Pliokogia apenninica, Collareta & Fulgosi & Bianucci, 2019

Collareta, Alberto, Fulgosi, Franco Cigala & Bianucci, Giovanni, 2019, A new kogiid sperm whale from northern Italy supports psychrospheric conditions in the early Pliocene Mediterranean Sea, Acta Palaeontologica Polonica 64 (3), pp. 609-626 : 612-621

Pliokogia apenninica sp. nov.

Figs. 2–8 .

1996 “a fragmented skull of kogiinae ( Cetacea : Odontoceti)”; Cigala Fulgosi 1996: 303.

ZooBank LSID: urn:lsid:zoobank.org:act:27C92259-D30E-4F02-BB6F-CB13067302EA

Etymology: From Apennines, the mountain range that includes the type horizon.

Holotype: MSNUP I-17603 , a partial cranium, one detached tooth, one vertebra, and one fragment of rib.

Type locality: Sant’Andrea Bagni , about 20 km WSW of the town of Parma , Parma Province, Italy. Indicative geographic coordinates: 44°43’ N, 10°05’ E. Precise locality data are available on request from the authors .

Type horizon: Zanclean marine mudstone (“Argille Azzurre” sensu lato): characterised by the presence of fossilised teeth and denticles of deep-water squalean sharks such as Centrophorus squamosus , Chlamydoselachus sp. , Deania sp. , Pristiophorus sp. , Scymnodalatias aff. garricki , Scymnodon ringens , and Zameus squamulosus ( Cigala Fulgosi 1986, 1996). The geological age of the type horizon has been constrained between 5.08 and 5.04 Ma by means of foraminiferal biostratigraphy, calcareous nannoplankton biostratigraphy, and magnetostratigraphy ( Channel et al. 1994; Cigala Fulgosi 1996).

Diagnosis.— Pliokogia apenninica is a small-sized physeteroid, similar in skull length to Kogia breviceps . It is recognised as a member of Kogiidae based on the following features: estimated bizygomatic width much smaller than 400 mm, presence of a sagittal facial crest, and external nares greatly asymmetric ( Lambert et al. 2017a). It is recognised as a member of Kogiinae by the right antorbital notch whose posterior end opens onto the supracranial basin this work).

The cranium of Pliokogia apenninica differs from other kogiids by the following presumed autapomorphies: (i) presence of a proportionally long rostrum, accounting for more than three fifths of the reconstructed total length of the cranium, whose flat dorsal surface is not invaded by the supracranial basin; (ii) presence of two well-distinct fossae on the right side of the supracranial basin, including an elongated peripheral maxillary fossa on the caudal portion of the right maxilla, posterior and posteromedial to the corresponding antorbital notch; and (iii) presence of a sagittal vomerine sulcus, reflected in a V-shaped transverse section of the dorsal surface of the vomer along the mesorostral groove.

Pliokogia apenninica further differs from Aprixokogia , Koristocetus , Nanokogia , and Scaphokogia by the thicker and blunter lateral maxillary crest. It further differs from Aprixokogia and Koristocetus by the presence of a subdivided right posterior dorsal infraorbital foramen and by the lower temporal fossa. It further differs from Aprixokogia , Scaphokogia , and Thalassocetus by the right antorbital notch opening onto the supracranial basin. It further differs from Aprixokogia and Kogia by the smaller angle between the frontal–maxilla suture line and the coronal plane, along the supraorbital process, with skull in lateral view. It further differs from Aprixokogia and Thalassocetus by the slit-like geometry of the antorbital notches. It further differs from Aprixokogia by the smaller skull length. It further differs from Kogia , Nanokogia , and Praekogia by presenting no lateral expansion of the postnarial eminence of the right premaxilla. It further differs from Kogia by the proportionally smaller lacrimojugal complex. It further differs from Kogia breviceps , Kogia sima , and Koristocetus by the absence of any constriction of the right premaxilla at the level of the external bony nares. It further differs from Kogia breviceps and Kogia sima by the supracranial basin not being laterally expanded, by the thinner and not inflated lateral maxillary crest, and by the higher temporal fossa. It further differs from Kogia pusilla , Kogia sima , and Koristocetus by the greater skull length. It further differs from Kogia sima and Scaphokogia by the lack of a left premaxillary foramen. It further differs from Kogia sima by the presphenoid which does not extend anteriorly within the mesorostral groove. It further differs from Koristocetus and Nanokogia by the less obliquely oriented frontal groove. It further differs from Nanokogia and Scaphokogia by having a wider mesorostral groove in the posterior portion of the rostrum. It further differs from Praekogia by having the left premaxilla that does not reach the sagittal facial crest. It further differs from Scaphokogia by the rostrum having a dorsal surface that is not semicylindrical, the postnarial eminence that significantly contributes to the sagittal facial crest, the sagittal facial crest that is not significantly dislocated towards the left side of the skull, and the supracranial basin that is not spoon-shaped.

Description.—Cranium: The skull is incomplete, lacking the basicranium, the left part of the supracranial basin, the left lateral portion of the rostrum, all the ear bones, the mandibles, and all the teeth but one ( Figs. 2–7 ). The dorsal aspect of the cranium is generally well preserved, although some surfaces are moderately abraded and the left part of the supracranial basin is lost ( Fig. 2 ). In ventral and lateral views, the cranium is poorly preserved, which makes the relationships between the different bones locally unclear ( Figs. 3 , 4 ). Various fractures, sometimes filled by gypsum, are observed throughout the cranium.

The maximum preserved length of the cranium is 380 mm, which closely approximates the condylobasal length, here estimated at about 400 mm. Such a reconstructed skull length is similar to that of Kogia breviceps , smaller than that of Aprixokogia , and greater than those of Kogia pusilla , Kogia sima , Koristocetus , and possibly Nanokogia .

The neurocranium is dorsally concave, forming a wide supracranial basin (a key feature of Physeteroidea); it is also strongly asymmetric, exhibiting strongly uneven bony nares (i.e., the left naris is much wider than the right one) that are significantly displaced leftwards, as well as an obliquely directed sagittal facial crest (a diagnostic character of Kogiidae ) ( Fig. 2 ). With respect to extant Kogia , the skull of Pliokogia appears as strongly elongated anteroposteriorly ( Figs. 2–5 ), due to the presence of a 250 mm long rostrum (measured between the level of the antorbital notches and the anteriormost tip of the skull). The ratio between the length of the splanchnocranium (i.e., the rostrum) and the condylobasal length is thus estimated at around 0.63 in Pliokogia , that is, indistinguishable from that estimated for K. pusilla (ca. 0.63, but the sole known skull is strongly distorted by diagenesis), vs. 0.51 in Nanokogia and about 0.45–0.50 in living kogiids and Koristocetus . As the rostrum of K. pusilla is markedly concave dorsally ( Bianucci and Landini, 1999), the long and dorsally flat splanchnocranium of Pliokogia is unique among the living and extinct kogiids for which this feature is known. Sutures between adjacent cranial bones are fused in MSNUP I-17603, thus indicating that it belonged to a physically adult individual e.g., Chen et al. 2011).

Bioerosional modifications can be locally observed on MSNUP I-17603. On the dorsal surface of the right maxilla, anterior to the antorbital notch, four prominent elongated incisions are present ( Figs. 2 , 6 ). These traces consist of substraight gouges with no serrations, disposed roughly orthogonal to the lateral margin of the bone, ranging in length between 11 mm and 32 mm. Their morphology cannot easily match any bioerosional feature originating from the action of marine invertebrates, being in turn perfectly consistent with the bone modifications due to biting by sharks provided with large, labiolingually flattened, smooth-edged tooth crowns (e.g., Bianucci et al. 2010, 2018; Govender 2015). In the light of the morphological-genetic classification scheme proposed by Cigala Fulgosi (1990) and then emended by Bianucci et al. (2010) and Collareta et al. (2017a), these traces can be interpreted as due to type I (i.e., production of a subrectilinear or weakly curved mark by impact of the tooth edge from above downward) or type II (i.e., production of a more or less elongated incision by dragging of the tooth edge in parallel with the dental axis) biting actions. In addition, an indeterminate circular bioerosion scar is present on the palatal surface of the left maxilla ( Fig. 3 ).

Premaxilla: Both premaxillae are lost in the anteriormost quarter of the rostrum; moreover, the left premaxilla is not preserved posterior to mid-length of the left bony naris. In dorsal view, the lateral margins of the premaxillae are substraight along the dorsal surface of the rostrum and weakly converge towards the anteriormost tip of the skull ( Fig. 2 ). At the level of the bony nares, the premaxillae are subequal in transverse width; anterior to this level, the right premaxilla is slightly but distinctly wider than the left premaxilla throughout their preserved length along the rostrum. Medial to the premaxillae, and partially overhung on by them, the mesorostral groove opens approximately 40 mm anterior to the base of the rostrum. The mesorostral groove widens progressively towards the apex of the rostrum as the medial borders of the premaxillae weakly diverge forwards. The posterior half of the mesorostral groove is more widely open dorsally than in Nanokogia and Scaphokogia (a condition reminiscent of that observed in Aprixokogia , Kogia breviceps , and Koristocetus ); however, moderate erosion of the medial margins of the premaxillae may have accentuated this character. In the posterior third of the rostrum, just anterior to the posterior end of the mesorostral groove, the dorsal surfaces of both premaxillae are slightly concave transversely ( Fig. 5 ); they progressively flatten in the central third of the rostrum. At about two thirds of the length of the rostrum (measured from its base), the dorsal surface of the left premaxilla is gently convex, whereas the dorsal surface of the right premaxilla is flat; MSNUP I-17603 thus differs from Aprixokogia and Kogia spp. , in which the dorsal surface of the rostrum is distinctly concave throughout. The lateral face of the left premaxilla (i.e., the premaxilla-maxilla suture) can be observed for most of the length of the rostrum thanks to the loss of the adjoining portion of the left maxilla; it is flat and subvertical throughout. As in Aprixokogia , Kogia breviceps , K. pusilla , Koristocetus , and Nanokogia , there are no foramina on the left premaxilla ( Fig. 2 ). Conversely, a rather large (12 mm long and 8 mm wide) premaxillary foramen is present on the right premaxilla, at the level of the posterior end of the mesorostral groove ( Figs. 2 , 6 ); this foramen is followed posteriorly by a 15 mm long groove. Medial and posteromedial to the right premaxillary foramen, the dorsal surface of the right premaxilla is high and bulging. The external bony nares are situated at the level of the antorbital notches. As observed in all physeteroids, they are strongly asymmetrical and distinctly displaced towards the left side of the skull. Only the anterior and medial walls of the large (i.e., more than 20 mm wide) left naris are preserved; they suggest an oval-shaped opening, situated in an anteroposteriorly oriented, funnel-shaped depression a feature reminiscent of Kogia spp. and Koristocetus ). The right naris is significantly smaller than the left and roughly circular, having a diameter of 9 mm; its anterior, lateral, and posterior walls are formed by the right premaxilla. Differing from Kogia breviceps , K. sima , and Koristocetus , no constriction of the right premaxilla is observable at the level of the external bony nares. Posterior to this level, as observed in all Kogiidae described to date, the right premaxilla is expanded posteromedially as a strip of bone (i.e., the postnarial eminence sensu Whitmore and Kaltenbach 2008) that forms most of the right dorsolateral face of the sagittal facial crest

Figs. 2 , 3 , 5 ); in this respect, MSNUP I-17603 markedly differs from Scaphokogia , in which the right premaxilla constitutes only a small fraction of the short and strongly leftwards displaced sagittal facial crest. The lateral margin of the postnarial eminence is almost straight and posterodorsomedially oriented, thus differing from the condition observed in Aprixokogia , Kogia breviceps , K. sima , Nanokogia , and Praekogia . Differing from extant Kogia , the postnarial eminence of MSNUP I-17603 does not overhang laterally over the right side of the supracranial basin. The anterior portion of the postnarial eminence, lateral and posterolateral to the right naris, is shaped as a rather deep fossa (i.e., the premaxillary fossa sensu Barnes 1973), whose floor represents the lowest point of the exposure of the right premaxilla within the supracranial basin (see description of the maxilla below) and, more generally, on the whole dorsal surface of the skull. The position and extent of the premaxillary fossa recall a similar depression observed in Praekogia ; in the latter taxon, however, this depression extends onto the adjoining lateral margin of the right maxilla, whereas in MSNUP

I-17603 it is defined laterally by a steep wall just medial to the premaxilla-maxilla suture. The premaxillary fossa is oval-shaped, rather large (ca. 60 mm longitudinally long and 30 mm transversely wide), and parallels the lateral margin of the postnarial eminence. In our opinion, this depression is homologous to the fossa observed on the right dorsolateral face of the sagittal facial crest of Kogia spp. , Koristocetus , and Thalassocetus (but see Barnes 1973 and Whitmore and Kaltenbach 2008 for different interpretations of the premaxillary fossa of Praekogia ). If this homology is correct, the premaxillary fossa of MSNUP I-17603, being situated close to the sagittal plane of the skull, likely hosted the spermaceti chamber accommodating the fibrous case that surrounds the spermaceti organ of extant Kogiidae ( Thornton et al. 2015) . The sagittal facial crest is only partially preserved, lacking its dorsal (i.e., subhorizontal) edge; in turn, the lower portion of the anterior (i.e., subvertical) edge of the sagittal facial crest is preserved, being observed at the posterior termination of the dorsal exposure of the presphenoid, medial to the bony nares. The preserved portion of the sagittal facial crest, facing rightwards, is flat and slopes anterodorsally. The posterior termination of the sagittal facial crest reaches (or almost reaches) the nuchal crest, thus differing from the condition observed in Aprixokogia . The premaxillae are not exposed on the ventral surface of the cranium ( Fig. 3 ); however, they are not preserved at (and close to) the anteriormost tip of the skull. The premaxilla is observed, in ventral view, on skulls of the extant kogiids Kogia breviceps and K. sima .

Maxilla: Both maxillae are incompletely preserved. The right maxilla is not preserved in the anteriormost fourth of the rostrum, and the ventral tip of the antorbital process is also lost ( Figs. 3 , 4 ). The left maxilla is mostly lost, except for part of its palatal surface. In dorsal view, the dorsal exposure of the maxilla tapers transversely towards the anterior termination of the skull ( Fig. 2 ). Through the rostrum, the lateral margin of the right maxilla is anteromedially directed; it is not substraight as seen in Kogia breviceps and K. sima , but rather exhibits a marked constriction at about three fifths of the rostrum length, thus recalling the condition observed in Koristocetus and, especially, Nanokogia . The right maxilla is slightly but constantly wider than the adjoining premaxilla along the rostrum; however, the former bone is not preserved at (and close to) the anterior end of the rostrum, a region where the right maxilla is wider than the premaxilla in some extinct kogiids (e.g., Nanokogia ). Medial and posteromedial to the right antorbital notch, the dorsal surface of the right maxilla is high, bulging, and rugose; then it flattens progressively forwards. The antorbital notch is deep, narrow, teardrop-shaped, and slit-like in its anterior portion, thus recalling Kogia spp. in this respect. The posterior end of the right antorbital notch opens onto the supracranial basin, a condition reminiscent of that observed in Kogia spp. , Koristocetus , Nanokogia , and Praekogia . Lateral to the antorbital notch, the right lateral maxillary crest reaches its maximum height and width ( Figs. 4 , 5 ); it is high and transversely thick, not as inflated as in Kogia breviceps and K. sima , but in contrast to the thin lateral maxillary crest of Aprixokogia , Koristocetus , Nanokogia , and Scaphokogia . The right lateral maxillary crest does not overhang either the supracranial basin or the orbital region; it points towards the apex of the rostrum and forms part of the right lateral margin of the supracranial basin. Due to the loss of the left maxilla and premaxilla posterior to the left bony naris, only the right portion of the supracranial basin (i.e., right to the sagittal facial crest), is preserved in MSNUP I-17603. As in all Kogiidae except for Scaphokogia , the supracranial basin generally faces anterodorsally ( Fig. 5 ). Similar to Koristocetus , Nanokogia , and Scaphokogia , the supracranial basin does not extend into the region above the rostrum ( Fig. 2 ); conversely, in Aprixokogia and Kogia spp. the anterior part of the supracranial basin is open onto the dorsal surface of the rostrum. Similar to Nanokogia , the right posterolateral edge of the supracranial basin slightly overhangs the underlying temporal region (see description of the frontal bone below). The preserved lateral and posterolateral margins of the ascending process of the right maxilla suggest a roughly oval outline for the supracranial basin, thus recalling the condition observed in Nanokogia and Praekogia rather than the more circular and posterolaterally expanded supracranial basin of extant Kogia . Posterior and posteromedial to the right antorbital notch, and posterolateral to the premaxillary fossa, a major depression is observed on the facial surface of the right maxilla, in the right posterolateral region of the supracranial basin ( Figs. 2 , 5 ). This depression, for which the name “peripheral maxillary fossa” is here proposed, is limited anteriorly by the bulging region of the maxilla medial to the right antorbital notch, medially by a broad and substraight ridge running just lateral to the right border of the postnarial eminence, and laterally by the right lateral maxillary crest. The peripheral maxillary fossa is almost completely preserved, its posteromedial termination (sited where the sagittal facial crest and the nuchal crest likely joined to each other) being lost; it is lobe-shaped, larger than the premaxillary fossa (i.e., ca. 110 mm long and 55 mm wide), and deeply excavated. The floor of this depression, which displays a fibrous aspect, includes the lowest point of the exposure of the right maxilla within the supracranial basin. The peripheral maxillary fossa of MSNUP I-17603 is strongly reminiscent of a similar kidney-shaped depression located in the right posterolateral region of the supracranial basin of Aprixokogia . The latter was interpreted by Withmore and Kaltenbach (2008) as homologous to the larger right maxillary fossa of modern kogiids, which occupies most of the facial aspect of the right maxilla and expands onto the caudal termination of the right antorbital notch (thus differing from the condition observed in Pliokogia ). In extant Kogia , this fossa hosts the vocal chamber where the phonic lips and vocal caps (two soft-tissue structures, part of the echolocating system of Kogia breviceps and K. sima ) are situated ( Thornton et al. 2015). If this homology and the above interpretation of the premaxillary fossa of MSNUP I-17603 are correct, then the topology of the echolocating system of this fossil kogiid would have been likely similar to that of the extant species K. breviceps and K. sima (see e.g., Thornton et al. 2015: figs. 2–5). Several dorsal infraorbital foramina are observed on the right maxilla ( Figs. 2 , 6 ). The right anterior dorsal infraorbital foramen is located 40 mm anterior to the posteriormost end of the right antorbital notch, near the maxilla-premaxilla suture; it is anteroposteriorly elongated, 5 mm wide, and followed anteriorly by a deep, 45 mm long groove extension. As observed in Kogia breviceps , K. sima , Nanokogia , Praekogia , Scaphokogia , and Thalassocetus , the right posterior dorsal infraorbital foramen is subdivided, and at least three large openings are observed on the supracranial basin posteromedial to the right antorbital notch. The anteriormost opening is located 10 mm posteromedial to the right antorbital notch; it is elliptical, anterolaterally oriented, 8 mm long and 4 mm wide, and bears no groove extension. Medial and slightly posterior to this foramen, another elliptical foramen is observed 5 mm lateral to the maxilla-premaxilla suture, close to the lateralmost margin of the premaxillary fossa. This foramen is anteroposteriorly elongated, 11 mm long and 7 mm wide, and is followed posteromedially by a short and wide groove; it is connected by a wide branch of the infraorbital canal with the underlying right ventral infraorbital foramen (see below). The posteriormost opening observed on the dorsal surface of the right maxilla is located 60 mm posterolateral to the right bony naris. It opens into the left portion of the peripheral maxillary fossa, 5 mm lateral to its medial border. This foramen is anteroposteriorly elongated, 15 mm long and 5 mm wide, and is not followed by a groove. Medial to the right antorbital notch, where the poorly preserved dorsal surface of the maxilla is rough and bulging, the strong abrasion of the cortical portion of the bone prevents the unambiguous detection of foramina, but three or four small (i.e., smaller than 5 mm in diameter) additional openings might be present in this area. In lateral view ( Figs. 4 , 5 ), the right lateral maxillary crest exhibits a high and subvertical face that comprises the supraorbital and antorbital processes of the maxilla (the anteroventral tip of the latter is missing). Medial to the anterior portion of the lateral maxillary crest, the lateral surface of the rostral portion of the right maxilla is high and adpressed to the right antorbital process. Anterior to this level, the lateral margin of the maxilla becomes slightly thinner dorsoventrally and regularly rounded. In ventral view

Fig. 3 ), the palatal surface of the maxilla is poorly preserved. Throughout the rostrum, the ventral surface of the maxilla is significantly convex transversely in its medial part, whereas it becomes flat to weakly concave close to the margins of the rostrum. A few badly defined circular depressions, having a mean diameter of ca. 8 mm, are locally observed on both maxillae, where the bone surface is better preserved. These depressions are reminiscent of shallow dental alveoli such as those observed on the bony palate of Koristocetus ; in turn, deeper and large upper dental alveoli are present in Aprixokogia , whereas a relict alveolar groove is observed in Kogia spp. , Nanokogia , and Scaphokogia . Along the right maxilla, these putative alveoli are situated roughly at mid distance between the lateral edge of the rostrum and the sagittal plane of the skull, where the flat and convex portions of the palatal surface of the maxillae meet each other. Among the individuated depressions, the posteriormost ones are observed on the left maxilla, in the posterior third of the rostrum. It is unclear, however, whether these putative alveoli were well distinct from each other all along the rostrum. Moreover, although the preservation state of the palatal surface of the rostrum might partially obliterate their original depth, it seems unlikely that, in the living animal, the maxillary dental alveoli would have been deep enough to bear functional teeth, especially when considering the slen- der and elongated morphology that characterises the sole preserved tooth of Pliokogia (see below). The right and left maxillae are separated medially by the ventral exposure of the vomer, except for the mid part of the palate, where the maxillae contact each other medially for about 50 mm, thus recalling the condition observed in Nanokogia . Due to the poor preservation state of the ventral surface of the skull, the presence of palatine foramina and sulci could not be ascertained. In ventral view, the right antorbital notch penetrates less posteriorly than observed in dorsal view, i.e., the notch itself is shorter posteriorly than it appears on the dorsal view. Posterior to the base of the rostrum, the maxilla projects lateral to the palatine and medial to the lacrimojugal complex and frontal to form the anterior and anteromedial walls of the ventral infraorbital foramen. The latter is elliptical, anterolaterally directed, and very large (i.e., 29 mm long and 15 mm wide), although its size could have been exaggerated by peripheral erosion of its walls. Anterior to the ventral infraorbital foramen and medial to the antorbital notch, a rather deep depression is observed on the posterior portion of the right maxilla. This fossa is anteroposteriorly stretched, ca. 55 mm long, and parallels the adjoining palatine, thus recalling the condition observed in Scaphokogia . Similar depressions, located anterior or anterolateral to the ventral infraorbital foramen, have also been observed in Kogia breviceps , K. sima , Koristocetus , and Nanokogia . Following the interpretation proposed by Vélez-Juarbe et al. (2015) and Collareta et al. (2017a), we regard this fossa as related to the anteriormost portion of the pterygoid sinus complex.

Nasal: The right nasal is absent. The same condition is observed in the vast majority of physeteroids, with the significant exception of the late Miocene Peruvian species Acrophyseter robustus ( Lambert et al. 2017a) . Due to the loss of the posterior wall of the left bony naris ( Fig. 2 ), it is not possible to ascertain whether MSNUP I-17603 possessed the left nasal (absent in all kogiids known to date).

Palatine: Because of the loss of most of the pterygoids, both palatines are largely exposed on the ventral surface of the skull, located posteromedially to the maxillae and anterolaterally to the choanae ( Fig. 3 ). They are long, anterolaterally bent (i.e., reniform), gently convex transversely, and almost contact each other medially. Their medial margin is provided by the ventral exposure of the vomer. The left palatine contributes to the anterior border of the left choana. Otherwise, the palatines are not well defined.

Lacrimojugal complex: Only the right lacrimal is partly preserved ( Figs. 3–5 ). In lateral view ( Fig. 4 ), the lacrimal is large and roughly hook-shaped: indeed, it exhibits an elongate posterodorsal corner (posterodorsal process sensu Muizon 1988) that is wedged between the frontal and the maxilla; a similar feature has been observed, more or less pronounced, in all Kogiidae except for Aprixokogia (condition unknown in Thalassocetus ). The posterodorsal process ofthelacrimalislongandslender;itapparentlyextendsmore posteriorly than in Kogia , Koristocetus , and Nanokogia , thus recalling the condition observed in Praekogia . As observed in Kogia , Koristocetus , Nanokogia , and Scaphokogia , the lacrimal-maxilla suture is sigmoidal: it is distinctly convex dorsally in its anterior half and gently concave dorsally in its posterior half, thus roughly paralleling the dorsal profile of the right lateral maxillary crest. The anteroventral tip of the lacrimal bone is not preserved. In ventral view ( Fig. 3 ), as observed in Nanokogia and Praekogia , the lacrimojugal complex extends posteromedially, towards the anterolateral termination of the ventral infraorbital foramen, but its boundaries are not well defined.

Frontal: Only the right frontal is partly preserved. This bone is mostly exposed in lateral and ventral views ( Figs. 3 , 4 ), its dorsal surface being indeed covered by the right maxilla. When viewed laterally ( Fig. 4 ), the frontal-maxilla suture along the supraorbital process is inclined at about 20° with respect to the coronal plane, thus strongly differing from Aprixokogia and Kogia (in which much higher angles are observed) and resembling instead Thalassocetus in this respect. The preorbital and postorbital processes are not preserved, and only a central segment of the orbit roof is preserved. The frontal also comprises the upper portion of the roof of the temporal fossa. The latter is only incompletely preserved; its higher point is situated slightly higher than the highest point of the orbit but ca. 40 mm lower than the overlying margin of the supracranial basin, thus contrasting with both the high temporal fossa of Aprixokogia and Koristocetus and the low temporal fossa of extant Kogia , recalling instead the condition observed in Nanokogia . The outline of the preserved portion of the temporal crest could suggest that the temporal fossa did not strongly differ from that of Nanokogia . When viewed ventrally ( Fig. 3 ), the supraorbital process displays a wide frontal groove (representing the distal extension of the optic canal) running from the unpreserved optic foramen towards the lateral termination of the supraorbital process of the frontal (i.e., the roof of the orbit). The frontal groove moderately widens towards its distal end; it is oriented anterolaterally at a smaller angle than observed in Koristocetus and Nanokogia .

Vomer: The vomer reaches the anterior end of the skull ( Figs. 2–5 ). In dorsal view ( Fig. 2 ), the vomer is widely exposed along the rostrum and forms the floor of the mesorostral groove. The dorsal surface of the vomer consists of two flat to slightly convex bony walls, facing dorsomedially, whose medial junction forms a straight, anteroposteriorly oriented sulcus, for which the name “sagittal vomerine sulcus” is here proposed; as such, the transverse section of the dorsal surface of the vomer along the mesorostral groove is V-shaped ( Figs. 2 , 5 ). The presence of such a sagittal vomerine sulcus appears to be unique of Pliokogia among Kogiidae . In ventral view ( Fig. 3 ), the vomer is exposed as a narrow but distinctly carinated slit running between the maxillae for most of the rostrum length. At the base of the rostrum, the vomer is exposed between the palatines, where it displays a prominent, anteroposteriorly elongated ventral keel. As in most kogiids except Praekogia , the vomer does not cover the ventral surface of the presphenoid medial to the choanae.

Sphenoid: Following Ichishima (2016), we identify the bony septum that separates the external nares medially as belonging to the presphenoid. This septum ( Fig. 2 ) is transversely thin but flares mediolaterally at the posterior termination of the mesorostral groove, where it is exposed for about 20 mm between the premaxillae. Differing from most physically mature specimens of Kogia sima and some specimens of K. breviceps , the presphenoid does not further extend anteriorly within the mesorostral groove. In ventral view ( Fig. 3 ), the presphenoid is partially exposed between the choanae. A long and thin slice of bone, running parallel and contiguous to the anteromedial margin of the right frontal groove towards the posterior wall of the right ventral infraorbital foramen, is here regarded as possibly representing the orbitosphenoid, following the interpretation proposed by Collareta et al. (2017b) for a similar feature in Koristocetus .

Pterygoid: Only a small fraction of the pterygoids is tentatively identified on MSNUP I-17603. An anteroposteriorly elongated stripe of bone, taking place between the right palatine and the adjoining portion of the maxilla lateral and anterolateral to the right bony naris, is here interpreted as a portion of the right pterygoid ( Fig. 3 ). This bone seems also to cover the lateral margin of the right palatine.

Occipital: A small portion of the supraoccipital is preserved, posterior to the apex of the temporal fossa and below the posterolateral margin of the supracranial basin ( Fig. 4 ). It contacts the frontal anteriorly; the supraoccipital-frontal suture runs roughly subvertical.

Dentition: The only preserved tooth ( Fig. 7 ) is slender, streamlined, delicate, and almost complete, lacking only the basalmost tip of the elongated root; it measures 35 mm in maximum preserved dorsoventral height and 5 mm in maximum transverse diameter. It displays a very weak curvature, weaker than generally observed in the teeth of extant Kogia and in more robust isolated fossil teeth, possibly belonging to Kogiidae , described and figured by Pilleri (1987: pl. 15) from the Italian Pliocene locality of Orciano Pisano. The uppermost 7 mm of the tooth are comprised of a dark-coloured cusp, having a roughly circular base (with a mean diameter of ca. 4 mm) and a conical outline. As in most extant adult individuals of Kogia breviceps and K. sima ( Plön 2004) , the sole preserved tooth of Pliokogia completely lacks enamel. The tooth apex exhibits a small, barely convex, slightly polished facet that is here interpreted as due to abrasive wear. This wearing facet is subvertically oriented, and as such, it provides the tooth apex with a spatula-like appearance. Similar abrasion surfaces, sometimes tilted medially or laterally, are often observed on the teeth of the great sperm whale Physeter macrocephalus , where they are explained by the repeated passage of water laden with prey items Lambert et al. 2013); tooth wear modifications, mostly resulting in the loss of the neonatal enamel cap, have also been reported for extant kogiids, but their origin is still not clear Plön 2004; Stephanie Plön, personal communication 2018). Although the only preserved tooth of Pliokogia was found in close proximity of the tip of the rostrum of MSNUP I-17603 (personal observation by AC and FCF), the absence of deep alveoli on the ventral surface of the maxillae suggests its identification as a mandibular tooth; only mandibular teeth are observed in extant Kogiidae , but functional maxillary teeth were presumably present in Koristocetus Collareta et al. 2017b ). Pliokogia apenninica represents the first extinct kogiid species based on significant cranial material for which at least one tooth is known.

Ribs: A single fragmentary and slightly distorted anterior rib is preserved ( Fig. 8A ). The head, neck, tubercle, and distal termination of the rib are unfortunately lost. The shaft has a minimum preserved width of 30 mm and ranges from 7 mm to 12 mm in thickness. Based on its general shape and curvature at the angle, this bone is tentatively identified as the second right rib.

Vertebrae: Only one 21 mm long partial vertebra is preserved ( Fig. 8B ). This vertebra appears to have been deformed by diagenetic compression, and consequently its centrum is shaped as an oblique circular cylinder. The proximal portions of both pedicles are preserved: the left pedicle is relatively more complete than the right and points anterodorsolaterally. The position of the pedicles indicates that, originally, the neural arch was rather wide transversely. The general proportions and size of this vertebra, as well as its moderate anterolateral thickness and the presence of pedicles, suggest its identification as a thoracic. Both the anterior and the posterior vertebral epiphyses are fused with the centrum. Given that, in extant odontocetes, the progression of vertebral epiphyseal ankylosis seemingly terminates in the thoracic and lumbar regions ( Galatius and Kinze 2003), this observation confirms that MSNUP I-17603 represents a full-grown cetacean individual.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 2. Cranium, in dorsal view, of the kogiid sperm whale Pliokogia apenninica gen. et sp. nov. (MSNUP I-17603, holotype), from the lower Pliocene of Sant’Andrea Bagni (Northern Apennines, northern Italy). Photograph (A1), explanatory drawing (A2). The black areas correspond to openings in the skull. The grey vertical stripes denote abraded or broken areas. Dashed lines are for the sagittal vomerine sulcus and the approximate border of the supracranial basin, premaxillary fossa, and peripheral maxillary fossa.

Fig. 3. Cranium, in ventral view, of the kogiid sperm whale Pliokogia apenninica gen. et sp. nov. (MSNUP I-17603, holotype), from the lower Pliocene of Sant’Andrea Bagni (Northern Apennines, northern Italy). Photograph (A1), explanatory drawing (A2). The black-shaded areas correspond to openings in the skull. The grey vertical stripes denote abraded or broken areas. Dashed lines for approximating the position of several sutures and borders.

Fig. 4. Cranium, in lateral view, of the kogiid sperm whale Pliokogia apenninica gen. et sp. nov. (MSNUP I-17603, holotype), from the lower Pliocene of Sant’Andrea Bagni (Northern Apennines, northern Italy). Photograph (A1), explanatory drawing (A2). The grey vertical stripes denote abraded or broken areas. Dashed line for approximating the position of the frontal-lacrimojugal complex suture.

Fig. 5. Cranium of the kogiid sperm whale Pliokogia apenninica gen. et sp. nov. (MSNUP I-17603, holotype), from the lower Pliocene of Sant’Andrea Bagni (Northern Apennines, northern Italy); in anterodorsal (A1) and right anterodorsolateral (A2) views.

Fig. 6. Close-up of the dorsal surface of the cranium of the kogiid sperm whale Pliokogia apenninica gen. et sp. nov. (MSNUP I-17603, holotype), from the lower Pliocene of Sant’Andrea Bagni (Northern Apennines, northern Italy), showing the shark bite marks that affect the right maxilla (arrows).

Fig. 7. Detached tooth of kogiid sperm whale Pliokogia apenninica gen. et sp. nov. (MSNUP I-17603, holotype), from the lower Pliocene of Sant’Andrea Bagni (Northern Apennines, northern Italy), in four different views (A1–A4). The dashed line denotes the extent of the wear facet.

Fig. 8. Postcranial bones of the kogiid sperm whale Pliokogia apenninica gen. et sp. nov. (MSNUP I-17603, holotype), from the lower Pliocene of Sant’Andrea Bagni (Northern Apennines, northern Italy). A. Fragmentary right rib in dorsal (A1), posterior (A2), and ventral (A3) views. B. Fragmentary thoracic vertebra in anterior (B1), posterior (B2), and dorsal (B3) views.