Platyscaphokogia landinii, Collareta & Lambert & Palomino & Urbina & Bianucci, 2020

COLLARETA, Alberto, LAMBERT, Olivier, MUIZON, Christian de, BENITES PALOMINO, Aldo Marcelo, URBINA, Mario & BIANUCCI, Giovanni, 2020, A new physeteroid from the late Miocene of Peru expands the diversity of extinct dwarf and pygmy sperm whales (Cetacea: Odontoceti: Kogiidae), Comptes Rendus Palevol 19 (5), pp. 79-100 : 83-93

Platyscaphokogia landinii n. sp.

( Figs 2-6; 8A)

urn:lsid:zoobank.org:act:FC3BE988-0F09-4262-911C-5B50070D1B97

ETYMOLOGY. — The specific name honours Walter Landini, Italian vertebrate palaeontologist, for his substantial contribution to the knowledge and understanding of both the Miocene vertebrate assemblages of the East Pisco Basin (e.g., Landini et al. 2017a, b, 2019) and the Italian fossil record of physeteroids (e.g., Bianucci & Landini 1999, 2006).

HOLOTYPE. — MUSM 3405 , an incomplete skull lacking the tip of the rostrum and the basicranium.

PARATYPE. — MUSM 3291 , an incomplete skull, preserving only the rostrum and the right portion of the neurocranium.

TYPE LOCALITY. — The holotype was found at Cerro Hueco la Zorra, Ica desert, c. 30 km south of the town of Ica, southern coast of Peru ( Fig. 1B); geographic coordinates of the finding site: 14°26’44.35”S, 75°42’24.05”W.

TYPE HORIZON. — The holotype was found in deposits belonging to the silty upper portion of the P2 allomember of the Pisco Formation (as defined by Di Celma et al. 2017, 2018a) ( Fig. 1B, C). Stratigraphically, MUSM 3405 is located between the P2-8 and the P2-9 horizons of Di Celma et al. (2017, 2018a), which are prominent and laterally continuous ledges at the top of sheer cliff faces. At the nearby locality of Cerro Blanco, a tephra layer placed just below the P2-8 horizon has been recently dated to 7.084 ± 0.044 Ma by means of Ar-Ar radiometric analyses ( Bosio et al. 2020b). An age younger than 7.084 Ma is thus proposed for the horizon from which the holotype was collected. Considering also that no marine deposit younger than the Miocene is known from the western side of the lower Ica valley, MUSM 3405 is here regarded as Messinian (latest Miocene) in age.

LOCALITY AND HORIZON OF THE PARATYPE. — The paratype was found at Cerro los Quesos (Ica desert, c. 50 km south of the town of Ica, southern coast of Peru ( Fig. 1B, C); geographic coordinates of the finding site: 14°30’54.00”S, 75°43’08.90”W), in deposits belonging to the P2 allomember of the Pisco Formation.

MUSM 3291 was indicated with the field number O14 and identified as belonging to “aff. Scaphokogia sp. ” in the fossil map published by Bianucci et al. (2016a). The horizon from which the paratype was collected belongs to the stratal package known as “Member F” ( Di Celma et al. 2016a), within the silty upper portion of the P2 allomember; it is further located c. 35 m above a tephra layer, the Mono key bed of Di Celma et al. (2016a), that has been dated to 6.93 ± 0.09 Ma by means of Ar-Ar radiometric analyses. A stratigraphically higher volcanic ash bed, cropping out in the uppermost portion of the Cerro Los Quesos succession (i.e., 17 m above the paratype), has been dated to an age equal or older than 6.71 ± 0.02 Ma ( Di Celma et al. 2016a). As such, the horizon from which the paratype originates can be constrained to a short time interval within the Messinian, i.e., between 6.93 Ma and 6.71 Ma. Such an age estimate finds support in the biostratigraphic data, which indicate an age younger than 7.35 Ma for this portion of Member F ( Gariboldi et al. 2017).

Several fossil vertebrates from Cerro Los Quesos were found in strata belonging to Member F, including mysticetes (balaenopterids and cetotheriids), odontocetes (e.g., the holotype of the beaked whale Chavinziphius maxillocristatus , the macroraptorial stem physeteroid Acrophyseter sp. , and the phocoenid cf. Lomacetus ginsburgi ), pinnipeds, crocodiles, seabirds, and sharks ( Bianucci et al. 2016a, b; Lambert et al. 2017a).

DIFFERENTIAL DIAGNOSIS. — Platyscaphokogia landinii n. gen., n. sp. differs from Scaphokogia cochlearis by the following combination of characters: in dorsal view, skull roughly drop-shaped, due to the remarkable tapering of the rostrum towards the anterior tip of the skull; sagittal facial crest longer, less displaced laterally, dividing the supracranial basin in two different sub-basins; supracranial basin moderately shallower and slightly more anterodorsally oriented; in lateral view, rostrum vertically much thinner (i.e., dorsoventrally compressed) and not exceeding in height the lateral maxillary crests at its base; long axis of the rostrum projecting anteriorly rather than anteroventrally; in ventral view, presence of larger, well-individualised, likely functional upper dental alveoli (right maxilla presenting 10 alveoli).

DESCRIPTION, COMPARISONS, AND REMARKS

The following description is based on the holotype, unless otherwise specified.

General outline of the cranium

The holotype skull ( MUSM 3405; Figs 2-5) is incomplete, lacking the basicranium, part of the floor of the supracranial basin (coinciding with the circumnarial basin of Mead & Fordyce 2009; the former term is here preferred for consistency with several recent works on the skull anatomy of extinct kogiids), all the ear bones, the mandibles, and all the teeth. Both the dorsal and the ventral aspects of the cranium are generally well preserved, although a few delicate features (e.g., the thin, plate-like lateral maxillary crests and the medial halves of the premaxillae along the rostrum) are locally broken or abraded and some regions of the cranium (e.g., the narrow antorbital notches and most of the mesorostral groove) are covered or infilled by hardened sediment. Unambiguous evidence of diagenetic deformation is not found on MUSM 3405, and even the most delicate features (e.g., thin and salient bony walls such as the lateral maxillary crests and the sagittal facial crest) do not appear to have significantly suffered compaction, displacement, or distorsion. The paratype cranium ( MUSM 3291; Fig. 6) is distinctly less complete, only preserving the rostrum, the anterolateral portion of the supracranial basin, and the right supraorbital region. Abrasion and weathering are ubiquitous on this skull, except for the well-preserved supraorbital region; moreover, hardened sediment infills most of the mesorostral groove. Sutures between adjacent bones are generally not distinguishable on this specimen, with the significant exception of the right supraorbital region, where the relationships between the maxilla, lacrimojugal complex, and frontal are clear.

The skull of Platyscaphokogia n. gen. is anteroposteriorly elongated and distinctly teardrop-shaped ( Figs 2; 3), thus strongly differing from the shorter and wider skull of extant Kogia . It is also strongly asymmetric, as highlighted by the presence of bony nares that are dissimilar in size (i.e., the right naris is much smaller than the left one) and significantly displaced towards the left side of the cranium ( Figs 2, 5). Both nasals are absent ( Figs 2, 5); the same condition is observed in all kogiids known to date. The dorsal surface of the neurocranium is distinctly concave, forming a wide supracranial basin (a diagnostic character of physeteroids) that is crossed by a sagittal facial crest (a key feature of Kogiidae ) formed by the coalescence of the medial margin of the right maxilla with the upturned medial margin of the left maxilla posterior to the bony nares ( Figs 2, 5). The dorsal and ventral aspects of the skull are strongly reminiscent of those of Scaphokogia , the most obvious differences between these two genera being in the degree of tapering of the rostrum (almost negligible in Scaphokogia , conspicuous in Platyscaphokogia n. gen.) and in the length and position of the sagittal facial crest (short and appressed to the left margin of the supracranial basin in Scaphokogia , longer and located more medially in Platyscaphokogia n. gen.) ( Muizon 1988; Figs 2, 3). The lateral view highlights further major differences between the crania of these two genera: for instance, whereas in Scaphokogia the rostrum is deflected downwards, dorsoventrally high, and clearly exceeds in height the lateral maxillary crests at its base, the rostrum of Platyscaphokogia n. gen. is roughly oriented horizontally, not inflated dorsoventrally, and its dorsalmost portion is located distinctly ventral to the highest point of the lateral maxillary crests ( Fig. 4). Nonetheless, even in lateral view, the skulls of these two genera still exhibit clear affinities, as highlighted by the observation of a concave upper portion of the occipital shield that forms an angle distinctly greater than 90° with the long axis of the rostrum, as well as by the peculiar morphology of the supraorbital region ( Figs 4, 6C).

Skull measurements of MUSM 3405 and MUSM 3291 are reported in Table 1.

Premaxilla

In dorsal view ( Fig. 2), the premaxillae appear as two bands of bone running subparallel to the long axis of the skull medial to the maxillae. All along the rostrum, the lateral margins of the maxillae are slightly wavy and converge very weakly towards the anterior termination of the skull. Medial to the premaxillae, the mesorostral groove dorsally opens approximately 35 mm anterior to the level of the posterior end of the bony nares. The dorsal opening of the mesorostral groove progressively widens forwards, as the medial borders of the premaxillae weakly diverge from each other. Consequently, in the posterior half of the rostrum, the mesorostral groove is more widely open dorsally than in Nanokogia and, especially, Scaphokogia . At about mid-length of the rostrum, the left premaxilla contacts and seemingly slightly overhangs the medial margin of its right antimere, thus roofing dorsally the mesorostral groove for c. 50 mm; this feature is reminiscent of the condition observed in Scaphokogia , where overlapping of the premaxillae occurs closer to the base of the rostrum ( Muizon 1988). In this region, the transverse width of the left premaxilla is roughly twice that of its right homologue; otherwise, the two premaxillae are subequal in width all along the rostrum. Anterior to the area of overlapping, the medial borders of the premaxillae weakly diverge again, thus causing the mesorostral groove to reach its maximum transverse width (c. 40 mm) at the apex of the rostrum. In the posterior half of the rostrum, each premaxilla is transversely narrower than the corresponding maxilla, whereas in the anterior half of the rostrum the width of each premaxilla is equal or greater than that of the corresponding maxilla. Anterior to the antorbital notch, the dorsal surface of the right premaxilla is generally flat, turning gently convex transversely close to the anteriormost tip of the rostrum. A similar condition is observed on the left premaxilla; however, the latter exhibits a transversely concave dorsal surface in proximity of the corresponding naris. Differing from Aprixokogia , Kogia breviceps , Kogia pusilla , Koristocetus , Nanokogia , and Pliokogia , a premaxillary foramen is present on the left premaxilla, at roughly the same level as the anterior termination of the antorbital notches. The right premaxillary foramen is placed at a slightly more posterior level than the left and is followed anteriorly by a deep, 30 mm long groove extension. Both the left and the right premaxillary foramina are anteroposteriorly elongated, c. 8 mm long and 4 mm wide. The external bony nares are located at the level of the posterior end of the antorbital notches. As observed in all physeteroids, the bony nares are distinctly asymmetrical, dissimilar in size, and displaced leftwards ( Figs 2, 5). The left naris is 22 mm long and 16 mm wide; it takes place in a funnel-shaped, anteroposteriorly oriented depression (a condition reminiscent of Kogia spp. , Koristocetus , and Pliokogia ). The rounded posterior end of the left premaxilla is located just behind the posterior wall of the left naris. The right naris is 13 mm long and 12 mm wide; it is also placed at the bottom of a funnel-like depression and its anterior, lateral, and posterior walls are formed by the right premaxilla. As in all kogiids known to date, posterior and posterolateral to the right bony naris, the right premaxilla exhibits a lobe-shaped expansion (the postnarial eminence, sensu Whitmore & Kaltenbach 2008) that enters the right portion of the supracranial basin (see description of the maxilla below). Differing from most other kogiids but Scaphokogia , the postnarial eminence constitutes here a small fraction (i.e., less than 1/ ª) of the total area of the supracranial basin ( Fig. 2). The right premaxilla does not reach the posterior margin of the supracranial basin, its distal termination being located at the level of the left posterior dorsal infraorbital foramen. As observed in Scaphokogia , the posteromedial portion of the postnarial eminence contacts the presphenoid and the right maxilla, thus contributing to the anteriormost part of the sagittal facial crest (see description of the maxilla below); in all other kogiids known to date, a much larger portion of the sagittal facial crest is contributed by the right premaxilla. Differing from all kogiids but Nanokogia and Scaphokogia , no distinct fossa takes place on the postnarial eminence, either within the borders of the right premaxilla (e.g., Kogia breviceps , K. sima , Koristocetus , and Thalassocetus ) or affecting both the right premaxilla and maxilla (e.g., Praekogia ). Differing from the condition observed in the living species of Kogia , the premaxillae are not exposed on the ventral surface of the skull.

Maxilla

Both maxillae reach the anterior end of the skull. In dorsal view ( Fig. 2), throughout the rostrum, the lateral margins of the maxillae point anteromedially, with a marked constriction at about three fifths of the total rostrum length, thus recalling the condition observed in Koristocetus and Nanokogia . Along the rostrum, the dorsal surfaces of both maxillae are flat to weakly convex and roughly subhorizontal. In dorsal view, the antorbital notch is deep and narrow (i.e., slit-like), and c. 75 mm long on both sides of the skull; in extent and morphology, it is strongly reminiscent of the condition in Scaphokogia . Both antorbital notches are set completely outside of the supracranial basin ( Figs 2, 5), thus differing from the derived condition that characterises all the members of the subfamily Kogiinae . Lateral to the antorbital notch, the lateral maxillary crest is shaped as a transversely thin, dorsally high, plate-like stripe of bone that runs almost parasagittally. On the right side of the holotype skull, the lateral maxillary crest has a thickness ranging from c. 5 to 10 mm and reaches a maximum height of 22 mm at mid length of the corresponding antorbital notch.The vertical, sharp maxillary crest of Platyscaphokogia n. gen. contrasts with the greatly thickened crest of extant Kogia , being in turn strongly reminiscent of the condition observed in Scaphokogia . At the level of the bony nares, similar to Scaphokogia , the dorsal surface of the right maxilla exhibits a very abrupt change in steepness, forming an almost right angle along an anteromedially oriented line. For this feature, which extends to the lateral half of the right premaxilla, the name “supracranial step” is here proposed. A supracranial step is also present in Scaphokogia and constitutes a synapomorphy of Scaphokogiinae . Just posterior to the supracranial step, the dorsal surface of the right maxilla faces posterodorsally: therefore, the supracranial step marks the right anterolateral border of the supracranial basin, the latter not extending into the rostrum (a condition reminiscent of Koristocetus , Nanokogia , Pliokogia , and Scaphokogia ). Similar to Scaphokogia , the supracranial basin is large, spoon-shaped, floored by the maxillae (except for a small area that comprises the postnarial eminence of the right premaxilla), and expanded posteriorly (so that its posterior border overhangs the occipital region) and towards the left side of the cranium; with respect to the supracranial basin of Scaphokogia , however, the supracranial basin of Platyscaphokogia n. gen. is shallower and faces slightly more anterodorsally. The supracranial basin is divided in two parts, hereinafter referred to as the right and the left sub-basins, by the presence of a sagittal facial crest, which represents a synapomorphy of Kogiidae . The right and left sub-basins of Platyscaphokogia n. gen. are clearly homologous to similar depressions observed on the dorsal surface of the neurocranium of living kogiids and known as the right and left maxillary fossae ( Thornton et al. 2015), which correspond to the right and left facial fossae of Mead & Fordyce (2009). However, the facial fossa is defined as the concave surface of the ascending process of the maxilla that lies between the nasal process of the premaxilla and the supraorbital margin of the maxilla ( Mead & Fordyce 2009, and references therein). As the depression that comprises the right portion of the supracranial basin of the scaphokogiines extends also over part of the right premaxilla, here we prefer to refer to this feature and to the corresponding depression of the left half of the skull using the term sub-basin. Differing from the condition observed in extant Kogia , however, the right sub-basin of Platyscaphokogia n. gen. also extends over the posterodorsal portion of the right premaxilla; therefore, for purposes of clarity, here we prefer not to use the term “maxillary fossa”. The posterior termination of the sagittal facial crest is placed where the posterior border of the supracranial basin meets the sagittal plane; from this level forwards, the sagittal facial crest runs anterolaterally to end anteriorly at the posteriormost level of the bony nares. In this respect, Platyscaphokogia n. gen. thus differs from Scaphokogia , where the sagittal facial crest is much shorter, more strongly displaced leftwards (thus strongly limiting the areal extent of the right sub-basin), and distinctly anteromedially oriented. Both the anterior (i.e, subvertical) and the dorsal (i.e, subhorizontal) edges of the sagittal facial crest are sharp, thin and straight; in dorsal view, they form a rounded angle of c. 90° at their junction. Throughout its length, the sagittal facial crest dips rightwards and partially overhangs the left sub-basin. The latter is anteroposteriorly elongated, tongue-shaped, and its floor is located at a higher dorsoventral level than the floor of the right sub-basin. The deeper right sub-basin has a roughly oval outline, its medial and lateral boundaries being made by the flat, rightwards dipping dorsolateral surface of the sagittal facial crest and the vertical medial surface of the right maxillary crest, respectively. Posteriorly, the border of the right sub-basin is marked by a subhorizontal bony plate, displaying a maximum anteroposterior breadth of c. 20 mm, formed by the posteriormost portion of the ascending process of the right maxilla. The function of such an idiosyncratic feature is substantially obscure, as it might be interpreted either as an area of origin for some facial muscles or as providing insertion for the organs of the forehead. Interestingly, in Physeter Linnaeus, 1758 , Heyning (1989) described a large right longitudinal muscle originating from the right portion of the occipital crest and inserting on the “museau de singe” (i.e., the phonic lips region) and right blowhole. It seems thus reasonable to hypothesise that the subhorizontal bony plate that comprises the posterior border of the right sub-basin of Platyscaphokogia n. gen. was the area from which a powerful and efficient right longitudinal muscle originated – an interpretation that evokes the presence of a highly specialized echolocation system. Although the central portion of the floor of the right sub-basin is missing, the lowermost point of the dorsal exposure of the maxillae should have been located in this area, thus recalling Scaphokogia in this respect. Similar to Scaphokogia , but differing from all other kogiids known to date, no distinct premaxillary/supracranial fossa (sensu Barnes 1973 and Vélez-Juarbe et al. 2015) or peripheral maxillary fossa (sensu Collareta et al. 2019) is observed within the borders of the supracranial basin; this condition represents a synapomorphy of Scaphokogiinae . In the light of the interpretation of the above fossae proposed by Thornton et al. (2015) and Collareta et al. (2019), the position of the melon and spermaceti organs that take place above these structures cannot be resolved for Platyscaphokogia n. gen. and Scaphokogia ; it is likely, however,that the spermaceti organ of Scaphokogiinae was larger than that of extant Kogiinae and displaced backwards, as suggested by the posterior extension of the supracranial basin ( Muizon 1988). On the left maxilla, a single posterior dorsal infraorbital foramen is observed within the left sub-basin, at the level of the base of the postorbital process (preserved on the right side). Similar to Scaphokogia , this foramen is large, elliptical, 19 mm long and 8 mm wide, and roughly parallels the left border of the supracranial basin; it is followed posterolaterally by a 15 mm long groove extension. In extant Kogia , the left posterior dorsal infraorbital foramen is commonly subdivided (e.g., duplicated), and a similar condition is observed in Aprixokogia and Nanokogia . Five left anterior dorsal infraorbital foramina are also observed anterolateral to the bony nares ( Fig. 2). They are small, fissurelike (i.e., they range between 6 and 11 mm in length and between 2 and 4 mm in width), elongated parallel to the suture between the maxilla and the premaxilla, and are in the medial half of the dorsal surface of the bone. Three of these foramina are located medial to the lateral maxillary crest and, out of them, two take place within the funnel-shaped depression that hosts the left bony naris, close to the maxilla-premaxilla suture. The remaining two left anterior dorsal infraorbital foramina are located anteromedial to the anteriormost point of the left antorbital process. No groove extensions are associated to the left anterior dorsal infraorbital foramina. In Scaphokogia , the left anterior dorsal infraorbital foramen lies anterior to the antorbital notch, whereas several openings are present in the same region in the skull of extant Kogia . On the right maxilla, across the supracranial step, a cluster of three small posterior dorsal infraorbital foramina is observed. One of these openings is located just posterior to the supracranial step, where the maxilla exhibits a posterodorsally oriented surface; it is roughly circular, with a diameter of c. 8 mm. On the anterior side of the supracranial step, one circular foramen having a diameter of c. 5 mm is observed at mid width of the exposure of the maxilla medial to the lateral maxillary crests. A third foramen of similar size is present, at approximately the same anterodorsal level, appressed to the locally concave lateral margin of the right premaxilla.Two anterior dorsal infraorbital foramina are also observed on the rostral surface of the right maxilla. Both these foramina are ellipitical, anteroposteriorly elongated, c. 7 mm long and 4 mm wide, and located in proximity of the maxillapremaxilla suture. None of the right dorsal infraorbital foramina exhibits a groove extension. The pattern of the right dorsal infraorbital foramina of Platyscaphokogia n. gen. closely resembles that of Scaphokogia , in which multiple small-sized openings are observed, along the right maxilla, between the anterior portion of the supracranial basin and a level slightly anterior to the base of the rostrum.

In lateral view ( Fig. 4), the ventral margin of the maxilla on the rostrum is substraight and not deflected downwards, thus departing from the clinorhynchy that characterises the skull of Scaphokogia . The maxillary flange, which is laterally hidden by the adjoining lateral maxillary crest, parallels the long axis of the rostrum. Anterior to the antorbital notch, the lateral surface of the maxilla is high, appressed to the right antorbital process, and dips laterally, whereas the ventral surface of the maxilla is regularly convex. From a short distance from the antorbital process forwards, the lateral margin of the maxilla is thinner dorsoventrally and somewhat keeled. In the anteriormost third of the rostrum, the lateral margin of the maxilla becomes regularly rounded in cross section.

In ventral view ( Fig. 3), ten well-individualised dental alveoli are observed on the palatal surface of each maxilla, starting 90 mm anterior to the anterior margin of the palatines forwards. They are located roughly at mid distance between the lateral margin of the rostrum and the suture between the maxilla and the vomer. Medial to the alveolar row, the palatal surface of the maxilla is distinctly convex transversely, thus forming a keel that tapers forwards; lateral to the alveolar row, the palatal surface of the maxilla is flat and dips ventromedially. Each dental alveolus is elliptical, anteroposteriorly elongated, c. 9 mm long and 6 mm wide, with a depth around 5-6 mm. The presence of well-individualised dental alveoli on the maxillae of Platyscaphokogia n. gen. is shared with Aprixokogia and, to some extent, to Pliokogia and Scaphokogia (in which discrete yet likely relic dental alveoli are present), whereas just a faint alveolar groove is present in Kogia and Nanokogia . Given their size and depth, we regard the dental alveoli of Platyscaphokogia n. gen. as most likely bearing functional teeth. The left alveolar row is followed posteriorly by a 55 mm long sulcus that seemingly bears no distinct alveoli. In ventral view, the right antorbital notch is shorter posteriorly than it appears on the dorsal view, as well as distinctly teardrop-shaped. Posterior to the antorbital notch, the ventral exposure of the maxilla wedges between the lacrimojugal complex and the palatine towards the large ventral infraorbital foramen. The latter is elliptical and anterolaterally directed; it measures 25 mm in length and 13 mm in width. Anterior to the ventral infraorbital foramen and medial to the lacrimojugal complex, a shallow fossa excavates the ventral surface of the maxilla, paralleling the palatine. This depression is strongly reminiscent of a similar, yet deeper and longer, fossa observed in the same region of the skull of Scaphokogia ; furthermore, as in Scaphokogia , this depression is deeper and better defined on the right side of the skull than on the left. Similar fossae have also been reported on skulls of Kogia breviceps , K. sima , Koristocetus , Nanokogia , and Pliokogia . Following the interpretation proposed by Vélez-Juarbe et al. (2015), this fossa is here regarded as related to the anteriormost portion of the pterygoid sinus complex (see Fraser & Purves 1960, for data on the extent of the pterygoid sinus and corresponding osteological features in extant Kogia ).

Palatine

Due to the loss of the pterygoids, both palatines are widely exposed on the ventral surface of the cranium, posteromedial to the maxillae ( Fig. 3). Each palatine displays a roughly oval, anteromedially oriented, blocky, and distinctly bulging (i.e., strongly transversely convex) anterior portion that is located anterior to the corresponding choana. Here the palatines contact each other medially, being separated anteromedially by a cusp-like wedge of the maxillae (a condition reminiscent of that of Scaphokogia ). Two anterolaterally elongated bands of bone lying lateral to the choanae are also identified as belonging to the palatines, possibly representing their medial laminae. They are gently bowed laterally (i.e., roughly C-shaped); they contact the orbitosphenoid posterolaterally and the presphenoid posteriorly. Alternatively, these bands of bone might be interpreted as representing shreds of the pterygoids.

Lacrimojugal complex

As in all physeteroids for which these elements are known, the lacrimal and the jugal are fused in a compound lacrimojugal complex. The latter is rather large and, in lateral view, it appears as roughly triangular ( Fig. 4). As in Kogia , Koristocetus , Nanokogia , Pliokogia , Praekogia , and Scaphokogia , the posterodorsal corner of the lacrimojugal complex is shaped as a pointed posterodorsal process (sensu Muizon 1988) that wedges between the maxilla and the frontal; this process is slightly shorter than observed in the holotype of Scaphokogia cochlearis (specimen MNHN.F.PPI229). As observed in Kogia , Koristocetus , Nanokogia , Pliokogia , and Scaphokogia , the suture between the lacrimojugal complex and the maxilla is somewhat wavy, thus recalling also the condition observed in the stem sperm whales Acrophyseter robustus and Livyatan , among other physeteroids; in particular, this suture is substraight in its anterior half and distinctly sigmoidal in its posterior half. The ventralmost tip of the lacrimojugal complex is preserved on the right side of the paratype skull ( Fig. 6B, C). It is robust, rounded, awl-shaped, and points slightly posteroventrally. When viewed ventrally, the lacrimojugal complex wedges between the frontal and the maxilla, tapering posteromedially ( Fig. 3). Differing from extant Kogia , where the ventral surface of the lacrimojugal complex is bulbous and creates a large attachment site for the masseter muscle ( Schulte & Smith 1918: fig. 8), the lacrimojugal complex of Platyscaphokogia n. gen. appears as somewhat transversely compressed in ventral view, thus recalling most other extinct kogiids in this regard.

Frontal

The frontals are mainly exposed in lateral and ventral views, their dorsal surfaces being largely hidden by the overlying maxillae. In lateral view ( Fig. 4), the suture between the frontal and the maxilla along the supraorbital process slopes, with respect to the coronal plane, at a lower angle than observed in Scaphokogia , being reminiscent instead of the condition observed in Pliokogia and Thalassocetus . Similar to Scaphokogia , when viewed laterally, the preorbital process is blunt and globose. In turn, in lateral view, the postorbital process is more slender and distinctly projects posteroventrally in the paratype skull ( Fig. 6C; the distal tip of the postorbital process is not preserved in the holotype skull); it is much less ventrally extended than observed in Aprixokogia and Kogia . In lateral view, the frontal also comprises the preserved upper portion of the temporal fossa. The highest point of the latter appears as distinctly higher than the highest point of the orbit, thus contrasting with the low temporal fossa of extant Kogia and recalling instead the condition observed in Scaphokogia . The ventral surface of the frontal ( Fig. 3) displays a wide frontal groove that represents the distal extension of the optic canal departing from the optic foramen (not preserved). The frontal groove runs anterolaterally, thus contrasting with the almost mediolaterally directed frontal groove of extant Kogia .Towards the roof of the orbit, the frontal groove flattens and widens progressively in a way reminiscent of Scaphokogia .

Vomer

In dorsal view ( Fig. 2), the vomer is observed forming the floor of the mesorostral groove at the apex of the rostrum. In ventral view ( Fig. 3), the vomer is exposed along the rostrum as a narrow slit that runs parallel to the long axis of the skull between the maxillae. In this region, the vomer is not keeled, thus differing from the condition observed in Pliokogia . Reflecting the flattened morphology of the rostrum, the vomer of Platyscaphokogia n. gen. is dorsoventrally compressed along the splanchnocranium ( Figs 3, 4); this greatly contrasts with the condition observed in Scaphokogia , the vomer of which is greatly pachyosteosclerotic and exhibits a wedge-shaped transverse section along the rostrum. The vomer of Platyscaphokogia n. gen. is also exposed posterior to the palatines, as a slice-like bony projection that coats the left surface of the presphenoid, thus contributing to the medial wall of the left choana. This condition resembles that observed in most kogiids except for Praekogia , in which the ventral surface of the presphenoid is partially wrapped by the vomer.

Sphenoid

In dorsal view, as in all kogiids, the presphenoid forms the sharp septum that separates the bony nares ( Figs 2, 5). Differing from K. sima and some specimens of K. breviceps , the presphenoid does not extend anteriorly to partially fill the mesorostral groove ( Fig. 2). In ventral view ( Fig. 3), the presphenoid is partially exposed between the choanae, where it forms an anteroposteriorly elongated, strongly carinated bony septum that is bordered laterally by the vomer. Posterior to the choanae, the presphenoid is robust and flares posterolaterally. A thin, elongated plate of bone running anterior and parallel to the anteromedial margin of the frontal groove is here identified as part of the orbitosphenoid.

Occipital and endocranial cavity

Only the uppermost portion of the supraoccipital is preserved in the holotype skull. In lateral view ( Fig. 4), the preserved portion of the occipital shield is concave and forms an angle of c. 140° with the long axis of the rostrum. Although this value might have been slightly accentuated by diagenetic compression (the evidence for which is otherwise lacking), this angle would have surely been distinctly greater than 90° – a character that, among Kogiidae , is only observed in Scaphokogia . Posterior to the temporal fossa, the suture between the occipital and the frontal is concave backwards and runs roughly subvertical. When viewed ventrally, the holotype skull allows for observing the roof of the endocranial cavity ( Fig. 3). The latter is gently concave on its left half but distinctly convex on its right half, thus evoking strongly asymmetrical cerebral hemispheres, much more dissimilar from each other than reported by Marino et al. (2003) in extant Kogia sima . Different from the condition observed in Koristocetus , no ossified falx cerebri is present within the endocranial cavity.