Balaenoptera

Boessenecker, Robert W., 2013, A new marine vertebrate assemblage from the Late Neogene Purisima Formation in Central California, part II: Pinnipeds and Cetaceans, Geodiversitas 35 (4), pp. 815-940 : 854-856

cf. Balaenoptera

REFERRED MATERIAL. — UCMP 219136, a right petrosal, collected by R.W. Boessenecker from UCMP locality V99851 .

STRATIGRAPHIC OCCURRENCE. — Lowermost part of the San Gregorio section of the Purisima Formation, latest Miocene (6.4-5.6 Ma; Messinian equivalent; Fig. 2 ).

DESCRIPTION

Ŋis large partial petrosal exhibits a low, and ventrally flattened pars cochlearis that is roughly oval-shaped in ventral aspect and somewhat elongated dorsally ( Fig. 20 ; Table 7 ). A small, thin, and bladelike posterior cochlear crest occurs posteroventrally on the pars cochlearis. Ŋe fenestra cochleae is small and circular and occurs within a small fossa. Ŋe anterior process is acutely triangular, transversely thick, and a longitudinal keel occurs on the ventral side at its apex. Ŋe anterior pedicle sits atop a rectangular plateau on the medial surface of the anterior process.A well- developed and knob-like lateral tuberosity is present on the lateral margin of the anterior process, anterior to the position of the fenestra vestibuli.Ŋe oval-shaped fenestra vestibuli is positioned at the posteroventral end of the pars cochlearis, and a posteriorly oriented facial sulcus is present dorsolaterally to the fenestra vestibuli. A small, rectangular, and concave fossa for the stapedial muscle is positioned lateral to the posterior cochlear rest and posterior to the fenestra vestibuli. A relatively small, oval shaped, and deep suprameatal fossa occurs on the anterodorsal side of the pars cochlearis. Ŋe vestibulocochlear canal and facial canal are located on the posterodorsal portion of the pars cochlearis. Both canals are circular and small, closely appressed, with a thin crista transversa. Ŋe endolymphatic and perilymphatic foramina are positioned posterior to the vestibulocochlear nerve canal, and occur in a deep fossa which is deeper at the dorsal end near the endolymphatic foramen ( Fig. 20D ). Both foramina are slit-like and open dorsally. Ŋe shallowly concave stylomastoid fossa is small, rectangular, and positioned dorsal to the fenestra cochleae. Ŋe dorsal side of the petrosal and anterior process is convex and rugose.

REMARKS AND COMPARISONS

Ŋis petrosal is identified as a balaenopterid because of the relatively wide and triangular anterior process and the medially elongated pars cochlearis. It differs from “ Balaenoptera ” cortesi “var.” portisi (including UCMP 219135, described above) and Megaptera novaeangliae in lacking extreme medial elongation of the pars cochlearis, and in having a transversely thick anterior process. Ŋe cranial nerve canals of UCMP 219136 are located on the posterodorsal portion of the pars cochlearis, like extant Balaenoptera spp. and M. novaeangliae ( Ekdale et al. 2011) ; this differs from many extinct balaenopterids including “ B. ” cortesi “var.” portisi, Diunatans luctoretmurgo , and Plesiobalaenoptera quarantellii , in which the cranial nerve canals are centrally positioned on the dorsal face of the pars cochlearis ( Bosselaers & Post 2010; Bisconti 2010a). Ŋis specimen is tentatively referred to Balaenoptera owing to several features. Ŋe pars cochlearis of UCMP 219136 is more similar to extant Balaenoptera than to Megaptera or any described extinct balaenopterids; however, the anterior process differs from every modern species within Balaenoptera in being dorsally rather than anteriorly directed. Among extant Balaenoptera , the morphology of the pars cochlearis is closest to Balaenoptera acutorostrata ; although the anterior process is of similarly short length between B. acutorostrata and UCMP 219136, it is anteriorly directed in extant B. acutorostrata and anterodorsally deflected in UCMP219136. UCMP 219136 also differs from all extant species of Balaenoptera in this feature. Ŋis indicates that at least two balaenopterids are represented in the San Gregorio assemblage based on petrosal morphology. UCMP 219136 is nearly identical in morphology to a fossil balaenopterid (SDNHM 80102) formerly identified as Balaenoptera davidsonii (Deméré et al. 2005) , which now appears to represent a separate balaenopterid taxon (T. A. Deméré pers. comm. 2011). Ŋe skull of SDNHM 80102 lacks squamosal creases and has elongate zygomatic processes, unlike Balaenoptera bertae n. sp., and it is thus unlikely that UCMP 219136 is a petrosal of B. bertae n. sp.

Figures

FIG. 2. — Stratigraphy of the Purisima Formation: A, Stratigraphic column of the San Gregorio section of the Purisima Formation (in meters), showing stratigraphic position of UCMP vertebrate localities, dated ash beds, diatom floras, and species of Carcharodon; thickness measurements start over above missing section in uppermost part due lack of overlapping strata; epoch and stage boundaries marked as "?" are estimated; modified from Boessenecker (2011a); Pliocene-Pleistocene boundary after Hilgen et al. (2012); B, photograph of 3-4 m thick ash within Purisima Formation at mouth of San Gregorio Creek correlated with 2.5 Ma Ishi Tuff (Powell et al.2007); C, trace fossils in pervasively bioturbated sandstone,Purisima Formation,mouth of San Gregorio Creek; D, swaley cross-stratified sandstone, mouth of San Gregorio Creek. Abbreviations: Qt, Pleistocene terrace deposits; Tp, Purisima Formation; C, claystone; St, siltstone; Ss, sandstone. Scale bars: A, 2 m; B, C, 10 cm.

FIG. 20.— Right petrosal of cf.Balaenoptera (UCMP 219146);A, medial view;B, lateral view;C, ventral view;D, dorsal view.Scale bar:3 cm.