BALAENOPTERIDAE Gray, 1864

Tanaka, Yoshihiro, Nagasawa, Kazuo & Taketani, Yojiro, 2020, A new skull of an early diverging rorqual (Balaenopteridae, Mysticeti, Cetacea) from the late Miocene to early Pliocene of Yamagata, northeastern Japan, Palaeontologia Electronica (a 12) 23 (1), pp. 1-27 : 2-4

Family BALAENOPTERIDAE Gray, 1864 cf. Miobalaenoptera numataensis Tanaka and Watanabe, 2019

( Figures 2–5 , Tables 1 and 2)

Remarks. YPM 11851 is comparable to Miobalaenoptera numataensis by having similarities such a straight and sharp lateral ridge of the fovea epitubaria (Character 273 in this study, see discussion), a dorsoventrally thin pars cochlearis, a straight and horizontally expanded subtemporal crest of the squamosal, and strongly posterolaterally diverged basioccipital crest, which were used as diagnoses for M. numataensis in Tanaka and Watanabe (2019a). In this study, YPM 11851 is identified as cf. M. numataensis preliminary, because there are several differences from the holotype of M. numataensis (see discussion and also Table 3). It cannot be identified as the species because of limited knowledge on ontogenetic and other variations of the species.

Emended diagnoses for Miobalaenoptera numataensis . Minor emendation including an identification of the lateral ridge of the fovea epitubaria and adding measurements is made here. Among the Balaenopteridae , Miobalaenoptera numataensis uniquely has the lateral ridge of the fovea epitubaria, which is straight and sharp, running to the anterior pedicle of the ventral surface of the periotic (see discussion). The ridge is located more anterior than that of the modern balaenopterids. M. numataensis also uniquely has a dorsoventrally thin pars cochleais (especially medial edge). The medial edge of the pars cochlearis is thinner than the diameter of the internal acoustic meatus (9.1 mm thick and 12.1 mm in diameter, respectively, on the holotype). Tanaka and Watanabe (2019a) mentioned differences with other balaenopterids as below. M. numataensis is different from other balaenopterids except “ Balaenoptera ” ryani by a feature, an incipient lateral tuberosities of the periotic. M. numataensis is different from other balaenopterids except “ Megaptera” miocaena, Protororquals cuvieri, and Megaptera novaeangliae by features, a slender zygomatic process, and a strongly excavated lateral margin of the orbit in dorsoventral view. M. numataensis is different from other balaenopterids except Fragilicetus velponi by a feature, strongly diverged basioccipital crests. M. numataensis is different from the other extinct balaenopterids except “ Megaptera” hubachi by a feature, a broadly or more or less horizontally expanded subtemporal crest. M. numataensis is different from the extant taxa and “ Megaptera” miocaena by a feature, a deep suprameatal fossa of the periotic. M. numataensis is different from the extinct taxa except Fragilicetus velponi by a feature, a long caudal tympanic process of the periotic. Here, we add differences between M. numataensis and supposed closely related Incakujira anillodefuego . M. numataensis is different from I. anillodefuego and also Diunatans luctoratemergo by some features, higher basioccipital crests, and straight subtemporal crest in ventral view. M. numataensis is different from I. anillodefuego by several features such as, a more anteriorly located foramen pseudovale, a strongly excavated orbit in dorsoventral view, more robust zygomatic process, larger postorbital process in lateral view, straight ventral margin of the anterior tip of the zygomatic process, a more slen- der caudal process of the periotic, and deeper area lateral from the fenestra rotunda and dorsal from the posteroventral margin of the pars cochlearis. M. numataensis is different from D. luctoratemergo by several features such as, more laterally projected basioccipital crest, a blunt anterior tip of the anterior process of the periotic in mediolateral view, dorsoventrally thinner and more flat pars cochlearis.

TANAKA, NAGASAWA, & TAKETANI: FOSSIL RORQUAL FROM JAPAN

Figures

FIGURE 2. 1-4, Skull of cf. Miobalaenoptera numataensis (YPM 11851) in anterior view (1, 2), posterior view (3, 4), photos (1, 3), line drawings (2, 4).

FIGURE 3. 1-2, Skull of cf. Miobalaenoptera numataensis (YPM 11851) in ventral view., photo (1), line drawing (2).

FIGURE 4. 1-6, Skull of cf. Miobalaenoptera numataensis (YPM 11851) in right lateral view (1 and 4), dorsal view (2, 5), left lateral view (3, 6), photos (1-3), line drawings (4-6).

FIGURE 5. 1-7, Periotics of cf. Miobalaenoptera numataensis (YPM 11851) in right side (1, 2, 4, 5), left side (3, 6, 7), anterior view (1), ventral view (2, 6, 7), lateral view (3), medial view (4, 5), photos (1-4, 6), line drawing (5, 7).