Balaenoptera edeni Anderson, 1879

Tanaka, Yoshihiro & Taruno, Hiroyuki, 2017, Balaenoptera edeni skull from the Holocene (Quaternary) of Osaka City, Japan, Palaeontologia Electronica 85 (4), pp. 1-13 : 4-9

Balaenoptera edeni Anderson, 1879

( Figures 2-9 , Table 1)

Diagnosis. OMNH-QV4816 is identified as Balaenoptera edeni based on having a large alisphenoid, which contacts with the squamosal widely ( Oishi, 1999). B. brydei and B. omurai differ from B. edeni having a much smaller alisphenoid ( Wada et al., 2003; Oishi et al., 2009). B. borealis differs from B. edeni and B. acutorostrata having the alisphenoid, which forms the medial part of the optic canal ( Muller, 1954; Oishi, 1999).

General description

Ontogeny. The posterior process of the periotic is in situ but is not fused to the squamosal in ventral view. Fused posterior process of the periotic to the skull had been suggested as a condition of adults ( Kasuya, 1973; Geisler and Luo, 1996). The posterior process of OMNH-QV4816 suggests that the individual is not a fully adult. Thus, OMNH-QV4816 is subadult.

Maxilla. The preserved maxillae are separated into rostral (1060 mm long) and orbital regions (181 mm long) ( Figure 3 ). The rostral part shows more or less straight lateral margin with a posteriorly widen triangular anterior tip. On the dorsal side, dorsal infraorbital foramina open anteriorly as deep grooves. There is a weakly excavated and anteroposteriorly long area for the premaxilla on the medial surface. In ventral view, anteroposteriorly long and deep palatal foramina and sulci run between the median line and the palatal margin. The ventral side is excavated and shows at least three anteroposteriorly long palatal sulci. In the cross section, the rostrum part of the maxilla is medially high, laterally thin triangular. The medial margin of the maxilla (70 mm high) shows weakly excavated plain for the contact of the premaxilla.

The preserved posterior part of the maxilla shows well-projected lateral process. Anterior to the process, there is a very shallow antorbital notch. Posterior to the antorbital notch, there are

PALAEO- ELECTRONICA.ORG depressed areas for the frontal anteriorly and lacrimal posteriorly. Lateral to the antorbital notch, a supposed most posteriorly located dorsal infraorbital foramen opens laterally and its shape is anteroposteriorly long elliptical (26.5 mm long and 10.5 mm height). Most of the infraorbital plate is broken away.

Premaxilla. Only a part of left premaxilla is preserved ( Figure 3 ). The preserved premaxilla is anteriorly weakly convex to dorsal and posteriorly narrow. Its posteromedial surface forms a vertical wall, which constitutes a part of the nasal fossa, which is a place for the depressor alae nasi ( Mead and Fordyce, 2009; Buono et al., 2015; Marx et al., 2016). Thus, the preserved premaxilla probably is a middle portion. Ventrally, the premaxilla is excavated dorsally and forms a dorsolateral part of the mesorostral groove. Medial surface is anteriorly flat, but has anteroposteriorly long depression for the contact with the maxilla.

Frontal. The frontal is laterally wide fan shape ( Figure 4 ). The dorsal surface is slightly concaved. In dorsoventral view, the preorbital and postorbital processes show weakly excavated larteral margin of the orbital region medially. These processes are blunt. The postorbital process has a fossa for the anterior tip of the zygomatic process. There are low preorbital and postorbital ridges on the ventral surface of the frontal, which run from these processes to the medial side. The posterolateral margin of the posterior process is extended posteriorly. In lateral view, the postorbital process is triangle and projects posteroventrally. The process is higher than the preorbital process, which is also triangular.

Vomer. The vomer is separated into a long rostral portion (1690 mm long) ( Figure 3 ) and dorsoventrally thin posterior portion under the brain case ( Figure 6 ). The anterior portion of the vomer is thinner than the posterior part, and dorsally opened Ushape in the cross section. Anterior part of the ventral surface is flat and exposes in ventral view between the maxillae. Posterior part of the rostral portion of the vomer is widened and shows several anteroposteriorly long grooves.

The posterior portion of the vomer is dorsoventrally low sheet and has more or less parallelsided lateral margins in ventral view. The posterior broken section shows the vomerine crest at the center (26.5 mm maximum height and 114.0 mm

TANAKA & TARUNO: A JAPANESE HOLOCENE WHALE width). The posterior end of the vomer wraps the basisphenoid.

Parietal. The preserved posterior part of the parietal forms the posteromedial part of the temporal fossa, which is vertical and weakly excavated ( Figure 9 ). The parietal contacts with the squamosal posteriorly, and the large alisphenoid ventrally.

A small exposure of the parietal locates lateral to the alisphenoid, anterior and medial to the squamosal. The parietal faces the fossa for the periotic medially. The part includes a few laterally excavated pits.

Squamosal. The squamosal has a long and anteriorly tapered zygomatic process and posteroventrally projected blunt postglenoid process.

The anterior part of the zygomatic process projects anteriorly, and is narrowing gradually in ventral and lateral views. The ventral surface of the zygomatic process is weakly convex, which is an area for the masseter deep ( Marx et al., 2016), and its anterior tip is tilted ventrally a bit. A faint supramastoid crest runs on the dorsal surface of the zygomatic process. Anterior to the base of the zygomatic process, at a posterior part of the temporal fossa, a deep squamosal cleft runs transversely. At the posterior part, the lateral surface of the zygomatic process shows faint shallow fossae, which are the fossae for the sternocephalicus. The posteroventrally projected postglenoid process is narrowing ventrally.

In ventral view ( Figure 6 ), the squamosal has a low spiny process, which is the border for the periotic. The falciform process is absent. An almost closed foramen pseudovale locates lateral to the pterygoid sinus fossa, and is formed by the both squamosal and pterygoid. The almost closed foramen pseudovale is filled by an additional pterygoid bone, and is more or less closed. The mandibular fossa is flat and smooth. Posterior to the postglenoid process, the squamosal forms a ventrally widely opened external auditory meatus (27 mm long and 242 mm wide). The posterior part of the squamosal contacts to the long and anteroposteri-

TANAKA & TARUNO: A JAPANESE HOLOCENE WHALE orly short plate-like fused posterior processes of the periotic and bulla.

Pterygoid. The pterygoid locates medially to the squamosal. Anteriorly, the pterygoid has a broken base of the hamular process and its broken section shows laterally convex crescentic shape. The pterygoid has an anteroposteriorly long elliptical, huge, and shallow pterygoid sinus fossa (130 mm long and 85 mm wide). The fossa contains a few foramina.

Alisphenoid. The large squared lateral exposure of the alisphenoid contacts with the squamosal widely laterally, parietal dorsally, presphenoid medially and the pterygoid ventrally ( Figure 7 ). The lateral exposure of the alisphenoid is rugose, flat and slightly far back compare to the other bones’ surfaces. The squamosal cleft continues to the ventral border of the alisphenoid. The alisphenoid forms an anterior part of the braincase.

There is a ventral exposure of the alisphenoid, lateral to the pterygoid, posterior to the foramen pseudovale but is not contacted. Posteriorly, the alisphenoid forms an anterior border of the fossa for the periotic.

Basisphenoid/presphenoid. The basisphenoid/ presphenoid suture is fused. The presphenoid locates anterior to the basisphenoid ( Figure 7 ), dorsal to the vomer. In anterior view, the presphenoid shows mediolaterally wide elliptical. The basisphenoid locates posterior to the vomer, and its overall shape is anteriorly wider trapezoid ( Figure 6 ). The posterior border of the basisphenoid with basioccipital is completely fused suture. The lateral borders contacts with the pterygoid.

Supraoccipital. The preserved ventral part of the supraoccipital has wide lateral borders, which are weakly curved nuchal crests ( Figure 8 ). The dorsal surface of the supraoccipital is flat. The external occipital crest was worn, but its broken surface shows that the crest runs dorsoventrally on the median line.

Basioccipital. The basioccipital is posteriorly wide trapezoid in ventral view with posteriorly just swollen (non peddicle) occipital condyles, which are ventrally slightly wider elliptical ( Figure 6 ). Between the condyles, there is a shallow intercondyloid notch. At the lateral border of the basioccipital, there are a couple of swollen basioccipital crests. Between the crests, the basioccipital basin is flat. The posterolateral end of the basioccipital forms widely opened jugular notch with the exoccipital.

Exoccipital. In dorsal view ( Figure 5 ), the lateral ends of the exoccipital extend posterolaterally. In ventral view, the exoccipital is 58 mm long and 270 mm wide, and contacts to the posterior process of the periotic. On the ventral surface of the exoccipital, there is a mediolaterally long triangular paroccipital concavity. The paroccipital process narrows at the lateral ends. The exoccipital/squamosal suture is wavy and opened.

Periotic and bulla. The posterior processes of the periotic and bulla preserve as fused condition only a mediolaterally long posterior process in situ ( Figure 6 ). The posterior process is dorsoventrally high and anteroposteriorly thin plate.

TABLE 1. Measurements in mm of OMNH-QV4816 skull and periotic. For skull and mandible, distances are either horizontal or vertical..

Measure Skull (mm)

Preserved length of skull 537

Bizygomatic width 1170

Bicondylar width 195

Preserved frontal length 352

Preserved frontal width 565

Posterior process of periotic maximum 226 mediolateral length

Posterior process of periotic maximum 26 anteroposterior length

Figures

FIGURE 2. Skull, OMNH-QV4816 in dorsal view. 1, Restoration of the skull based on a photo, which was taken just before specimen was damaged by pulling up from the locality. 2, Line art was made from the photo. The outline of the skull is anteroposteriorly extended at the level of the antorbital process (arrow), result of a slide between the lose frontal and maxilla.

FIGURE 3. Maxilla, premaxilla and vomer, OMNH-QV4816. 1, left anterior part of maxilla and premaxilla. 2, left middle part of maxilla. 3, left preorbital region of maxilla. 4, right anterior part of maxilla. 5, right preorbital region of maxilla. 1- 5, in dorsal view. 6, right anterior part of maxilla. 7, right preorbital region of maxilla. 8, left anterior part of maxilla and premaxilla. 9, left middle part of maxilla. 10, left preorbital region of maxilla. 6-10, in ventral view. 11 to 13, vomer. 11, in dorsal view. 12, in left lateral view. 13, in posterior view.

FIGURE 4. Frontals, OMNH-QV4816. 1, left in dorsal view. 2 and 3, right in dorsal view. 2, line art. 3, photo. 4 and 5, right in ventral view. 4, photo. 5, line art. 6. right in ventral view. 7 and 8, in right lateral view. 7, photo. 8, line art.

FIGURE 5. Posterior part of skull, OMNH-QV4816 in dorsal view. Top, photo. Bottom, line art.

FIGURE 6. Posterior part of skull, OMNH-QV4816 in ventral view. Top, photo. Bottom, line art.

FIGURE 7. Posterior part of skull, OMNH-QV4816 in anterior view. Top, photo. Bottom, line art.

FIGURE 8. Posterior part of skull, OMNH-QV4816 in posterior view. Top, photo. Bottom, line art.

FIGURE 9. Posterior part of skull, OMNH-QV4816 in right lateral view. Top, photo. Bottom, line art.