial corner of the squamosal fossa. A small foramen possibly for an emissary vein from the dorsal cerebral vein (petrosquamous sinus) is situated on the squamosal/parietal suture anterior to the nuchal crest. On the endocranial surface the squamosal/parietal suture meanders anteroposteriorly near the base of the lateral wall of the braincase immediately dorsal to the petrosal/squamosal suture. The squamosal/ parietal suture is highly oblique in cross section with the squamosal extensively overlapping the parietal laterally. The anteroventral corner of the squamosal is sutured to the alisphenoid anteromedially. A small portion of the pterygoid likely sutured to the squamosal dorsomedial to the foramen ‘‘pseudovale’’ as in Zygorhiza (Kellogg, 1936: fig. 31c) and Georgiacetus . We follow Luo and Gingerich (1999: 49) in recognizing that foramen—the external opening for the mandibular branch of the trigeminal nerve—as the foramen ovale ( fig. 6 : fo). In Carolinacetus the foramen ovale is within the squamosal, and its ventral margin is incomplete.

The squamosal is sutured to the supraoccipital medially along the lateral margin and base of the nuchal crest and to the exoccipital posteriorly. In the holotype of Carolinacetus the exoccipital and the squamosal are separated by a narrow gap for most of their length and only sporadically contact each other. The lateral portion of the posterior surface of the squamosal is tightly sutured to the mastoid process of the petrosal.

The external surface of the squamosal bears rough­textured rugosities and relatively smooth depressions, is generally concave, and is the origin for the posterior half of the temporal muscle and fascia. A broadly elevated region, herein termed the squamosal rise ( fig. 5 : sqr), ascends anterodorsally to join an elongate elevation on the parietal homologous to the parietal ridge of Georgiacetus (Hulbert et al., 1998: 912) . The squamosal rise descends posteroventrally across the face of the squamosal. Below the squamosal rise the squamosal is deeply excavat­ ed, forming the squamosal fossa. Above it the bone slopes posterodorsally toward the nuchal crest. In the center of the squamosal rise there is a dorsoventrally oriented depression that separates an elevated region along the squamosal/parietal suture from a textured tuberosity anterior to the posterior margin of the temporal fossa. That tuberosity is not homologous to the squamosal prominence of Sanders and Barnes (2003).

In the holotype cranium, the preserved structures on the ventral side of the squamosal are the external auditory meatus, the posttympanic process, the sigmoid fossa, the postglenoid foramen, the falciform process, the medial edge of the glenoid fossa, and the foramen ovale ( fig. 6 ). The postglenoid process, most of the glenoid fossa, and the zygomatic process were not preserved.

The external auditory meatus extends from the tympanic cavity posterolaterally to the edge of the skull and is a deep, narrow trough flaring slightly at its lateral end. Its posterior margin is formed by the ventrally extended posttympanic process of the squamosal. A small sigmoid fossa is situated immediately anterior to the medial end of the external auditory meatus and is separated from the latter by a small ridge ( fig. 7 : sgf). The sigmoid fossa received the sigmoid process of the tympanic as in basilosaurids (Luo and Gingerich, 1999). The postglenoid foramen occurs in the squamosal/petrosal suture anteromedial to the sigmoid fossa ( fig. 7 : pgf). On the medial edge of the squamosal immediately anterior to the sigmoid fossa there is a small groove that leads dorsally into the postglenoid foramen. The falciform process of the squamosal is situated anterior to the postglenoid foramen and probably articulated with the anterior margin of the tympanic as in Georgiacetus and Protocetus . The falciform process is broad and blocky, unlike that of odontocetes, in which it is long, narrow, and ventrally extended. In the holotype of Carolinacetus the medial surface of the falciform process is rugose, probably for the attachment of the tympanic bulla. Although the anterior edge of the falciform process is missing, it appears to have partially floored the foramen ovale, which is 7.6 mm in diameter.

ALISPHENOID: Posteriorly, the alisphenoid is sutured to the squamosal and the parietal. On the ventral half of its medial surface there is a broad sutural surface that probably received the pterygoid. Externally, there is a broad longitudinal ridge at the juncture of the basicranium and the lateral wall of the cranium. Anteriorly, the alisphenoid becomes very thin as it approaches the sphenorbital fissure, which is confluent posteriorly with the fossa for the piriform lobe of the cerebrum. There is no indication of the foramen ovale on the preserved portion of the alisphenoid in this specimen. The foramen rotundum is absent in other archaeocetes (Kellogg, 1936).

OCCIPUT: In certain areas of the occipital region of the holotype cranium the bones are fused, thus obscuring the sutures between them. In posterior view, the supraoccipital forms a dorsally elongate surface from which originated deep epaxial neck musculature that supported the skull ( fig. 9 ). The dorsal edge of the supraoccipital is thrust posteriorly, causing the upper two­thirds to face posteroventrally. The lower third is directed posteriorly and bears a prominent median ridge dorsal to the foramen magnum. Herein termed the supracondylar ridge ( fig. 9 : scr), this prominence probably anchored the nuchal ligament, which separates the right deep neck muscles from the corresponding left muscles in the bovid Bos taurus (Dyce et al., 1996) . The left half of the ridge is missing in the holotype of Carolinacetus , but immediately lateral to the base of the right half is one of presumably a pair of nuchal tubercles as in Pakicetus inachus (Gingerich and Russell, 1981) . Lateral to the ridge for the nuchal ligament there is a deep, dorsally elongate fossa for the rectus capitus dorsalis muscle, which is probably homologous to the recti postici muscle in extant cetaceans ( Schulte, 1916; Schulte and Smith, 1918). The ventral half of the rectus capitus dorsalis fossa contains numerous vascular foramina. On either side of the midline near the dorsal edge of the supraoccipital are two shallow fossae from which the semispinalis capitus probably originated. The ventral margin of the right fossa is V­shaped, points ventrally, and extends posteriorly along the inner edge of the lambdoidal crest. The left fossa appears larger and more wedge­shaped, but its exact shape and extent is unclear due to breakage. In posterior view, the dorsal margin of the supraoccipital forms a tightly rounded arc, somewhat horseshoe­like in shape ( fig. 9 ). The posteriormost point of the lambdoidal crest is thickened and sharply directed inward. Breakage at the vertex of the skull reveals that the supraoccipital is thickest anteroposteriorly along the midline and wedges between the posterior ends of the parietals. The dorsal surface of the supraoccipital is covered by thin, posterior extensions of the parietals.

The endocranial surface of the occipital contains fossae for various lobes of the cerebellum. Posterior to the median tentorial projection, a dorsoventrally elongate, rugose fossa accommodated a dorsal portion of the lobus medius cerebelli (sensu Dart, 1923), also known as the cerebellar folia ( Sisson, 1921). Medial to the dorsoventral fossa, a sagittal ridge divided the cerebellum into right and left halves. Ventral to the dorsal fossa for the lobus medius cerebelli, a transverse ridge 32 mm in length forms the anterior margin of a fossa for another lobe of the cerebellum seated immediately anterior to the roof of the foramen magnum. The posterior wall of the braincase dorsal to the foramen magnum is massively thick, measuring 36 mm along the midline.

Lateral to the foramen magnum, the exoccipital broadens laterally and extends to the lateral edge of the mastoid process of the petrosal, thus obscuring the latter in posterior view. The exoccipital is sutured to the squamosal dorsally and anteriorly, to the mastoid process of the petrosal (lateral to the squamosal) dorsally and anteriorly, and to the posterior process of the tympanic (ventral to the mastoid process) anteriorly and ventrally. The triple junction of the supraoccipital, exoccipital, and squamosal occurs dorsolateral to the broken base of the right occipital condyle. At that junction there is a small pit­like fossa that we interpret as the supracondylar fossa. Extending laterally from that fossa there is a fissure­like separation between the exoccipital and squamosal, terminating at the posterodorsal exposure of the mastoid process of the petrosal. Herein termed the posterodorsal tongue of the petrosal ( fig. 9 : pdt), this exposed portion is sutured to the dorsal edge of the exoccipital, which is notched to receive it. This portion of the mastoid process is 30 mm in transverse diameter and 14 mm in vertical diameter. At the lateral end of the fissure between the squamosal and exoccipital suture there is a foramen that we interpret as the mastoid foramen. We use the term ‘‘mastoid foramen’’ as defined by MacPhee (1994) to specify a foramen that transmits an emissary vein from the sigmoid sinus. The mastoid foramen in Carolinacetus is formed by the exoccipital ventrally, anteriorly, and laterally, while the roof of the foramen is formed by the squamosal. Continuing from that foramen is a vascular channel that curves ventromedially within the suture between the squamosal and the exoccipital and opens endocranially posterodorsal to the pars cochlearis of the petrosal. Medial to the endocranial opening for the mastoid foramen there is a series of parallel vascular grooves for the occipital diploic veins, which travel posterodorsally into the squamosal/exoccipital suture to enter cancellous bone of the occipital.

The external surface of the exoccipital has several fossae that presumably were the origin or site of insertion for musculature of the neck. Although speculative, myological interpretations can be implied from the morphology of extant cetaceans (Carte et al., 1868; Schulte, 1916; Schulte and Smith, 1918; Howell, 1927) and artiodactyl outgroups ( Sisson, 1921; Woodburne, 1968). In the holotype of Carolinacetus the right condyle of the skull and most of the condylar neck are missing. A small foramen and a vascular groove of unknown function perforate the base of the condylar neck 22 mm dorsal to the hypoglossal foramen on the posterior surface of the occipital. Ventrolateral to the condylar neck there is a notch in the ventral edge of the exoccipital that forms the dorsal margin of the jugular foramen. The hypoglossal foramen is in the medial wall of the jugular foramen. Lateral to the jugular foramen there is a partially preserved, bulbous paroccipital process. Dorsal to the paroccipital process there is a semicircular depression containing two pits, ca. 9 mm in diameter each, which may mark the insertion of the rectus capitus lateralis muscle. The ventral pit is considerably deeper than the dorsal one and contains a pair of minute foramina. Lateral to those depressions the exoccipital is more posteriorly directed. Lateral to the paroccipital process an ovate prominence of bone forms the base of the exoccipital and defines the edges of two muscular fossae. Dorsal to this region a broad fan­shaped fossa tapers ventrally and marks the likely point of insertion for the obliquus capitus anterior muscle, which inserts in a similar position in the collared peccary ( Woodburne, 1968). The exoccipital extends to the lateral edge of the skull and thus excludes the mastoid process of the petrosal and the postmeatic portion of the squamosal as a surface for muscle attachment. Instead, a vertical row of four muscular fossae occurs on the lateral edge of the exoccipital. The top three are roughly equal in size and the ventral one is twice the size of the others. From dorsal to ventral position they are interpreted as fossae for the longissimus capitus, the splenius mastoideus, sternomastoideus, and the cleidomastoideus muscles. The latter two may be reversed as in extant cetaceans ( Schulte, 1916; Howell, 1927).

A fragment of the right side of the basioccipital bearing the basioccipital crest is preserved but cannot be reattached to the skull. However, its position relative to the rest of the skull can be estimated because a matching portion of the hypoglossal foramen was preserved. This fragment indicates that the dorsoventral thickness of the basioccipital rapidly increases laterally. The ventrolateral border is formed by a rounded, protruding basioccipital crest estimated to be 31.5 mm in length and 19 mm in greatest thickness. The dorsal surface of the basioccipital crest has numerous fine pits to which connective tissue to the ectotympanic bulla and petrosal were attached. On the dorsal surface of the basioccipital anterior to the hypoglossal foramen there is a broad, laterally orient­ ed excavation for the peribullary sinus. Anterior to that excavation the dorsoventral thickness increases and the carotid foramen and canal perforate the lateral side of the basioccipital crest, unlike extant cetaceans in which the carotid foramen is within the basisphenoid ( Schulte, 1916; Wible, 1984; Fordyce, 1994).

JUGAL: The left jugal is represented only by its posterior portion and the right one only by its anterior portion ( fig. 3 ). The sutural surface for the articulation of the lacrimal is present on the anterior end of the right jugal. The dorsal surface of that bone is flattened adjacent to the orbit, is not sharply recurved, and apparently there was not a postorbital process. Medially, the sutural surface for articulation with the maxilla is present above the lower edge of the right jugal.

PETROSAL: Although only the right petrosal is preserved in the holotype of Carolinacetus , it is by far the best preserved petrosal from the archaeocete families of Pakicetidae , Ambulocetidae , Remingtonocetidae , and Protocetidae . The ventral portion of that bone was found separate from the skull, thus allowing both the ventral and dorsal surfaces of the petrosal to be studied before restoration to its original position. In describing the petrosal we employ the term ‘‘mastoid process’’ in preference to ‘‘posterior process of petrosal’’ because the former stresses homology with other ungulate taxa. We suggest that usage of the term ‘‘posterior process of petrosal’’ be restricted to petrosals that have decoupled from the skull (see Fraser and Purves [1960] for a detailed explanation of petrotympanic decoupling).

The ventral view of the petrosal is dominated by a gently convex promontorium that houses the spiral cochlear canal ( figs. 7 , 8 ). The medial edge of the promontorium is defined by a broad area (3–5 mm) of rugose bone, the lateral edge by the epitympanic recess, the anterior edge by the groove for the tensor tympani muscle, and the posterior edge by the peribullary sinus. The posteromedial third of the promontorium is flat, while the remaining portion is rounded, convex, and protrudes slightly more ventrally. At the posterior end of the rugose bone and medial to the fenestra rotunda there is a small, pointed process herein termed the medial tuberosity of the promontorium ( fig. 7 : mt). From this tuberosity a short medially direct­ ed ridge ascends to the perilymphatic foramen. A narrow sulcus extends from the perilymphatic foramen paralleling the posterior margin of the tube of the internal acoustic meatus (IAM). Anterodorsal to this tuberosity there is a prominent pit within which there is a small foramen of unknown utility. The fenestra rotunda opens onto the posterior face of the pars cochlearis and is highly elliptical, with the long axis being 4.6 mm in length and oriented transversely and horizontally. A shallow groove extends medially from the fenestra rotunda and terminates posterior to the medial tuberosity of the promontorium. Immediately lateral to the fenestra rotunda there is a small posteriorly projecting ledge of bone that might be homologous with the caudal tympanic process of other mammals as defined by MacPhee (1981) and Wible (1990). Lateral to the promontorium is the fenestra vestibuli, which is situated medial to the epitympanic recess. The fenestra vestibuli faces lateroventrally ( fig. 8 : fv) and is not visible in a directly ventral view ( fig. 7 ). It is elliptical with the long axis oriented anteroposteriorly and is 2.8 mm in length. In the holotype of Carolinacetus the base of the stapes is still lodged within the fenestra vestibuli and appears as a ring of broken bone. Lateral to the fenestra vestibuli is the tympanic aperture of the canal for the facial nerve ( fig. 8 : VII). The sulcus for the facial nerve begins at its tympanic aperture and proceeds posteriorly along the roof of the epitympanic recess. A deep fossa for the stapedial muscle is located posterior to the fenestra vestibuli, medial to the facial nerve sulcus, and lateral to the fenestra rotunda. The stapedial muscle fossa is at least twice as deep as the facial nerve sulcus.

The tegmen tympani, which is homologous to the superior process of Kellogg (1936) (see Luo and Gingerich, 1999), is the part of the petrosal lateral to the facial nerve canal and has extensive contact with the medial side of the squamosal. The epitympanic recess is situated on the ventral surface of the tegmen tympani and bears a large circular fossa for the head of the malleus, which is situated anterolateral to the tympanic aperture for the facial nerve canal. The presence or absence of the fossa for the incus cannot be determined because that area of the petrosal is missing. The tegmen tympani extends dorsally (dorsal Schuppe of tegmen tympani of Pompeckj, 1922), forming a small portion of the endocranial wall of the braincase. The dorsal edge of tegmen tympani is not flush with the squamosal but instead is slightly raised and is penetrated by multiple fossae/ foramina. The anterior foramen transmitted the cranio­orbital sinus, the adjacent posterior canal contained the capsuloparietal emissary vein (i.e., postglenoid vein), and the posterior fossa connected to the sigmoid sinus ( fig. 7 : cos, etc).

As in other cetaceans, the petrosal of Carolinacetus has a distinct anterior process (anterior extension of the tegmen tympani; Luo and Gingerich, 1999). In ventral view, most of the anterior process is obscured by the squamosal, which wraps around the anterior end of the former. In basilosaurids the squamosal overlaps the lateral side of the anterior process but does not override it (Luo and Gingerich, 1999). A pronounced deep groove for the tensor tympani muscle extends along the ventromedial edge of the anterior process. This groove begins as a deep fossa posteriorly and narrows to a shallow groove at its anterior end ( fig. 7 : gtt). Medial to the tensor tympani fossa on the ventromedial edge of the anterior process there is a small hiatus fallopi for the greater superficial petrosal nerve. The posterior end of the anterior process projects toward the tympanic cavity, is flattened, and faces primarily ventrally but slightly posteriorly. That portion of the process likely articulated with the tympanic bulla as in Protocetus (Geisler, personal obs.). Anterior to the probable articulating surface with the ectotympanic there is an anterodorsal­to­posteroventral fossa that receives a posterior projection from the falciform process of the squamosal. The postglenoid foramen is visible lateral to the posterior end of the anterior process. The foramen also occurs in basilosaurid archaeocetes but was misidentified by Kellogg (1936: fig. 6 ) as the fossa epitubaria. The dorsal two­thirds of the anterior face of the anterior process is flat and faces primarily anteriorly but slightly ventrally. The anterior process tapers to a point, homologous to the anterodorsal angle of other taxa (see Fordyce, 1994; Luo and Marsh, 1996; Geisler and Luo, 1996) ( fig. 7 : ap). This point is homologous to, but not as well developed as, the dorsal spine of Remingtonocetus (Kumar and Sahni, 1986) .

The mastoid process of the petrosal is elongate and is wedged between and sutured to the squamosal anteriorly and to the exoccipital posteriorly. It has extensive exposure on the lateral and posterior sides of the skull, with a significant portion of the ventrolateral exposure undoubtedly being ectotympanic and not petrosal. Viewed laterally, the mastoid process has a thick wedge­shaped exposure of heavily rugose bone that narrows dorsally to a thin layer between the squamosal and the exoccipital. It then thickens adjacent to the dorsolateral corner of the exoccipital and forms the posterodorsal tongue of the petrosal ( fig. 9 ). In posterior view, that portion of the petrosal protrudes into the exoccipital/squamosal suture ca. 10 mm medial to the lateral edge of the exoccipital. The ventral surface of the mastoid process is almost completely obscured by the posterior process of the tympanic ( fig. 6 ). That observation, based on a cross­sectional break through the mastoid process in the holotype and upon careful comparison with the basicranium of Georgiacetus , demonstrates that in Carolinacetus the apparent mastoid process of the petrosal is actually a compound structure composed of the posterior process of the tympanic and the mastoid process of the petrosal as in mysticetes (Kasuya, 1973; Geisler and Luo, 1996). In ventromedial view, the proximal end of the mastoid process is constricted to form the neck of the mastoid process (homologous to neck of posterior process, sensu Luo and Marsh, 1996). The degree of constriction is unclear because a fragment of the petrosal is missing. The petrosal/tympanic suture is situated at the anteromedial end of the mastoid process of the petrosal and anterolateral to the paroccipital process. Adjacent and posterior to that suture there is a short groove that is interpreted as a continuation of the facial nerve sulcus on the ventral surface of the mastoid process as reported in other cetaceans (e.g., Geisler and Luo, 1996; Dooley et al., 2004).

In dorsal view, the rim of the internal acoustic meatus in Carolinacetus is elevated to form a cranially elongate tube that projects a maximum of 20 mm from the edge of the pars cochlearis ( fig. 7 : tiam). The tube is somewhat volcano­shaped, and the meatus itself is analogous to the crater on a volcano’s summit. The dorsal lip of the internal acoustic meatus tube projects considerably farther than the ventral lip and has a needle­like process in the posteromedial corner. The internal acoustic meatus tube is present in Georgiacetus but is not as well developed as in Carolinacetus . The rim of the internal acoustic meatus is elevated in many mysticetes (Geisler and Luo, 1996; Dooley et al., 2004), although this morphology is presumably convergent to the one in Carolinacetus and Georgiacetus .

The internal acoustic meatus tube transmitted both the vestibulocochlear nerve (cranial nerve VIII) and the facial nerve. The tractus spiralis foramina, for branches of the cochlear nerve, are centered in the IAM at its deepest point. The foramen singulare for the vestibular nerve is slightly anterolateral to the tractus spiralis foramina. The cranial aperture for the facial nerve canal is concealed by sediment, but a break repaired during preparation revealed its position anterior to the foramen singulare ca. 9 mm from the lip of the IAM. That break also exposed a very thin dividing wall (ł 0.25 mm) between the facial nerve canal and the IAM proper. The ventralmost surface of the IAM tube is smooth, broadly rounded anteroposteriorly, and forms a nearly flat medial sulcus for the inferior petrosal sinus (Geisler and Luo, 1998: fig. 8 ). The anterior process of the petrosal and the anterior edge of the IAM tube form, respectively, the lateral and posterior borders of the piriform fenestra, through which the inferior petrosal sinus exited the cranial cavity. The medial side of the piriform fenestra was probably enclosed by the basioccipital and basisphenoid as in other protocetids (e.g., Protocetus and Georgiacetus ). The endocranial surface of the petrosal contains two large fossae: the suprameatal fossa (sensu Luo and Eastman, 1995) dorsal to the IAM, and a second one, located posterior to the IAM, herein termed the postmeatal fossa ( fig. 7 : pmf). Those two fossae are probably excavations for the endocranial portion of the peribullary sinus. A subarcuate fossa is not present. Posterodorsal to the IAM tube and between the postmeatal and suprameatal fossae there is a prominent dorsomedially projecting process herein termed the suprameatal process ( fig. 7 : smp) Between the base of the suprameatal process and the base of the IAM tube there is a small cleft for the endolymphatic foramen.

ECTOTYMPANIC BULLA: Both ectotympanic bullae of ChM PV5410 are preserved, with the right one being more complete than the left. Our description of the bulla of Carolinacetus is a composite based on both specimens. In ventral view, a broad anteroposteriorly oriented median furrow divides the bulla into lateral and medial eminences ( fig. 10 ). The thickened and inflated medial edge and eminence is termed the involucrum. In ventral view, the median furrow wraps around the posterior face of the bulla and forms a broad rounded notch on its posterior edge. The medial margin of the involucrum is deeply indented 47.7 mm anterior to its posterior end, marking the opening for the eustachian tube. That indentation is also evident in medial view where the dorsoventral thickness of the involucrum abruptly thins. The posterior surface of the involucrum is covered with small depressions and rugosities. That area and a similar one along the medial side of the involucrum are points of attachment for tough connective tissue that anchored the bulla to the paroccipital process of the exoccipital and to the basioccipital crest, respectively. In medial view, a deep groove runs dorsally on the surface of the involucrum 8.0 mm anterior to its posterior edge. This groove deepens dorsally and turns anteriorly before reaching the dorsal surface of the involucrum, then terminates 15 mm anterior to the posterior margin of the involucrum.

On the posterior face of the bulla there is a vertical tympanic cleft between the involucrum, or inner lip, and the outer lip of the bulla. A broad sulcus originates near the ventral end of the outer lip and extends dorsally along the lateral margin of the cleft and then enters the tympanic cavity at the ventral edge of the cleft. Broken fragments from the holotype bullae indicate that the lateral edge of the bulla thinned dorsally. The presence of a sigmoid fossa on the squamosal strongly suggests that a large sigmoid process was present but was not preserved. Other features of the outer lip also were not preserved.

As mentioned above, what appears to be the mastoid process is actually a compound process composed ventrally of the posterior process of the ectotympanic and dorsally of the mastoid process of the petrosal. The posterior process of the tympanic is broad, elongate, and almost completely covers the ventral surface of the mastoid process of the petrosal and is sutured to it ( figs. 6 , 7 : ppt). The distal end of the posterior process is exposed on the lateral side of the skull between the exoccipital and squamosal ventral to the mastoid process of the petrosal ( fig. 3 ). The posterior process increases in thickness posterolaterally, and its ventral surface is generally smooth with a few shallow transverse grooves. In the holotype right bulla there are two broken surfaces near the proximal end of its ventral surface; in Georgiacetus and in basilosaurids, the inner and outer pedicles of the posterior process of the tympanic connect at similar points. Multiple pits occur in the space between the broken bases of the pedicles and might have been continuous with the sulcus lateral to the vertical tympanic cleft.

MANDIBLE

The posterior regions of both mandibles are preserved, but only the medial halves of the anterior ends were found. The posterior portion of the left mandible is intact as far as the anterior margin for the alveolus of the third molar but is missing that portion of the dentary between the first molar and the canine ( fig. 11 ). Only 114 mm of the anterior end is preserved, including a portion of the alveolus for i1 and the alveolus for i2. The posterior region of the right mandible is preserved from the condyle to the anterior edge of the alveolus for p4, but it is missing portions of the coronoid process and the entire angular region ( fig. 12 ). The preserved portion of the anterior end of the right mandible extends from the posterior end of the alveolus for i3 to the middle of the alveolus for i1, a distance of 180 mm. The anterior ends of both mandibles are tightly sutured along the symphysis but are not ankylosed ( fig. 13 ).

The posterior end of the alveolus for i1 is situated immediately anteroventral to the alveolus for i2, with the posterior wall of the former forming the anterior wall of the latter.

(A) and medial (B) views. Scale bar is 10 cm in length. See appendix 3 for anatomical abbreviations.

Although the anterior part of the alveolus for i1 is missing, it is clear that this tooth was considerably smaller than i2, the alveolus of which is approximately 33 mm in anteroposterior diameter. Posterior to the alveolus for i2 there is a shallow, rounded depression approximately 27 mm in anteroposterior diameter that we think to be the medial wall of the alveolus for i3. The alveolus for the canine lies almost directly behind the alveolus for i3 and has an anteroposterior diameter of approximately 50 mm. The medial wall of the canine alveolus is extremely narrow, measuring only 10.5 mm ( fig. 13 ). Centered on the medial wall of the alveolus of i2 and of the canine there is a low, rounded ridge, suggesting a corresponding groove on the medial surface of these teeth.

The most striking feature of the mandible is its extreme depth in the region of the mandibular fossa, where it is 265% greater than the depth at p4 (table 2). The posterior increase in mandibular depth is caused by the steep ascending (coronoid) process and by the abrupt downward slope of the mandible’s ventral edge, which begins at a point below the margin of the mandibular foramen and ends at the angular process ( fig. 11 ). The ascending process begins posterior to p4 then rises sharply in the region of m2. The lateral surface of the mandible is highly convex with a broadly rounded longitudinal ridge bisecting the mandible into dorsal and ventral halves. In lateral view, the dorsal edge of the coronoid process begins 15 mm posterior to the posterior edge of the alveolus for m3, rises sharply at an angle of approximately 458, gradually curves to become nearly horizontal, and finally curves sharply ventrally at its posterior terminus. The anteroposterior length of the coronoid process is roughly twice its dorsal height, and the anterior half of its lateral surface forms a shallow temporalis fossa. The base of the dorsal edge of the coronoid is in line with the lateral edge of the alveolus for the m3. The coronoid process is relatively thin, and the dorsal edge adjacent to the temporalis fossa is curved laterally. Medial to the base of the coronoid and 23 mm posterior to the edge of the alveolus for m3 there is small foramen that leads to an enlarged opening into the mandibular canal.

The medial side of the mandible is dominated by a large rectangular mandibular fossa ( fig. 11 , 12 : mfs). This fossa is continuous anteriorly with a cavernous mandibular foramen. The medial surface of the mandible is flat to slightly concave anterior to the mandibular foramen and highly concave in the region of the mandibular fossa. The mandibular fossa and foramen presumably contained an intramandibular fat body as in extant odontocetes ( Norris, 1980). The dorsoventral depth of the mandibular foramen is nearly the height of the mandible at m3. The medial and lateral walls of the mandibular canal are extremely thin just anterior to the mandibular foramen, having an average thickness of 3 and 4 mm, respectively. In medial view, the medial edge of the mandibular foramen is relatively straight and is oriented vertically. This edge is continuous with longitudinal ridges that define the dorsal and ventral edges of the mandibular fossa. The ventral edge of the mandible adjacent to the mandibular fossa bows ventrally in medial view and ends with a posteriorly projecting point that forms the mandibular angle. That angle terminates at a point almost directly ventral to the posterior end of the coronoid process. A rugose ridge extends anteriorly from the mandibular angle for a distance of 51 mm. This ridge and the flattened ventral edge adjacent to it probably served as a point of attachment for the pterygoideus internus muscle. The condyloid process projects approximately 56 mm posteriorly from the plane of the posterior edge of the coronoid process and the tip of the mandibular angle. The condyle is separated from the posterior edge of the coronoid process by the mandibular notch and from the mandibular angle by the curved posterior edge of the mandible. The condyloid process is stout and projects posteriorly approximately along the longitudinal axis of the mandible. On its medial face there is a wide longitudinal fossa for the insertion of the pterygoideus externus muscle. In lateral view, a pronounced ridge separates the condyloid process into a dorsally facing shallow fossa and a broad, flat, ventrally facing surface. In posterior view, the condyle is roughly triangular and is directed ventrally. In dorsal view, the condyle is situated lateral to the long axis of the ramus. Its dorsal edge is approximately level with the dorsal edge of the alveolus for m2, and the edge that forms the mandibular notch is in line with the medial edge of the condyle.

DENTITION

Eighteen dental elements were preserved, including both upper canines, a probable right P1, a probable left p1, left p3, right p4, half of the left m1, right and partial left m2, right and left m3, and seven tooth fragments. In general, the enamel of all the teeth is poorly preserved ( fig. 14 ). The tips of the lower teeth bear numerous fine dorsoventral grooves, and the inner faces of both roots are bisected by a narrow, deep groove. The two teeth that appear to be the upper and the low­ er first premolars are relatively small, and each is missing the crown and the basal portions of the roots, but the size and the tightly coalesced roots of these teeth indicate that they are first premolars.

UPPER CANINE: The left upper canine is in situ and the right one was isolated ( figs. 4 , 14B ). The root and crown of the canine are transversely compressed, the crown occupying one­half the total length of the tooth on the lingual surface and one­fourth of the labial surface. The anterior half of the labial surface of the crown bears a prominent wear facet from occlusion with the lower canine. The root is finely grooved lengthwise and bears a distinct linear median depression on both the lingual and labial surfaces below the crown base. The root of the canine is directed posteriorly and lies almost parallel to the ventral margin of the maxilla, but the crown is angled anteroventrally.

LOWER THIRD PREMOLAR: The crown of the left p3 is transversely compressed with a broad and low protoconid ( fig. 14D ). The protoconid is inclined posteriorly and has poorly defined anterior and posterior carinae. The anterior carina forms a long angled cutting surface. Posterior to the protoconid is a posteriorly projecting cusp that is separated from the former on the lingual surface by a broad shallow fossa. In lingual view, the anterior edge of the crown bears a shoulder that separates the cutting edge from the ascending part of the crown below. Between the crown and the dorsal ends of the roots, the tooth is anteroposteriorly and transversely constrict­ ed. The tooth is double­rooted with clear separation up to the base of the crown. The anterior root is 7 mm longer than the posterior root and is considerably wider both in transverse and anteroposterior thickness. The lingual face of the anterior root is concave to flat, the posterior face is concave, and the remaining faces are convex. The posterior root is convex on all faces.

LOWER FOURTH PREMOLAR: The right p4 has a narrow pointed protoconid that is inclined posteriorly and a low posteriorly pointed cusp ( fig. 14C ). The apical end of the protoconid is recurved to a more vertical position. The crown is strongly compressed transversely. Both the anterior and posterior edges of the protoconid have carinae. The posterior carina descends abruptly posterior to the protoconid and continues onto the posterior cusp, which is low, dorsally convex, and hook­like in form. The protoconid and posterior cusp are separated by a shallow furrow on both the labial and the lingual faces. The tooth has two widely separated roots, with the anterior root being significantly longer and much more robust than the posterior one. The lingual sides of the roots are flattened, and the labial sides are strongly convex. Approximately midway down the anterior and posterior roots the transverse and anteroposterior diameters increase abruptly.

LOWER SECOND MOLAR: The complete right m2 was found in place in the mandible, but only the posterior half of the left m2 was preserved ( figs. 11 , 14A ). The protoconid is more than twice the height of the hypoconid. The anterior face of the protoconid is flattened, suggesting the original presence of lingual and labial carinae. The protoconid and the hypoconid are connected by a carina, the posterior half of which is also known as the cristid obliqua. In anterior aspect, the protoconid is angled farther lingually than the hypoconid. The hypoconid is directed posteriorly beyond the margin of the posterior root. The anterior root is inflated at the base. The posterior root is more transversely narrow than the anterior one.

LOWER THIRD MOLAR: Both lower m3s were found in place in the mandibles. The crown of the right m3 is better preserved than the left, but it is missing half of its posterior root. Although incomplete in both specimens, the protoconid is clearly not much higher than the hypoconid ( fig. 15 ). As in the lower premolars, both cusps are transversely compressed. A crenulated cingulum 3–5 mm in height wraps around the base and posterior edge of the hypoconid. It continues anteriorly for at least half of the length of the tooth on the lingual side, with the anterior portion remaining only as an eroded remnant. The cingulum has been almost completely removed by wear from the labial side of the tooth. The enamel of the cingulum and the hypoconid has fine, faint wrinkles that become progressively fewer and shallower apically. Along its entire anteroposterior length the hypoconid bears a carina with faint, widely spaced serrations. The hypoconid is separated anteriorly from the protoconid by a small carnassial notch 1 mm in height. The anterior face of the protoconid is formed by labial and lingual carinae that are widely separated at the base but converge dorsally near the apex of the protoconid. On the left m3 the lingual carina ends on a distinct anteriorly projecting cusp, which abuts anteriorly against the hypoconid of m2. The roots are separated by a narrow gap, with the anterior one being compressed anteroposteriorly and the posterior one compressed transversely. The lingual and labial faces of the posterior root are flat. The anterior face of the anterior root is concave, the labial face is convex, and the lingual surface is flat.

VERTEBRAE

Thirteen vertebrae were recovered, including the atlas, axis, and 3rd, 4th, 6th, and 7th cervical vertebrae, and the 1st, 2nd, and 4th– 8th thoracics. Three neural arches and six fragments of centra also were found. Each of the cervicals is described separately, but we have selected only the second and sixth thoracic vertebrae for description because most of the preserved thoracics are similar in morphology. Positional determinations of the thoracics were simplified by the preservation of vertebrae from the anterior portion of the series, in which the transitional elevation of the transverse processes posterodorsally can be readily observed ( fig. 16 ). Measurements of the cervical vertebrae are given in table 3; those of the thoracics appear in table 4. In measuring transverse diameters of the thoracics, measurements of the anterior face of the centrum were taken at the approximate centers of the demifacets, and those of the posterior face were taken at the lateral margins of the demifacets.

Epiphyses were missing from both faces of the fourth thoracic and from the posterior face of both the fifth and the seventh thoracics. In both of the latter two vertebrae, portions of the anterior epiphysis were not fully ankylosed and were subsequently glued onto the centrum. Hence, the holotype of C. gingerichi seems to have been a young adult and may not have reached the maximum size attained by this species.

FIRST CERVICAL: The atlas vertebra is approximately 215 mm in width and 101 mm in vertical dimension ( fig. 17 ). It is missing most of the ventral tubercle (hypophysis), the anterolateral portion of the transverse process, and the edges of the transverse processes ventrolateral to the transverse foramina. In general form it is robust and more nearly like the atlas vertebra in artiodactyls than in modern cetaceans. Hence, we have used certain osteological terms generally applied to land mammals, for example, ‘‘transverse foramen,’’ is usually termed ‘‘vertebrarterial canal’’ in the cetacean literature.

The width of the anterior face is approximately 125 mm, and its height at the midline is 56 mm. The surface is deeply concave with high and narrow lateral and dorsal edges. The anterior margin of the dorsal surface extends forward beyond the lower portion of the anterior face and is deeply emarginated at the midline by a broad notch, herein termed the supracondylar notch ( fig. 15C : sn). This notch is present in many land mammals, including artiodactyls, perissodactyls, canids, ursids, and hyaenids, inferring that its presence is primitive for cetaceans. Posterior to the notch the dorsal surface grades upward to the dorsal tubercle.

The right transverse process is well preserved, with most of its dorsal surface being depressed below the level of the dorsal tubercle. The transverse process is formed by a horizontal tabular portion and a vertically oriented descending portion, which in lateral view has the form of an inverted ‘‘L’’. The deeply excavated region immediately below the horizontal portion of the transverse process is termed the atlantal fossa in land mammals. The distal end of the transverse process is upturned and thickens posteriorly, achieving its greatest thickness at the junction of the horizontal and descending potions.

The right and left halves of the posterior face are separated by the semicircular trough for the reception of the odontoid process of the axis vertebra, herein termed the odontoid fossa ( fig. 17B : of). Each half has an ovoid articular surface with an upturned lateral edge that forms a thin lip that partially conceals the opening of the transverse foramen. This foramen is vertically elongate, with a maximum diameter of 8.9 mm. It opens anteriorly into the atlantal fossa along the lateral surface of the centrum and at that point is situated 18.5 mm posteroventral to the ventral opening of the alar foramen. This foramen perforates the base of the transverse process at a point 28.3 mm posterior to the dorsal edge of the anterior face of the centrum. At that point it is directly opposite to the mouth of the lateral vertebral foramen, which opens into the neural canal. This canal is broadest (59.5 mm) across its posterior opening and tapers ventrally into the odontoid fossa.

SECOND CERVICAL: The axis vertebra is complete except for the distal ends of the spinous and transverse processes, the lateral edges of the posterior face of the centrum, the right pedicle of the neural arch, and the lateral edge of the right transverse foramen ( fig. 18 ). The right and left articular surfaces for the atlas vertebra are flat dorsoventrally and slightly concave transversely. The odontoid process is bullet­shaped in ventral view and is 30 mm in transverse diameter and 29 mm in anteroposterior length. The ventral two­thirds of the odontoid surface is smooth and articulates with the atlas; the dorsal third does not. There is a sharp ridge that begins on the dorsal surface of the odontoid process and extends along the midline for the entire length of the centrum. It increases in height posteriorly and divides the dorsal surface of the centrum into two deep, elongate fossae. It also separates two pairs of foramina, a smaller pair at the base of the odontoid process and a second larger pair close to the posterior end of the centrum. The greatest length of the centrum, including the odontoid process, is 87 mm.

The posterior face of the centrum is poorly preserved, but the remainder shows that it was roughly ovate in shape except for its ventral edge, which is transversely horizontal. From near the posterior bases of the pedicles it slopes posteroventrally at an angle of approximately 398 to intersect the posterior margin of the ventral region of the centrum. The ventral surface bears a large hypophysis that forms an elongate tuberosity occupying most of the length of the centrum. This feature begins anteriorly as a low narrow ridge, 12 mm in width, behind the odontoid process. Posteriorly, it deepens dorsoventrally and widens until it reaches an estimated maximum transverse width of 34 mm at the posterior end of the centrum. Most of the dorsal surface of the hypophysis has been eroded, but its preserved surface is rugose. A deep fossa parallels the hypophysis on each side.

The transverse processes extend posterolaterally from the centrum. Their anterior faces are convex, and the posterior faces are concave. Their ventral edges are thickened and rugose, apparently for muscle attachment. The transverse foramen for the vertebral artery is preserved only on the left side of the holotype axis vertebra. In posterior view it is situated at the upper end of the base of the left transverse process and is ovoid with a vertical diameter of 20 mm and a transverse diameter of 14 mm. Posteriorly, it forms a short canal that curves and narrows anterolaterally. The anterior aperture is much smaller than the posterior one, with a dorsoventral diameter of 16.4 mm and a transverse diameter of 9 mm. This opening is obscured in anterior view by the lateral margin of the surface that articulates with the atlas. The lateral margin of the foramen is formed by a bony strut that joins the transverse process to the left pedicle of the neural arch. The left pedicle is robust, with a transverse width of 25 mm.

In anterior view the neural canal has the shape of an inverted heart. Its anteroposterior length varies considerably and is greatest (42 mm) along the roof of the neural arch. The postzygapophyses are nearly circular and face posteroventrally and slightly laterally. The distal end of the spinous process is missing, but its general shape can be discerned. Its base is anteroposteriorly long, is triangular in cross section, and is sharply hooked posterodorsally. On the posterior face of the preserved portion of the spinous process there is a shallow medial sulcus that extends into a narrow, slit­like foramen in the base of the neural arch just above the dorsal margin of the neural canal. This foramen is 6.7 mm in dorsoventral diameter and 1.9 mm in transverse diameter.

THIRD CERVICAL: This vertebra is nearly complete, and both epiphyses are firmly ankylosed. As seen in lateral view, the anterior and posterior faces of the centrum are not aligned. Instead, the posterior face is in a more ventral position. Whereas the anterior face is somewhat cordiform ( fig. 18D ), the posterior face is nearly circular. In lateral view, both faces are similar in having a sigmoidal profile with the dorsal two­thirds concave and the ventral third convex. A faint, dorsoventral groove ca. 9 mm in length is centered on the anterior epiphysis. Nutrient foramina on the dorsal side of the centrum are separated by a span of bone that straddles the sagittal plane, and the ventral side bears a hypophysis along its posterior margin.

As seen in anterior view, the base of the transverse process is directed ventrolaterally while its distal end is slightly recurved to point laterally. The anteroposterior thickness of the process thins distally, and its posterior face bears a deep and wide transverse sulcus. The ventral edge of the sulcus is delineated by an elongate tubercle. A short transverse canal perforates the base of the transverse process. In anterior view, the canal is ovoid with the long axis being oriented dorsoventrally. Within the canal, the lateral side of the centrum forms a deep pit. The pedicles of the neural arch are stout and are joined to the transverse processes by bridges of bone that form the lateral walls of the transverse canals. The neural arch bears large prezygapophyses that face dorsomedially and large postzygapophyses that face ventrolaterally. The neural spine is absent, although a bump occurs in a homologous position, and the neural canal is ovoid.

SIXTH CERVICAL: The sixth cervical vertebra is well preserved, missing only the distal portion of the lateral branch of the right transverse process and the tip of the spinous process ( fig. 19A ). The shapes of the anterior and posterior faces of the centrum differ considerably. The anterior face is squarish, measuring 50.5 mm transversely and 51 mm dorsoventrally, and is bisected transversely by a shallow channel­like depression. The posterior face is ovate, is 60.5 mm in transverse diameter and 50.7 mm dorsoventrally, and is concave at its center. The ventral side of the centrum bears a low median keel. Along the midline of the dorsal surface an elevated hourglass­shaped platform of bone ca. 8 mm in width extends between the anterior and posterior faces of the centrum and separates a pair of bilaterally symmetrical foramina.

The transverse processes are bifid, with the lateral (upper) branch being much smaller than the ventral branch, which is robust and dorsoventrally compressed and extends for 79 mm ventrolaterally from the centrum. The anteroposterior width of the right ventral branch widens distally from 36 mm at its base to a width of 57 mm at its distal end. On the ventral side of each ventral branch there is an anteroposteriorly elongate tuber­

(ChM PV5401). Scale bars are 5 cm in length. See appendix 3 for anatomical abbreviations.

cle for muscle attachment situated approximately 26 mm distal to the centrum. The tubercle on the left side is much better developed than the one on the right side. The wellpreserved left lateral branch of the transverse process is short (16.2 mm), projects laterally, is hooked slightly dorsally, and its dorsoventral depth exceeds its anteroposterior width (12.1 mm).

The bridge of bone between the dorsal and ventral branches is anteroposteriorly compressed. Posterior to the junction of the dorsal and ventral branches there is an ovate muscular fossa, with its medial margin formed by a low ridge that ascends toward the base of the dorsal branch. That ridge forms the posterolateral margin of a broad sulcus leading into the transverse foramen, which perforates the base of the transverse process medial to its lateral branch and immediately lateral to the centrum. As a conduit for the vertebral artery, this foramen is nearly circular and has a maximum diameter of approximately 15 mm. The pedicles of the neural arch support a pair of large, flat prezygapophyses. The prezygapophyses are ovate, are 32–33 mm in anteroposterior length, and the dorsal plane of each is concomitantly sloped dorsomedially and posteroanteriorly. Both facets face dorsomedially, and the transverse plane of their dorsal margins intersects the peak of the roof of the neural canal. Immediately posterior to the prezygapophyses the postzygapophyses project posteriorly beyond the posterior face of the centrum. They are ovate, face ventrolaterally, and overhang the posterior entrance to the transverse foramen. The long axis of each postzygapophysis is oriented dorsolaterally, and the right one is 30.8 mm in length.

In anterior view, the neural canal is pentagonal with rounded corners and is 26.2 mm in greatest height and 37.6 mm in transverse diameter. The spinous process is incomplete, but its preserved portion shows that it was low and shaped like a spear point, with the apex being situated anteriorly. Laterally, it is flanked by narrow expanses of rugose bone bounded by steep posterodorsally and anteromedially directed escarpments that converge above the peak of the neural arch. The point of convergence is uneven, however, with the left escarpment being higher than the right one. Although not demonstrable within the light of present knowledge, the triangular shape of the area enclosing the spinous process may be diagnostic for the sixth cervical vertebra of Carolinacetus .

SEVENTH CERVICAL: This vertebra is complete except that it is missing the left prezygapophysis, the left transverse process, and the posterior epiphysis, which was not ankylosed ( fig. 19B ). The centrum of C7 rapidly increases in size posteriorly, resulting in its posterior face being much larger than its anterior face. The anterior and posterior faces are different in shape; the anterior face is square­shaped, except for a broadly rounded ventral side, while the posterior face is oval with a width 1.5 times its height. Centered on the anterior face is a narrow, dorsoventral groove ca. 10 mm in length. In lateral view, the anterior face of the centrum has a sigmoidal profile; the anterior two­thirds are concave while the ventral third is convex. Several small foramina lie on the centrum adjacent to the lateral margins of the anterior epiphysis. Other features of the centrum include: bilateral nutrient foramina on its dorsal side, a weak sagittal keel on its ventral side, and elevated capitular facets for the 1st rib on the lateral sides adjacent to the margins of the posterior face.

Extending dorsolaterally from the centrum is a wide, dorsoventrally compressed plate of bone that bifurcates into the transverse process and the pedicle for the neural arch. In posterior view, there is an oval pit on that plate between the base of the transverse process and the postzygapophysis. The transverse process is about 35 mm in length, is oriented ventrolaterally, and has a fairly flat distal end. The transverse foramen is absent. In anterior view, the neural canal is pentagonal in shape with rounded corners. Above the canal is a gracile neural spine that bears bilateral pits on the anterior face of its base, one on either side of the sagittal place. The pre­ and postzygapophyses are large; the left postzygapophysis has a transverse diameter of 33 mm, and whereas the prezygapophyses face dorsomedially, the postzygapophyses face ventrolaterally. Medial to the postzygapophysis and just inside the neural canal is an ovoid area of rugose bone probably for muscle attachment.

SECOND THORACIC: Most of the holotype second thoracic vertebra is preserved, but it is missing both transverse processes, the edges of the right metapophysis, and the distal half of the spinous process ( fig. 20 ). The anterior face of the centrum is slightly trapezoidal, with a transverse diameter of 60.6 mm and a vertical diameter of 48.8 mm. Most of the anterior face is gently convex except for the dorsalmost portion, which is concave transversely. Just above the ventral margin there is a low tuberosity on each side of the sagittal plane. The demifacets for the capitula of the second ribs are situated at the dorsal margins of the anterior face. Posteriorly, each demifacet is bordered by a shallow fossa. The posterior face of the centrum is much broader than its vertical height, having a transverse diameter of 76.4 mm and a vertical diameter of 44.1 mm. The left demifacet for the capitulum of the third rib is present, but the right one is less distinct.

The neural canal is ovate, with a transverse diameter of 34.1 mm and a vertical diameter of 24.3 mm. The left prezygapophysis is virtually complete; its articular surface is flat, faces dorsomedially, and is approximately 26 mm wide. The right prezygapophysis is missing its lateral and medial edges but is 20.2 mm in anteroposterior length, virtually the same measurement as its counterpart on the left. The distance between the two prezygapophyses was ca. 50 mm, estimated by doubling the distance from the inner margin of the left prezygapophysis to the midline of the neural canal. Posterior to the articular surface of each prezygapophysis there is an ovoid fossa at the base of the neural arch. Medial to each of these fossae there is a shallow angular recess bordering the dorsal margin of the neural canal.

The preserved portion of the spinous process is triangular in cross section, with the apex of the triangle being directed anteriorly. It is inclined posteriorly at an angle of ca. 258 from the vertical axis of the anterior face of the centrum. The posterior face of the process is fairly flat, but ventrally it grades into a narrow groove leading to a rectangular, concave fossa between the medial edges of the postzygapophyses. Each postzygapophysis is ovate, slightly concave, and faces ventromedially.

SIXTH THORACIC: This vertebra is virtually complete except for the distal end of the right transverse process and the anterior tip of the right prezygapophysis ( fig. 21B ). The vertebra is 206.5 mm in height from the ventral base of the centrum to the tip of the spinous process. It is approximately 115 mm in width between the lateral margins of the transverse processes. The centrum is 67.7 mm in greatest transverse width anteriorly and 83 mm posteriorly. The neural canal is circular and is 30.4 mm in vertical diameter and 32.4 mm transversely.

The anterior face of the centrum is nearly circular except for its slightly emarginated dorsal edge, and it is slightly concave at its center. The demifacets for the capitula of the sixth ribs are somewhat larger than the demifacets on most of the other preserved thoracics of Carolinacetus . The ventral surface of the centrum is featureless, and there is no pronounced median keel. On the dorsal surface there is a low median ridge flanked by vascular foramina. Unlike the more circular anterior face, the posterior face of the centrum has the shape of a transversely expand­ ed heart and is concave over most of its surface. The demifacets for the capitula of the seventh ribs are much larger than the anterior demifacets but are similar in size to the demifacets for the seventh ribs on the anterior face of the seventh thoracic.

The neural canal is circular in shape and relatively large, with its vertical diameter (30.4 mm) being 57% of the vertical diameter of the anterior face at the midline (53.5 mm). The pedicles are stout and thicken posteriorly. The prezygapophyses extend anteriorly from the base of the junction of the transverse processes with the neural arch and slope dorsolaterally. The left prezygapophysis is flat and circular and is 19 mm in anteroposterior diameter and 18.9 mm in transverse diameter. Immediately behind it a pair of ventrolaterally elongate fossae deeply indent the base of the transverse process. Posterior to the right prezygapophysis there is only a single shallow, circular fossa. The medial edges of the prezygapophyses extend posteromedially to join the anterior margin of the neural arch at the midline. Immediately above the midline of the arch there is a shallow, broadly triangular recess. At its center a hood­like transversely broadened projection extends anteriorly and overhangs the midline of the neural canal. We herein term this feature the supraneural prominence ( fig. 21 : snp).

The transverse processes extend dorsolaterally from the juncture of the pedicles and the neural arch. The distal end of the wellpreserved left process flares slightly upward at its outermost extremity, which is 57.5 mm from the median ridge at the base of the spinous process. The articulating facet for the tuberculum of the left sixth rib is vertically elongate, faces laterally and slightly ventrally, and is 39.7 mm in dorsoventral diameter. The vertical axis of the facet is directed anteriorly, so that the dorsal end of the process projects forward beyond the vertical axis of the anterior face. The left postzygapophysis is larger than the right one, which is virtually complete but may have lost the edge of its lateral margin through erosion. The left one is slightly concave and is quadrangular with rounded corners.

The spinous process is inclined posteriorly at an angle of approximately 458 from the vertical axis of the anterior face of the centrum and is 142.6 mm in length measured from the plane of the bases of the postzygapophyses. The proximal two­thirds of the process is triangular in cross section, with the posterior face as the base. The posterior face is irregular in width, ranging from 23 to 23.5 mm in transverse diameter. Proceeding from its margins, the lateral faces of the process

(ChM PV5401). Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

converge anteromedially to form the apex of the triangular configuration. Proximally, the narrow, blade­like anterior edge of the spinous process forms a wall dividing the left and right sides of the neural arch. Distally, the spinous process narrows and becomes elliptical in cross section, with the posterior face tapering from a width of 23.5 mm to 16.3 mm at a point 60.5 mm from the distal end. Along most of the posterior face there is a longitudinal median ridge. At the base of the spinous process between the postzygapophyses there is a cave­like recess floored by a shelf of bone that extends posteriorly and overhangs the posterior entrance to the neural canal. We herein term this projection the supraneural shelf. This feature is not present in the second thoracic vertebra, but the beginnings of it are evident in T4. It is further developed in T5, is most pronounced in T6 and T7, and has begun to disappear in T8.

OTHER THORACIC VERTEBRAE: The preserved portions of the other holotype thoracic vertebrae of Carolinacetus are similar in morphology to T6. Three trends are evident in the thoracic series. The shape of the neural canal ranges from ovate with a wide transverse diameter in the anteriormost thoracics (T2–T5) to nearly circular in T6–T8 and presumably in the remaining thoracics. As shown in table 4, the vertical diameter of the neural canal increases from 24.3 mm in T 2 to 30.4 mm in T6, resulting in its more circular shape in the latter vertebrae. That trend seems merely a reflection of the progressive increases in the height of the pedicles that accompanied the gradual elevation of the transverse processes and their facets for articulation of the capitula of the ribs. In most cetaceans the elevation of the rib facets reaches its peak at or about the middle of the thoracic series, after which they begin a sequential descent to the level of the transverse processes of the lumbar vertebrae.

As demonstrated by T2 and T4 ( fig. 22 ), the anterior face of the centrum in the anteriormost thoracics continues the squarish shape of that face in the cervicals, but in T5 the shape is more cordiform ( fig. 23 ). The anterior face of T4 is intermediate in morphology between that of T2, which is nearly square, and that of T6, which is nearly circular. Additionally, the supraneural prominence between the prezygapophyses is poorly defined in T2 and T4 and is most prominent in T5–T8, although it is somewhat erod­ ed in the latter vertebra ( fig. 23 ). Seemingly by more than sheer coincidence, the supraneural shelf between the postzygapophyses is most strongly developed in T5–T7, suggesting that it and the supraneural prominence of the following vertebra were common points for the attachment of connective tissue.

As shown in table 4, the spinous processes of the thoracics undergo a progressive posteroventral incline from the vertical axis of the posterior face of the vertebra, beginning at 258 in T2 and continuing to a slope of 458 in T6 and T7. The narrow, posteriorly inclined spinous processes are characteristic of protocetid vertebrae and are more typical of terrestrial ungulates than of crown cetaceans.

RIBS

The holotype ribs of Carolinacetus gingerichi consist of 15 partial elements with heads, two individual rib heads, and five shaft fragments ( fig. 24 ). All of the rib elements were randomly distributed among the rest of the skeletal remains, making it difficult to determine the correct anatomical position of individual specimens. Without careful study, even a provisional interpretation of the preserved material would not have been feasible.

The ribs of Carolinacetus are somewhat unusual in that a well­formed tuberculum persists through virtually the entire rib sequence, as we have determined it. In most cetaceans the posteriormost two, three, or even four ribs lack a well­defined capitulum and tuberculum, but only one of the preserved rib heads of Carolinacetus —the probable last (13th) rib—lacks these structures in their conventional form. In all of the preserved ribs of Carolinacetus except the latter one, the capitulum is globose and is much broader than the neck. The better preserved specimens can be assigned to the right or left side of the ribcage based on the curvature of the shaft and the direction in which the neck of the capitulum is twisted, that is, to the left in right ribs and to the right in left ribs. Among all of the rib elements for which assignments can be attempted, including the two detached rib heads, 10 left ribs and 7 right ones are represented. Measurements of the length of the ribs as preserved were taken on a straight line from the articular facet of the tuberculum to the distal end of the shaft as preserved, with the exception of three ribs (L3, R7, L8) in which the tuberculum is missing; in those ribs the measurement was taken from the dorsal side of the capitulum.

The change in angle (CIA) of the dorsal edge of the rib is preserved in 10 of the ribs and is quite prominent. That feature is common in varying degrees in the ribs of cetaceans (e.g., Kogia , Tursiops ) and in many artiodactyls (e.g., Cervus , Odocoileus ) and certain other land mammals, but it is especially well developed in Carolinacetus . In the holotype ribs of Carolinacetus the CIA becomes progressively more distal to the tuberculum from anterior to posterior through the probable 7th rib, but in the probable 8th rib, it begins a retreat toward the tuberculum that continues through the much shorter 9th and 10th ribs. Its position in the probable 11th and 12th ribs could not be determined because most of the shaft is missing in those specimens. Nevertheless, the position of the CIA seems clearly related to the length of the rib, that is, closer to the tuberculum in the shorter anterior and posterior ribs and farther from it in the longer ribs in the middle of the rib cage. Hence, this feature was used to determine the probable position that the preserved rib elements occupied in the sequence of dorsal ribs of Carolinacetus (table 5). For example, in L2 the CIA occurs only 50 mm distal to the base of the tuberculum, while in the probable R3 the CIA is 93 mm from the base of the tuberculum. In R3 and all subsequent ribs the CIA measurement was taken along the dorsal curvature of the rib.

In the two detached rib heads (i.e., the capitulum and its neck) the direction of the twist of one of them shows it to be from a right rib. Despite its fragmentary nature, the latter specimen can be assigned a provisional place in the rib series. The length of its neck (29.2) compares favorably with that of probable L4 (27.4), so we have tentatively referred this rib head to R4. The other detached rib head has only suggestions of the capitulum and the tuberculum and is clearly a posterior rib, possibly the last rib in the series.

The dorsoventral depth of the shaft at the base of the tuberculum was useful in placing the anterior ribs and in recognizing two partial ribs with shallow depths as probable L11 and L12.

The location of the CIA was the principal—and most reliable—means of determin­ ing probable positions in the rib sequence. That feature first appears in R2 and is well preserved in 11 of the 13 ribs apparently present in Carolinacetus , and, as seen in table 5, it provides a valuable landmark for placement of the ribs. The morphology of the dorsal surface of the rib immediately lateral to the tuberculum also is useful in recognizing anterior and posterior ribs. In the specimens that we have identified as the first five ribs the dorsal surface of the shaft adjoining the tuberculum is rounded, but in the rest of the ribs it is flat for a short distance (ca. 25 mm) extending laterally from the tuberculum. This feature is especially prominent in the probable 8th–12th ribs.

R1 R3 R4 R5 R6 R7 R8 R13 L1 L2 L3 L5 L8 L9 L10 L11 L12

FIRST AND SECOND RIBS: Although approximately half of the shaft is missing in the left first rib (L1) and nearly all of it in the right first rib (R1), these bones are easily recognized by the very large tuberculum and very short neck of the capitulum. Those features are characteristic of the first rib in cetaceans and in many land mammals (e.g., Bison , Equus ).

The preserved portion of the holotype L1 of Carolinacetus measures 152.6 mm in a straight line from the articular face of the tuberculum to the broken end of the shaft. In this rib the tuberculum actually exceeds the capitulum in size and robustness, with the former measuring 30.1 mm mediolaterally and 25.1 mm anteroposteriorly and the latter 27.2 mm vertically and 23.1 mm anteroposteriorly. On the dorsal surface of L1 there is an elongate, anteroposteriorly compressed tubercle at the base of the tuberculum. This feature is present also on R1 but is not as large as in L1. Among the other holotype ribs of Carolinacetus , that structure appears only on the one that we have identified as R2, where it is associated with the CIA, but it is not as robust as in the first ribs.

The holotype R1 is fragmentary, measuring only 72.5 mm from the broken base of the tuberculum to the broken edge of the shaft. The tuberculum is missing in R1, but the size of its base indicates that it was small­ er than its counterpart in L1. The capitulum in R1 is 30.9 mm vertically, but its anterior face has been eroded and yields an incomplete measurement of only 19.3 mm anteroposteriorly.

The probable R2 also lacks most of its shaft, measuring only 144.6 mm on a straight line from the articular face of the tuberculum to the broken end of the shaft. It is also missing the epiphysis of the capitulum. In R2 the tuberculum is 30.9 mm mediolaterally and 18.1 mm in anteroposterior diameter. The capitulum is 24.1 mm vertically and 19.4 mm anteroposteriorly, and its neck is 31.8 mm in length and is longer and more slender than the neck in the first ribs. The CIA of the dorsal side is preserved in R2 and is situated 46 mm distal to the tuberculum. Both the dorsal and ventral surfaces of the shaft are rounded at the base of the tuberculum dorsally and at the beginning of the neck of the capitulum ventrally.

THIRD, FOURTH, AND FIFTH RIBS: The ribs that we recognize as the probable L3, R3, L5, and R5 differ from the rest of the preserved ribs of Carolinacetus in having a greater depth of the shaft at the distal base of the tuberculum, averaging 34.9 mm in dorsoventral diameter at that point. A detached rib head is tentatively regarded as that of the right fourth rib.

Most of the probable right third rib is preserved and measures 251.5 mm on a straight line from the articular face of the tuberculum to the preserved distal end. This rib appears to be missing not more than 60 or 70 mm of the distal end of the shaft. The capitulum is 22 mm in vertical depth and 25 mm anteroposteriorly, and its ventral face is flattened. Though not nearly as robust as in L2, the tuberculum of R3 is large, measuring 22.8 mm in mediolateral length and 15.4 mm anteroposteriorly. The CIA is more distal than in L2, being 93 mm from the tuberculum. The probable L3 is missing the tuberculum and less than half of its shaft and measures 200.9 mm on a straight line from the dorsal side of the capitulum to the broken end of the shaft. The capitulum is 22.5 mm in vertical depth and 24 mm anteroposteriorly. When L3 is placed against R3 with the heads of the two ribs aligned evenly, the CIA of the two occurs at exactly the same point, and the dorsal and ventral degrees of curvature are almost identical, leaving little doubt that these two ribs are opposites. Using the head of R3 as a gauge, it is possible to obtain a measurement of the dorsoventral depth of L3 very close to the base of the missing tuberculum. In L3 the dorsoventral depth is 35.8 mm and is 36 mm in R3, adding further support to the placement of this rib as the opposite of R3. This measurement in L3 was not placed in table 5, however, because the position of the distal base of the tuberculum cannot be determined as precisely as in the ribs in which the tuberculum is present; nevertheless, we are confident that the measurement of the dorsoventral depth of L3 was taken very close to, if not immediately behind, the distal base of the missing tuberculum. In both of these ribs the dorsal surface immediately behind the tuberculum is round­ ed, and directly below that point the ventral surface is keeled, with the keel continuing for approximately half the length of the preserved portion of the shaft of the more completely preserved R3.

The detached rib head that we regard as probably belonging to R4 consists only of the capitulum and its neck and is 50.5 mm in total length from the articular face of the capitulum to the broken end of the specimen. The neck of the capitulum measures 29.2 mm from the medial remnant of the base of the missing tuberculum to the epiphysial suture of the capitulum. That measurement is close to the measurement of the neck of L4 (27.4 mm) and for that reason is thought to represent its opposite. The capitulum is 23 mm in vertical diameter and 20.8 mm anteroposteriorly.

The probable left fifth rib is missing approximately half of its shaft and is 192 mm in length on a straight line from the articular face of the tuberculum to the broken end of the shaft. The head is well preserved although slightly eroded in places. The capitulum is 22.4 mm in vertical depth and 22.7 mm anteroposteriorly, and its ventral face is flattened. The tuberculum measures 24.8 mm in mediolateral length and 15.3 mm anteroposteriorly. The dorsal surface of the shaft immediately behind the tuberculum is round­ ed, and directly below that point the ventral surface is sharply keeled along its anterior edge and rounded along its posterior margin. In that regard L5 differs from R3, in which the ventral keel is located medially instead of along the anterior edge of the shaft. The dorsoventral depth of the shaft at the distal base of the tuberculum is 34.6 mm. There is also a marked difference in the location of the CIA, which is 93 mm distal to the tuberculum in R3 and 140 mm in L5, as the CIA continues to move farther distally in the rib sequence.

The probable R5 is missing perhaps a third of its shaft and is 222.8 mm in length on a straight line from the articular face of the tuberculum to the broken end of the shaft. The capitulum is 21.5 mm in vertical depth and 22.2 mm anteroposteriorly. The tuberculum is poorly preserved and is missing its medial third, precluding a mediolateral measurement; its preserved portion is 12.1 mm anteroposteriorly, but that is an incomplete measurement. The dorsoventral depth of the shaft at the distal base of the tuberculum is 34.1 mm, and the CIA is 144 mm distal to the tuberculum, slightly farther than in L5 (139) but not appreciably so.

As we have determined them, the first five ribs differ from the remaining ones in having a deeper dorsoventral depth immediately distal to the tuberculum. This characteristic can be seen in the anterior ribs of other cetacean taxa and thus lends support to our placement of this portion of the holotype ribs of Carolinacetus .

SIXTH, SEVENTH, EIGHTH, AND NINTH RIBS: It is within this series that the ribs of Carolinacetus reach their greatest length and then begin to shorten as they approach the posterior proton of the rib cage. The probable right sixth and seventh ribs are the best preserved of all of the holotype ribs of Carolinacetus . The R7 is missing its tuberculum, however, so to obtain comparative lengths of R6 and R7 the overall measurement of each of these two ribs has been taken from the dorsal surface of the capitulum instead of from the articular face of the tuberculum.

The R6 measures 298.1 mm on a straight line from the dorsal side of the capitulum to the preserved distal end of the shaft. The projected terminus of the curvature of its anterior and posterior faces indicates that it is missing approximately 25 mm of its distal end, the addition of which would place its original length at approximately 323 mm. The capitulum is 23.4 mm in vertical depth and 24 mm anteroposteriorly. The tuberculum is missing its medial half, and thus a mediolateral measurement is not possible. Its anteroposterior diameter is 11.5 mm, though this may not be a complete measurement because of the breakage. The dorsoventral depth of the shaft at the distal base of the tuberculum is 28.9 mm, the first measurement at this point that does not exceed 30 mm. The CIA is 168 mm from the tuberculum, continuing the movement of the CIA distally from the proximal end of the rib.

The probable right seventh rib measures 302.6 mm on a straight line from the dorsal side of the capitulum to the preserved distal end of the shaft. Although the lateral edge of the distal end the shaft is missing, the medial edge is present and appears to be the natural end of this rib, and thus it seems to be essentially complete. It is clearly shorter than R6, indicating that it represents the beginning of the posterior rib series. It is in this rib that the CIA achieves its greatest separation from the tuberculum, a distance of 191 mm. Although the tuberculum is missing in R7, a remnant of the edge of its distal base can be detected, permitting a measurement of the CIA. The dorsoventral depth of the shaft at that point is 23.6 mm. The area immediately distal to the tuberculum is flattened, typical of the posterior ribs of Carolinacetus . The capitulum is 23.9 mm in vertical depth and 25.8 mm anteroposteriorly.

The probable left eighth rib is virtually complete, lacking only the tuberculum and less than 20 mm of its distal end. It is 243.3 mm in length on a straight line from the dorsal side of the capitulum to the preserved distal end of the shaft. The capitulum is 21.1 mm in vertical depth and 24 mm anteroposteriorly. Placement of this rib in correct alignment in juxtaposition to R8, in which the tuberculum is preserved, permits an approximate measurement of the CIA of L8 as 178 mm distal to the presumed lateral margin of its missing tuberculum. Thus, it is in the probable eighth rib that the CIA begins to retreat medially toward the tuberculum. Missing bone behind the tuberculum prohibits a measurement of the depth of L8 at that point.

As preserved, the probable R8 lacks part of the capitulum and most of its shaft, measuring only 130.4 mm on a straight line from the articular face of the tuberculum to the preserved distal end. The flattened area on the lateral side of the tuberculum identifies this specimen as a posterior rib. The absence of the CIA on the preserved portion places this rib between R7, in which the CIA is 191 mm distal to the tuberculum, and the probable L9, in which the CIA is just 58 mm from the tuberculum. When placed in juxtaposition with L8, the two ribs are virtually identical in curvature and lateromedial diameter, leaving little question that that these two ribs are opposites. The capitulum is 24.9 mm in vertical depth and 21.5 mm anteroposteriorly. The tuberculum is 19.5 mm in mediolateral length and 12 mm anteroposteriorly.

The probable left ninth rib evinces a pronounced shortening of the shaft characteristic of the last few ribs in the sequence. This rib is virtually complete, missing only the distalmost tip of the shaft, and as preserved it measures 205.6 mm on a straight line from the articular face of the tuberculum to the preserved distal end. There is a pronounced flattening of the dorsal surface immediately lateral to the tuberculum, a characteristic of the posterior ribs of Carolinacetus . At this point the dorsolateral depth of the shaft is 27 mm. Doubtless because the shortness of the rib, the CIA is located only 58 mm distal to the tuberculum. The capitulum is 20.5 mm in vertical depth and 25.5 mm anteroposteriorly. The length of its neck (30.9 mm) is slightly greater than the necks of the other posterior ribs, but this difference merely reflects the variation that occurs in the lengths of the necks throughout the rib cage of Carolinacetus . The tuberculum is somewhat eroded but yields measurements of 19.3 mm in mediolateral length and approximately 11 mm anteroposteriorly.

TENTH, ELEVENTH, TWELVETH, AND THIR­ TEENTH RIBS: The small sizes of the specimens that we think to be the probable 10th– 13th ribs clearly place them near the posterior end of the rib cage. Remarkably, the 10th–12th ribs of Carolinacetus all have as well­developed capitula and tubercula as any of the preceding ribs. All three of those ribs also display a prominent flattening of the dorsal surface of the shaft immediately lateral to the tuberculum.

The probable left 10th rib is virtually complete, lacking not more than 15 or 20 mm of its distal end. As preserved, it measures 162.8 mm on a straight line from the articular face of the tuberculum to the broken edge of the distal end. The head of this rib seems inordinately large compared to its relatively short shaft, with the capitulum measuring 22.2 mm in vertical depth and 24.7 mm anteroposteriorly. The tuberculum is 18.3 mm in mediolateral length and 16.2 mm anteroposteriorly. The flattening of the dorsal surface of the shaft immediately lateral to the tuberculum is quite pronounced in this rib, measuring 12.1 mm anteroposteriorly and 24.2 mm in its lateral extent. The dorsoventral depth of the shaft lateral to the base of the tuberculum is 26.3 mm. In this rib the CIA occurs 64 mm distal to the tuberculum.

The probable L11 is missing most of its shaft and measures only 79.6 mm from the medial edge of the tuberculum to the preserved end of the shaft. The capitulum is 21.4 mm in vertical depth and 23 mm anteroposteriorly. The tuberculum is considerably smaller than in the preceding rib, measuring 14.6 mm in mediolateral length and 14.1 mm anteroposteriorly. In L11 the dorsoventral depth of the shaft lateral to the base of the tuberculum continues its decrease in the posterior ribs and is just 24 mm in this rib.

The smallest of all of the preserved ribs with both capitulum and tuberculum, the probable L12 is also missing much of its shaft, but apparently not more than about 60%. It is 92.5 mm from the medial edge of the tuberculum to the preserved end of the shaft. The capitulum is 18.3 mm in vertical depth and 26.7 mm anteroposteriorly, remarkably large for such a small rib. The tuberculum is eroded but is approximately 15 mm in mediolateral length and about 12 mm anteroposteriorly. The dorsoventral depth of the shaft lateral to the base of the tuberculum is a mere 15.6 mm. The diminutive size of the shaft of this rib and the greatly reduced size of the tuberculum compared to that of the capitulum strongly suggest that it is the last of the ribs of Carolinacetus in which both of those structures are well defined.

One single­headed rib element was preserved among the holotype ribs of Carolinacetus . Only 32 mm in length, this specimen is strongly convex on one side (apparently the posterior face), and on the other side there is a median gutter separating two rounded elevations, with the larger presumably representing a relict capitulum and the smaller one seemingly being a remnant of the tuberculum. If this interpretation is correct, this side would be the posterior face and this rib head would thus be from the right side. Even on the opposite (anterior) side there is a narrow groove that outlines the presumed tuberculum and separates it from the presumed capitulum. We feel fairly confident that such is indeed the case, and accordingly we regard it as the head of the probable right 13th rib.

Whether there were additional singleheaded ribs in the rib sequence of Carolinacetus is, of course, impossible to determine with absolute certainty, but judging from the small size of the probable 12th and 13th rib elements it seems rather unlikely that the ribcage of Carolinacetus contained more than 13 ribs.

REFERRED SPECIMEN

An isolated probable sixth thoracic vertebra (ChM PV6088) from the Tupelo Bay Formation at the type locality of Carolinacetus is here referred to C. gingerichi . Both morphometrically and in its preservation, this specimen so closely agrees with the holotype thoracics of Carolinacetus that it could easily be mistaken for one of the holotype series, but it was collected at a different location within the Berkeley Quarry. The length/width ratio of the centrum of this vertebra (1.48) is only slightly less (0.07) than that of the holotype T6 (1.55) of Carolinacetus but is considerably greater than that of the T6 of Georgiacetus (1.34), which is also smaller in overall dimensions. Both the supraneural prominence and the supraneural shelf are as well developed as they are in the holotype sixth thoracic of Carolinacetus , suggesting that the referred specimen occupied a similar position in the thoracic series. All of the measurements of this specimen exceed those of the holotype T6, and the epiphyses are well ankylosed to the centrum, indicating that it belonged to a slightly larger and more mature individual than the holotype of Carolinacetus . The referred specimen differs from the holotype T 6 in having a less circular neural canal and a greater posterior slope (508) of the spinous process.

Although it is possible that PV6088 belongs to another as­yet­unknown taxon, the dimensions and morphology of this specimen are so similar to the thoracics of Carolinacetus that we feel safe in referring it to that form until its identity can be demonstrated otherwise.

Figures

Fig. 9. Line drawing (left) and photograph (right) of the cranium of Carolinacetus gingerichi (ChM PV5401) in posterior view. The posterodorsal tongue of the petrosal (pdt) is an autapomorphy of Carolinacetus. Scale bars are 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 11. Posterior portion of the left mandible of Carolinacetus gingerichi (ChM PV5401) in lateral (A) and medial (B) views. The great depth of the mandible between the coronoid process (cp) and the mandibular angle (man) is an autapomorphy of this genus. Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 3. Reconstructions of the holotype skull of Carolinacetus gingerichi (ChM PV5401) in (A) dorsal and (B) lateral views. Gray areas indicate portions of the skull actually recovered. Scale bar is 10 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 4. Anterior end of the rostrum of Carolinacetus gingerichi (ChM PV5401) in (A) dorsal, (B) lateral, and (C) ventral views. Note that the external nares (no) are above the canine. Scale bar is 10 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 5. Partial cranium of Carolinacetus gingerichi (ChM PV5401) in (A) dorsal and (B) lateral views. Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 6. Line drawing (A) and photograph (B) of the cranium of Carolinacetus gingerichi (ChM PV5401) in ventral view. Dashed line is approximate boundary between middle and posterior cranial fossae. Scale bar is 10 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 7. Stereopairs and line drawings of the right petrosal of Carolinacetus gingerichi (ChM PV5401). A. Stereopairs of the petrosal in ventral view with (B) corresponding line drawing. C. Line drawing of the endocranial side of the petrosal with (D) corresponding stereopairs. Note the elongate tube (tiam) that houses the internal acoustic meatus and postglenoid foramen (pgf) in the petrosal/ squamosal suture. Scale bars are 2 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 8. Ventrolateral view of the right petrosal of Carolinacetus gingerichi (ChM PV5401). Anterior is to the top of the page and the scale bar is 1 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 10. Right tympanic bulla of Carolinacetus gingerichi (ChM PV5401) in dorsal (A) and ventral (B) view. Most of the outer lip of the bulla was not preserved. Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 12. Posterior portion of the right mandible of Carolinacetus gingerichi (ChM PV5401) in lateral

Fig. 13. Sutured anterior ends of the right and left mandibles of Carolinacetus gingerichi (ChM PV5401) in dorsal (A) and lateral (B) views. The lateral sides of the mandibles are not preserved, thus exposing the alveoli for the incisors and canines. Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 14. Teeth of Carolinacetus gingerichi (ChM PV5401) in labial view: right m2 (A), right C (B), right p4 (C), and left p3 (D). Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 15. Left (A) and right (B) m3s of Carolinacetus gingerichi (ChM PV5401) in lingual view. Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 16. Lateral views of the thoracic vertebrae (i.e.. T4–T7) of Carolinacetus gingerichi (ChM PV5401). Scale bar is 10 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 17. Atlas vertebra (C1) of Carolinacetus gingerichi (ChM PV5401) in anterior (A), posterior (B), dorsal (C), and right lateral view (D). Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 18. Axis vertebra (C2) of Carolinacetus gingerichi (ChM PV5401) in anterior (A), posterior (B), left lateral view (C), and anterior view of third cervical vertebra (D). Scale bars are 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 19. Anterior views of sixth (A) and seventh (B) cervical vertebrae of Carolinacetus gingerichi

Fig. 20. Anterior view of second thoracic vertebra of Carolinacetus gingerichi (ChM PV5401). Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 21. Anterior views of the fifth (A) and sixth thoracic (B) vertebrae of Carolinacetus gingerichi

Fig. 22. Anterior view of the fourth thoracic vertebra of Carolinacetus gingerichi (ChM PV5401). Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 23. Anterior views of the seventh (A) and the eighth (B) thoracic vertebrae of Carolinacetus gingerichi (ChM PV5401). Scale bar is 5 cm in length. See appendix 3 for anatomical abbreviations.

Fig. 24. Posterior views of the ribs of Carolinacetus gingerichi (ChM PV5401). The left 4th, 6th, 7th, and 13th and the right 2nd and 9th–12th ribs were not preserved. Anterior is to the top of the page, and the scale bar is 10 cm in length. See appendix 3 for anatomical abbreviations.