Eubalaena, Gray, 1864

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 : 834-836

cf. Eubalaena sp. 1

REFERRED MATERIAL. — UCMP 219100, a small partial right tympanic bulla; and UCMP 219481, a small partial right tympanic bulla. 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

Except where noted, this description is based primarily on UCMP 219100. Ŋis tympanic bulla has a large involucrum with a greatly expanded medial surface which is anteriorly truncated ( Fig. 10 A-C). Several elongate, deep, and anterolaterally directed transverse creases occur on the involucrum. Posteriorly, the involucrum is much deeper dorsoventrally. A deep sulcus runs adjacent to the involucral ridge; the involucral ridge is slightly retracted from the ventral margin, and diverges posteriorly from the main ridge. Ŋe ventral surface is relatively smooth and convex and lacks a keel-like main ridge. Laterally adjacent to the main ridge there is an indistinct

involucrum

longitudinal furrow. Ŋe posteromedial portion of the bulla is rather robust and inflated in medial aspect. A fragment of the anterior portion of the outer lip is present, and it is broadly convex in anterior view. Ŋis results in a eustachian opening that is narrow; anteriorly this margin is slit-like and has an angle of c. 45° (between the involucrum and outer lip). UCMP 219481 also exhibits a narrow eustachian opening, although it has an anteroventral corner of the eustachian opening that is more acutely defined and not broadly curved as in UCMP 219100 ( Fig. 10F ).

REMARKS AND COMPARISONS

Ŋese tympanic bullae share many features with extant Eubalaena Gray, 1864 , including an elevated anterior lobe and a narrow eustachian opening ( Ekdale et al. 2011). Nevertheless, the involucral and main ridges are posteriorly divergent, whereas in extant Eubalaena they are parallel. Ŋey differ from balaenopterids in having a convex dorsal margin of the involucrum in medial and lateral aspect; balaenopterid bullae generally differ from those of extant balaenids in having a relatively straight medial margin of the involucrum that is not broadly arched in dorsal view ( Oishi & Hasegawa 1995b; Ekdale et al. 2011). Ŋese bullae differ from Balaena Linnaeus, 1758 in being relatively transversely broader; bullae of extant Balaena are transversely flattened relative to those of Eubalaena ( Ekdale et al. 2011) . Ŋe tympanic bulla of Caperea Gray, 1864 is similar in size, but transversely narrower than UCMP 219100; it exhibits a posteroventrally expanded involucrum and a rectangular eustachian opening, unlike UCMP 219100 and 219481 ( Ekdale et al. 2011). UCMP 219100 differs from Balaenella Bisconti, 2005 in having a rounded anteroventral margin that is not shaped into a corner ( Bisconti 2005), although this is seen in UCMP 219481 and Eubalaena . UCMP 219100 and 219481 differ from the tympanic bulla of B. balaenopsis Van Beneden, 1872 in exhibiting a narrow, triangular eustachian opening; UCMP 219100 further differs in exhibiting a more transversely inflated bulla, and UCMP 219481 differs in exhibiting a corner-like anteroventral margin. Ŋe tympanic bulla of Bal-

aenula astensis Trevisan, 1942 is more transversely compressed than UCMP 219100, and exhibits a eustachian opening that is less acutely triangular and slit-like than UCMP 219100 and 219481, although it is somewhat narrower than the broadly rounded eustachian opening in B. balaenopsis . A balaenid tympanic bulla (UCMP 29852; identified as Balaenula sp. by Barnes 1977) from the Purisima Formation at Point Reyes (Early Pliocene) is similar in size, but appears to be transversely thinner and exhibits a flatter medial surface of the involucrum relative to UCMP 219100 and 219481. Although they are relatively small ( Table 5 ), these specimens share enough features with Eubalaena to be tentatively identified to this genus.

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. 10. — Tympanic bullae of balaenid mysticetes:A, UCMP 219100,partial right tympanic bulla of cf. Eubalaena sp. 1 in medial view; B, UCMP 219100 in dorsal view; C, UCMP 219100 in ventral view; D, UCMP 219099, partial left tympanic bulla of cf. Eubalaena sp. 2 in medial view; E, UCMP 219099 in dorsal view; F, UCMP 219481, fragment of right tympanic bulla of cf. Eubalaena sp. 1 in medial view. Scale bar: 3 cm.