Balaenopteridae Gray 1864

REFERRED MATERIAL. — UCMP 219137, a right posterior process of a petrotympanic UCMP 219098, 219097, 219107, 219478, one left and three right tympanic bullae, and UCMP 219141, a fragment of the posterior end of a left mandible, collected by R. W. Boessenecker from UCMP localities V 99834, V 99835, V 99836, V 99840, and V 99851. STRATIGRAPHIC OCCURRENCE. — Lower and middle parts of the San Gregorio section of the Purisima Formation, latest Miocene to Early Pliocene (6.4 - 3.35 Ma; Messinian-Piacenzian equivalent; Fig. 2). DESCRIPTION Two balaenopterid morphotypes are represented by fossil tympanic bullae (Table 5). Morphotype 1 includes UCMP 219097 and 219478; except where noted, the description of this morphotype is based primarily on UCMP 219478 (Morphotype 2 described below). In medial and lateral aspect, the tympanic bulla is roughly oval (Fig. 21 A), with subequally sized anterior and posterior lobes. A vertical shelf or flange along the anteromedial margin is absent. Ŋe involucrum is smooth, gently convex, and tapers anteriorly. Ŋe involucrum bears some faint transverse striations (Fig. 21 A, D) and a well-developed dorsal posterior prominence (sensu Oishi & Hasegawa 1995 b; Fig. 21 C, F). In medial view, the tympanic cavity is anteriorly oval - shaped, and narrows posteriorly where it is pinched between the involucrum and sigmoid process. Ŋe conical process is blunt, low, and ventrally concave. Ŋe lateral furrow is broad and situated at the midpoint of the bulla. Ŋe sigmoid fissure (= posterior border of sigmoid process) is transversely oriented, deep, and ventrally shallows (Fig. 21 B). Ŋe involucral and main ridges are posteriorly divergent and faintly developed. Ŋe involucral ridge is separated from the main ridge along the ventral margin (Fig. 21 A, D). The tympanic bullae (UCMP 219098 and 219107) of Morphotype 2 are generally similar to those of Morphotype 1, but differ in a few characteristics, including their smaller size (Fig. 22; Table 5). No dorsal posterior prominence is developed. Slight transverse striations occur on the involucrum of UCMP 219098 (Fig. 22 A), while more deeply incised transverse creases occur in UCMP 219107 (Fig. 22 B). Ŋe involucral ridge is straight and retracted from the ventral margin (in dorsal aspect), and is roughly parallel with the main ridge. A vertical shelf or flange along the anteromedial margin is absent. Ŋe lateral surface of the outer lip is greatly inflated (Fig. 22 C), and tympanic bullae of Morphotype 2 are relatively transversely wider than in Morphotype 1. Ŋe conical process is acutely triangular in medial aspect (Fig. 22 A). Ŋe isolated posterior process of the petrotympanic (UCMP 219137) is large (Table 9), anteroposteriorly flattened, and slightly curved posterolaterally (Fig. 23). It tapers laterally to be- come bladelike, and exhibits a sharp ventral crest. Along the anterior side of this crest, a shallow facial sulcus is present. Medially, the posterior process turns anteromedially toward the neck. Ŋe mandible fragment (UCMP 219141) preserves part of the condyle, the angular process, and the ventral margin of the mandibular foramen, but lacks everything anterior to the mandibular foramen (Fig. 24). Ŋe mandibular condyle is large, hemispherical, and prominent laterally. Ŋe angular process is much smaller, and in medial and lateral views is shaped as a tabular process running ventral to the condyle along the ventral margin of the mandible. A transversely oriented groove separates the mandibular condyle from the angular process; the angular process is broadly visible in lateral view (Fig. 24 B). Ŋe mandibular foramen is posterodorsally oriented, large, and transversely broad. Ŋe posteriormost section of the mandible is straight and parasagittally oriented, while anteriorly, the mandible bends strongly laterally, indicating a large degree of bowing of the horizontal ramus (Fig. 24 A) as in Balaenoptera spp. and Megaptera novaeangliae. REMARKS AND COMPARISONS Ŋese tympanic bullae (morphotypes 1 and 2) are referred to the Balaenopteridae based on their oval outline in medial aspect, simple sub-cylindrical involucra, lack of a dorsoventrally expanded and medially flattened involucrum as in Balaenidae, and being dorsoventrally shallower and transversely wider than extant Eschrichtius robustus; furthermore, their much larger size and lack of a median furrow precludes their assignment to Cetotheriidae (Oishi & Hasegawa 1995 b; Ekdale et al. 2011). Morphotype 1 tympanic bullae share in common with extant Megaptera a large and robust dorsal posterior prominence, a feature which is lacking in most extant species of Balaenoptera as well as Plesiobalaenoptera, although some species of Balaenoptera exhibit a weakly developed dorsal posterior prominence. A slight dorsal posterior prominence occurs in Diunatans luctoretmurgo, although its bulla is substantially larger than UCMP 219478 and 219097 and differs in having a much more robust involucrum than the Purisima Formation specimens. Ŋese specimens differ in several regards from most extant Balaenoptera spp., including their smaller size (except Balaenoptera acutorostrata and Balaenoptera bonaerensis, which have smaller tympanic bullae). Most extant species of Balaenoptera exhibit an involucral ridge that is separated from the posterior margin (except Balaenoptera musculus), similar to Megaptera novaeangliae and these Purisima specimens. In dorsal view, the anterior portion of the involucra of these specimens and M. novaeangliae are much more constricted and have a large rounded eustachian opening; the latter feature is relatively smaller in Balaenoptera. Ŋese specimens are somewhat smaller than tympanic bullae of extant M. novaeangliae, and are transversely narrower with a less convex lateral surface. Due to the variability of bullar morphology in fossil and modern balaenopterids, Morphotype 1 bullae are only identified to the family level, despite similarities with Megaptera. It is unclear whether these specimens are referable to Balaenoptera bertae n. sp. Morphotype 2 tympanic bullae differ from those of Morphotype 1 (see above) and extant Megaptera in lacking a robust dorsal posterior prominence (Fig. 24). Ŋe present specimens differ from Balaenoptera, Diunatans, and Plesiobalaenoptera in lacking a transversely flattened anteromedial shelf, in having a substantially more inflated lateral surface of the outer lip, a sigmoid process that is positioned further posteriorly, and their much smaller size. UCMP 219098 further differs from Plesiobalaenoptera in lacking a distinctly convex anterior lobe in dorsal view and having a relatively larger eustachian opening; it further differs from Balaenoptera and Plesiobalaenoptera in having a smooth main ridge not developed into a slight ventral keel (Bisconti 2010 a). Ŋese specimens lack the derived features of most previously described balaenopterids, and are relatively archaic in comparison to these taxa. However, they differ from all known balaenopterid and stem-balaenopteroid (= cetotheres sensu lato) taxa in exhibiting an extremely inflated lateral surface and a posteriorly positioned sigmoid process. Additional tympanic bullae from California similar to Morphotype 2 bullae include UCMP uncataloged (Field number FP 302, UCMP locality V 90042) from an Upper Miocene (c. 5.33 Ma) locality in the Purisima Formation near Santa Cruz, and UCMP 88665 from the Upper Miocene San Luis Rey River Local Fauna of the San Mateo Formation (6 - 8 Ma) near Oceanside, California. Tympanic bullae agreeing in morphology to Morphotype 2 have occasionally been identified as “ Plesiocetus ” Van Beneden, 1859 (Barnes 1977; Oishi & Hasegawa 1995 a). However, as detailed by Deméré et al. (2005), the generic taxon “ Plesiocetus ” has been applied incorrectly to a large number of different taxa, to the point where the application of this taxon has been inconsistent and confusing. Because of confusion surrounding the taxon “ Plesiocetus ” (Deméré et al. 2005; Bosselaers & Post 2010), future identifications should be avoided and Morphotype 2 bullae are only identified to the family level herein. It is unclear whether these specimens are referable to B. bertae n. sp. Ŋe posterior process of a petrotympanic (UCMP 219137) shares with Balaenopteridae an elongate and strap-like posterior process with parallel dorsal and ventral margins (Fig. 23). It differs from herpetocetine mysticetes in being more elongate and larger in size and lacking a plug-like morphology with a flattened distal apex (Whitmore & Barnes 2008). It differs from Eschrichtius in being longer and lacking a ventrally expanded crest, and is precluded from assignment to the Balaenidae in being anteroposteriorly flattened, distally tapering in ventral view, and lacking a broad and well defined sulcus for the facial nerve. UCMP 219137 differs from B. bertae n. sp. its much larger size, and in lacking a slightly inflated distal apex that is not tapered and bladelike. Ŋe mandible fragment (UCMP 219141) is also not identifiable beyond the family level because of its incompleteness. It exhibits two balaenopterid features. Ŋe first feature, a concave lateral margin of the posterior part of the mandible, results from the sigmoid curvature of the mandible; and the posterior ⅓ of the mandible in balaenopterids is medially bowed (while the anterior ½ is laterally bowed). Ŋe second feature is a robust angular process that is visible laterally, but does not extend posteriorly to the mandibular condyle; the angular process is not developed in balaenids, and in Eschrichtius, it is indistinct and not separated from the condyle by a crease as in UCMP 219141 and all balaenopterids. Based on its size, it can be eliminated from B. bertae n. sp., which is perhaps one-half the size of UCMP 219141. It is possible that it may belong to cf. Balaenoptera, “ B. ” cortesi “ var. ” portisi, but is too incomplete to be identified beyond the family level.