Isoninia, Godfrey & Gutstein & Morgan Iii, 2021

Godfrey, Stephen J., Gutstein, Carolina S. & Morgan Iii, Donald J., 2021, A new odontocete (Inioidea, Odontoceti) from the late Neogene of North Carolina, USA, Fossil Record 24 (2), pp. 275-285 : 276-279

Genus Isoninia gen. nov.

Figs. 2 and 3

Type species

Isoninia borealis , sp. nov., herein designated.

Etymology

The genus name is a combination of “Ison”, the family (surname) name of Ronald Ison, who found and donated the type specimen to the Calvert Marine Museum, and Inia , the feminine generic epithet of the Amazon river dolphin ( Inia geoffrensis ), also known as the boto or pink river dolphin. The species name “borealis ” is Latin for “northern”, a reference to the type and only known specimen having been found in the Northern Hemisphere.

Holotype

The holotype is CMM-V-4061 ( Figs. 2 and 3). It consists of an incomplete skull, preserving the posterior facial region, from the lateral margins of the bony nares to the supraoccipital, including the vertex. Ventrally, part of the vomer/presphenoid complex is also preserved. This specimen was alluded to by Geisler et al. (2012) prior to it having been assigned a CMM catalogue number: “Other odontocete cranial elements are known from the same locality. However, they cannot be assigned to the present taxon either because they do not overlap with the aforementioned specimens that constitute the hypodigm or because of the presence of at least one other comparably sized but currently undescribed inioid from the same locality” ( Geisler et al., 2012, p. 201).

Type locality and horizon

The holotype was recovered by Ronald Ison (from the same location where the type and referred specimens of Meherrinia isoni were collected; Geisler et al., 2012) from the bed of the Meherrin River – a tributary of the Chowan River, Murfreesboro, Hertford County, North Carolina, USA (near 36 ◦ 27 0 N, 77 ◦ 5 0 W). More detailed locality information is available from the authors to qualified individuals upon request. Unfortunately, the specimen was not found in situ and was devoid of entombing sediment, as were all specimens of Meherrinia ( Geisler et al., 2012) . Nevertheless, it is considered to have been locally derived from the Cobham Bay Member of the Eastover Formation (upper Miocene, Messinian) (Lauck W. Ward, personal communication, 30 November 2009) ( Fig. 1). Because it was not collected in situ, it is possible that it could have come from the Pliocene Yorktown Formation, but according to Ward (personal communication, 30 November 2009) this derivation is less likely because of where along the river bed the specimen was recovered. Conservatively, the specimen can be considered to have come from a time interval encompassing the upper Tortonian (Miocene, i.e., the Claremont Manor Member of the Eastover Formation) to the lower Piacenzian (Pliocene). As in Geisler et al. (2012), another argument in favor of the derivation of Isoninia from the Eastover Formation is the observation that specimens of neither Isoninia borealis nor Meherrinia isoni have been found in the Yorktown Formation despite extensive collection and study of the fauna of the Yorktown Formation at the Nutrien Aurora Phosphate Mine (formerly known as the Lee Creek Mine, Aurora, North Carolina) for more than 40 years ( Whitmore and Kaltenbach, 2008).

Diagnosis

Isoninia is diagnosed on the basis of the following unique combination of features, most of which are not apomorphic: anteriorly retracted premaxillae and maxillae (i.e., no contact between premaxillae and nasals and maxillae retracted anteriorly from the sides of the frontals below the vertex); large posterior-most dorsal infraorbital foramen at approximately the same level with posterior margin of external bony nares; robust nasals, anteroposteriorly longer than transversely wide in dorsal view with ventrolateral margins turned ventromedially to form ridges deeply imbedded within corresponding troughs in frontals (italicized text represents an apomorphy); nasals large; nasals with transversely convex dorsal surface; dorsal surface of nasals not inflated; elevated, bilaterally compressed nearly symmetrical vertex, narrower than external bony nares; frontals form apex of the vertex; presence of os suturarum (or extra folds of the frontals or a projection of the interparietal) at the vertex; in ventral view, pronounced fossa on ventrolateral face of frontal below and behind postorbital process of frontal for the postorbital lobe of the pterygoid sinus (the postorbital recess).

Description

The bones that are preserved are thickened by comparison to a comparably sized Inia . These thickened bones along with Maximum width of bony nares 53

Maximum anteroposterior length of right nasal 43

Maximum width of nasals (combined) 43.5

Maximum width across maxillae (as preserved) 98

Minimum distance between maxillae across the 35 vertex

Maximum width between ethmoid foramina 50 (within cranial cavity)

Dorsoventral thickness of vertex through cerebral 52.5 midline of supraoccipital up through frontal

the degree to which the ventral surfaces of the maxillae and nasals deeply suture to the frontals ( Fig. 3c) indicate that the holotype of Isoninia borealis was a mature individual (as is also seen in the holotype of the inioid Stenasodelphis russellae, Godfrey and Barnes, 2008 , and in the holotype of the iniid Kwanzacetus khoisani, Lambert et al., 2018 ).

Vomer: the anterior-most preserved part of the skull is the prenarial process of the vomer ( Figs. 2 and 3), which widens conspicuously to the mesorostral canal. Posterior to it is the nasal septum.

Maxilla: only parts of the maxillae adjacent to the nares and nasals are preserved. Here, at least their lateral margins are incomplete. However, with the preserved portion it can be clearly observed that the maxillae are retracted anteriorly such that they do not overlap the frontals immediately adjacent to the vertex ( Figs. 2 and 3). Consequently, there is wide exposure of the frontal lateral to the vertex. Deep interdigitating sutures bind the maxillae and frontals ( Fig. 3c). A shallow trough on the anterior extremity of the preserved segment of the right maxilla is interpreted as the area that held the posterior extremity of the now-missing premaxilla ( Fig. 2). This suture indicates that the premaxillae were retracted rostrally such that the posterior-most reach of the premaxillae was approximately level with the mid-point in the length of the external bony nares. The maxillae form the posterolateral margin of the external nares. The thickened posteromedial margin of the maxilla broadly contacts and underlies the lateral margin of the nasal ( Fig. 3a and c).

Nasals: both nasals are well preserved in Isoninia borealis . They are robust elements, each sub-rectangular in outline in both dorsal and lateral views. In dorsal view, their conjoined anterior margin forms the essentially straight posterior margin of the external bony nares. From this position, their combined widest point, they slightly narrow gradually along their lateral borders before turning abruptly medially towards the apex of the vertex. The dorsal surface of the nasals slopes conspicuously towards the bony nares ( Fig. 3e). The pronounced undercut below the anterior margin of the nasals ( Fig. 3c) may have held the now-missing part of the presphenoid that formed the posterodorsal margin of the nasal septum. The lateral margin of each nasal is deeply imbedded ventromedially into the frontal ( Fig. 3c). The frontals are wedged between the posteromedial ends of the nasals.

Frontal: the frontals are the largest bones preserved in Isoninia borealis , occupying most of the dorsal area from the temporo-orbital plate to the back of the vertex ( Figs. 2 and 3). The posterodorsal margin of the apex of the vertex may have been slightly eroded postmortem but is otherwise complete. Immediately medial to the preserved lateral margin of the frontal is a conspicuous trough until the margin of the preserved portion of the specimen, interpreted here as homologous to the trough for the posterior-most dorsal infraorbital foramen (the actual foramen is not preserved due to the loss of the lateral portion of the maxilla).

The trough is (dorsal infraorbital canal; Fig. 3f) larger than in any other comparably sized odontocete, so the opening for the posterior-most dorsal infraorbital foramen was also thought to be correspondingly large, most similar to that seen in Isthminia (see Pyenson et al., 2015; fig. 3B). Immediately below this trough, ventrolaterally, a fossa for the postorbital lobe of the pterygoid sinus occupies the area on the ventrolateral face of the frontal below and behind the postorbital process of the frontal, (i.e., the postorbital recess in Fig. 3f).

Behind the thickened maxilla, the frontal gains wide dorsolateral exposure as it ascends the lateral face of the elevated vertex. The frontals form the narrow and pointed vertex posteromedially. Although the suture between contralateral frontals is located along the sagittal plane, below the vertex they are not symmetrical. A CT-scan image through the vertex transversely ( Fig. 3d) confirms that the right frontal is thicker than the left and its lateral face is not as concave as is that of its left antimere. A supernumerary ossification is present on the vertex, being similar to structures described in the delphinid Sotalia guianensis ( Simıes-Lopes, 2006) as os suturarum. It is visible both in photos of the partial skull and in CT scans ( Figs. 2, 3a and d). The CT images appear to indicate that it is a separate ossification distinct from the frontals, but lack of clarity engenders caution; it may simply be a fold in the left frontal. This separate ossification could represent an interparietal also present in Kwanzacetus and Inia ( Lambert et al., 2018) , as well as Samaydelphis ( Lambert et al., 2020) .

Below the vertex beyond the reach of the maxillae is a cluster of foramina that pass posteriorly and sub-horizontally through the frontal into the body of the supraoccipital ( Fig. 2). The course of at least one larger foramen on either side of the vertex can be followed through the supraoccipital where it turns medially towards the midline of the latter.

The ventral sides of the frontals preserve some of the endocranial cavity ( Fig. 3f). The ethmoid foramen passes from the anterior face of the cranial cavity anterolaterally towards the orbit. The dorsal rim of the optic canal is preserved posteroventral to the ethmoid foramen ( Fig. 3e and f).

S. J. Godfrey et al.: A new odontocete ( Inioidea , Odontoceti) from the late Neogene 279

Presphenoid and cribriform plate: most of the presphenoid ( Ichishima, 2016) is preserved in Isoninia borealis ( Figs. 2 and 3). On the posterior cerebral face of the cribriform plate dorsomedial to the ethmoid foramina are two small ovoid depressions, the long axis of which has an anteroventral– posterodorsal orientation. They may have held small olfactory bulbs (or their vestigial remnants) in life; in any case, these cavities are less well developed as compared to the tiny olfactory bulb cavities preserved in the platanistoid Araeodelphis natator ( Godfrey et al., 2017) . Extending from each of these depressions is a trough that merges into a single one dorsomedially. Minute foramina pass from these depressions and conjoining troughs through the cribriform plate to the nasal cavity ( Fig. 3b and f). Nevertheless, it is not known whether these tiny foramina through the cribriform plate actually conducted olfactory axons to olfactory receptor tissue within the nasal cavity (see Ichishima, 2016, p. 6). Unlike MGUH VP 3338 (an unnamed platanistoid odontocete from the latest Oligocene of Denmark; Hoch, 2000) and Squalodon sp. ( Godfrey, 2013), there is no development of crescentic foramina or ethmoturbinals on the anterior side of the ectethmoid in Isoninia . Therefore, Isoninia borealis seems to be more derived in the complete loss of these structures over the two aforementioned osmatic odontocetes. Although the derived condition in Isoninia does not pinpoint its geological age, it does suggest a younger Miocene or Pliocene age – a time when most, if not all odontocetes were anosmatic.

Supraoccipital: a robust supraoccipital is wedged deeply between and behind the frontals ( Fig. 3). In dorsal view, the supraoccipital gains wide exposure behind the vertex ( Fig. 3a). Posterodorsally, the top of the supraoccipital is a step down from the top of the vertex formed by the frontals ( Fig. 3e). In a posterior view of Isoninia , the broken bone suggests the presence of an external occipital crest just below the top of the supraoccipital. A transverse CT scan image through the vertex ( Fig. 3d) shows that the supraoccipital was wedged dorsally between the thickened frontals. In ventral view, the supraoccipital abuts the frontals along their posteromedial margins. Posteriorly, within the brain cavity, the medial part of the supraoccipital becomes increasingly ridge-like (i.e., the internal occipital protuberance; Fig. 3d and f) and in life is presumed to have partially separated the cerebral hemispheres posteriorly.