Numataphocoena yamashitai Ichishima and Kimura, 2000

Tanaka, Y & Ichishima, H, 2016, A new skull of the fossil porpoise Numataphocoena yamashitai (Cetacea: Phocoenidae) from the upper part of the Horokaoshirarika Formation (lower Pliocene), Numata Town, Hokkaido, Japan, and its phylogenetic position, Palaeontologia Electronica 19 (3), pp. 1-28 : 3-8

Numataphocoena yamashitai Ichishima and Kimura, 2000

Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Table 1

Emended diagnosis. Numataphocoena yamashitai differs from other phocoenids in having a raised area along an extended sulcus from the medial border of the posterior dorsal infraorbital foramen on the dorsal surface of the maxilla, the maxillary terrace (new term, see Discussion); narrower and sharper anterior part of the internal acoustic meatus; and a robust anterior process of the periotic. Numataphocoena yamashitai differs from later branching phocoenids (such as Lomacetus , Piscolithax spp , extant species) in absence of the maxillary crest (Character 28), and wide premaxillae against the maxillae at the level of the postorbital process (Character 35).

Description

Morphological terminology for the skull follows Mead and Fordyce (2009).

Ontogenetic age. The skull sutures are mostly closed but distinct in NFL 2074. Compared with the holotype of Numataphocoena yamashitai (NFL 7), which is a physically and sexually matured individual ( Ichishima and Kimura, 2000), NFL 2074 is around 20% smaller in size, based on the length of the maxilla (the distance between the anterior tip of the antorbital process of the maxilla to the posterior end of the ascending process. NFL 2074: 145 mm; NFL 7: 188 mm) ( Table 1). Ichishima and Kimura (2013) reported the referred specimen of Haborophocoena toyoshimai as a young individual on the basis of the incompletely fused supra/exoccipital suture, which is around 16% smaller skull size than the physically matured holotype of Haborophocoena toyoshimai . NFL 2074 is, therefore, most likely younger than the holotype.

Premaxilla. The distance between the tip of the premaxilla as preserved and the base of the rostrum at the level of the antorbital notch is 106 mm. Each premaxilla is flat anterior to the level of the antorbital notch, and posteriorly it rises dorsally as the premaxillary eminence, which projects dorsolaterally with a weak depression on the dorsal face (the premaxillary sac fossa). In dorsal view, the posterior end of the premaxillae is widest (27.0 mm on the left) at the level of the nares. A rounded end of the nasal process stops at the level of the posterior margin of the bony nares. Ventrally, the anterior

TANAKA & ICHISHIMA: A NEW SKULL OF NUMATAPHOCOENA part shows sutures with the lost maxilla laterally, and with the vomer medially.

Maxilla. The preserved cranial part of the left maxilla is dorsoventrally thin and rises gradually posterodorsally at the level of the bony nares. The base of the rostrum is wide and has a distinct antorbital notch. On the ventral face, the maxilla forms a part of the palate, which is flat anteriorly and has a weak palatal crest posteriorly. Two shallow palatine sulci run anteroposteriorly on the right maxilla. A rounded antorbital process projects anteriorly and forms a sharp and deep antorbital notch medially. Medial to the antorbital process, there is an anterior dorsal infraorbital foramen.

In dorsal view, the maxilla covers most of the frontal. The posteromedial surface of the maxilla is steep at the level of the bony nares. The posterior dorsal infraorbital foramen opens into a groove, which continues posterolaterally to an area, the maxillary terrace (new term, see Discussion), whose posterior margin reaches the lateral edge of the skull roofing over the temporal fossa. The maxillary intrusion (sensu Arnold and Heinsohn, 1996), a dorsal exposure of the maxilla medial to the premaxilla and anterior to the bony nares is uncertain. An incipient fossa for the inferior vestibule ( Mead, 1975) is just posterior to the nasal process of the premaxilla and lateral to the bony nares. The fossa for the inferior vestibule is circular and much shallower than that of modern phocoenids, which have a small expansion medially.

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In ventral view, just posteromedial to the lacrimojugal is the antorbital fossa, which includes the ventral infraorbital foramen anteriorly and the sphenopalatine foramen posteriorly.

Palatine. Ventrally, the left palatine shows a smooth anterior wall of the pterygoid sinus fossa, which is a dorsoventrally long elliptical fossa.

Pterygoid. The left anterior fragment of the pterygoid might be on the skull, just posterior to the palatine, but the suture is not clear.

Ethmoid. The ethmoid is used in the sense of Mead and Fordyce (2009), but note that Ichishima (2011) suggested that the mesethmoid is probably absent in Odontoceti . The structure of the preserved left ethmoid is unclear in the specimen because of erosion and damage. A thin imperforated cribriform plate forms the posterior wall of the nasal passage. The cribriform plate rises to the fossa for nasals on the frontal. The dorsal end shows a partially damaged osseous nasal septum.

Vomer. An anterior broken section of the vomer can be seen in the mesorostral groove, which is Vshaped in anterior view. The most posterior part of the vomer has been worn away.

Sphenoid. Posteromedial to the orbital rim, there is a shallow and mediolaterally long groove, which might be the frontal groove of the sphenoid. In general, the frontal groove runs from a combined large foramen of the orbital fissure and optic canal, but the medial part of the frontal groove is broken away on NFL 2074.

Frontal. The frontal contributes to the ventral surface of the orbit and the frontal boss at the vertex. The frontal boss is smooth and was originally bounded by the nasals and maxillae. The frontal forms the anterodorsal wall of the braincase and ventrally exposes around a half of the shallow and long orbit. Posterior to the frontal groove, there is a shallow fossa for the postorbital lobe of the pterygoid sinus, just anterior to the temporal fossa. The nasals are not preserved, and their articular surfaces on the frontal are shallow. The fossa for the nasal is anteroposteriorly longer than wide (around 2.0 cm long, 1.5 cm wide). The posteromedial corner of the nasal might be positioned more anterior than the posterolateral corner.

Lacrimojugal. The lacrimojugal is squared in ventral view and thin in lateral view. Its ventral surface

PALAEO- ELECTRONICA.ORG is eroded. Anteromedially, there is a broken base of the lacrimojugal (8 mm diameter). The anterior border of the lacrimojugal forms the most anterior part of the antorbital process. The tubacular posteromedial process is located posterolaterally. The medial process locates medial and just a bit anterior to the posteromedial process, and is covered by the maxilla medially.

Parietal. The preserved parietal is exposed as an anteroposteriorly narrow band dorsally, just posterior to the frontal. The parietal forms the dorsal part of the temporal fossa. The parietal/frontal suture is unclear at the posterolateral part of the skull. The nuchal crest of NFL 2074 is formed by the parietal dorsolaterally and might be formed by the frontal medially. The supraoccipital is lost in NFL 2074, which might also form the nuchal crest.

median line).

TANAKA & ICHISHIMA: A NEW SKULL OF NUMATAPHOCOENA

Figures

FIGURE 2. The skull, NFL 2074, referred specimen of Numataphocoena yamashitai in dorsal view. 1, photo, 2, line art.

FIGURE 3. The skull, NFL 2074, referred specimen of Numataphocoena yamashitai in ventral view. 1, photo, 2, line art.

FIGURE 4. The skull, NFL 2074, referred specimen of Numataphocoena yamashitai in lateral view. 1, photo, 2, line art.

FIGURE 5. The skull, NFL 2074, referred specimen of Numataphocoena yamashitai in anterior view. 1, photo, 2, line art.

FIGURE 6. Life restoration of Numataphocoena yamashitai. Art work by Tatsuya Shinmura (Ashoro Museum of Pale- ontology).