Phocoenidae, Gray, 1825

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 : 889-890

Phocoenidae unnamed genus 1

Phocoenidae species C – Barnes 1977: 321-343.

REFERRED MATERIAL. — From the San Gregorio Section of the Purisima Formation: UCMP 219480, an isolated left petrosal collected by R.W. Boessenecker from UCMP locality V99851 .

From the Santa Cruz Section of the Purisima Formation: UCMP 128285, partial skeleton including skull, UCMP 219504, nearly complete skull; UCMP 219486, petrosal; SCMNH 21221, petrosal; 21222, petrosal; 21224, petrosal.

STRATIGRAPHIC OCCURRENCE. — Lowermost part of the San Gregorio section of the Purisima Formation, latest Miocene to earliest Pliocene (6.4 to c. 5 Ma; Messinian- Zanclean equivalent; Fig. 2 ).

DESCRIPTION

UCMP 219480 is a medium-sized (28.5 mm in length; Table 11 ) odontocete petrosal from the base of the San Gregorio section of the Purisima Formation ( Fig. 41 ). Ŋe specimen has a large, prominent, and anteroposteriorly broad pars cochlearis. Ŋe internal acoustic meatus is teardrop shaped, transversely narrow, and anterolaterally oriented ( Fig. 41B ). Ŋe facial canal appears to be coalesced with the internal acoustic meatus due to the deeply recessed crista transversa. No dorsal crest or tuberosity is present; the dorsal face is smooth, rectangular, and slightly convex. A low parabullary ridge runs along the lateral margin of the petrosal from the base of the anterior process to the epitympanic hiatus; there is no anterior bullar facet ( Fig. 41A, B ). Toward the extremities of the anterior and posterior processes, the dorsal surface slopes ventrally (in medial aspect). Ŋe anterior process is large, blunt, and swollen; in ventral aspect, it is kidney shaped and curves anteromedially. In medial aspect it is bilobate and appears to curve ventrally. Ŋe anterior incisure forms a well-defined crease between the pars cochlearis and the anterior process; ventrally there is a broad, smooth, and concave fovea epitubaria. Ŋe short posterior process is dorsoventrally thick and in medial aspect, its posterior margin is broadly curved and lacks a sharp posterior crest. Although the distal part of the posterior process is abraded, the proximal part of the posterior bullar facet is ventrally deflected. In ventral aspect, the long axis of the pars cochlearis forms an angle of approximately 130° with the long axis of the pars cochlearis.

REMARKS AND COMPARISONS

Ŋis petrosal (UCMP 219480) exhibits one delphinoid feature (lateral side of petrosal smooth; Geisler & Sanders 2003), and two phocoenid synapomorphies: a posterior process that is ventrally deflected, and an angle between the posterior process and long axis of the pars cochlearis less than 135° ( Murakami et al. 2012a, b). Ŋis specimen differs from petrosals of Phocoenidae unnamed genus 2 (see below) in its larger size, larger, bilobate anterior process (rather than small and conical), and proportionately smaller posterior process. It further differs from another undescribed broadheaded phocoenid from the San Diego Formation (SDNHM 38340) in lacking a medially directed anterior process and circular internal acoustic meatus. UCMP 219480 appears to represent a taxon distinct from cf. Phocoena (based on petrosals of modern Phocoena spp.) and Phocoenidae unnamed genus 2 (= “ Phocoenidae new genus ” of Racicot et al. 2007), based on referred petrosals from the San Diego Formation ( Barnes 1973: fig 2g-j). Isolated petrosals from the Santa Cruz section of the Purisima Formation (UCMP 95963, 219486, UCMP uncatalogued, field no. FP-177, SCMNH 21221, 21222, 21224) share with this specimen a large, anteroposteriorly broad pars cochlearis with a small teardrop-shaped internal acoustic meatus, a small and ventromedially deflected posterior process, and an inflated anterior process that is bilobate at its apex. Other petrosals matching this morphology (listed above) consistently exhibit a sharp triangular projection at the posterolateral apex of the posterior process; this appears abraded in UCMP 219480. Ŋis feature is noteworthy as it is unique to petrosals of this morphotype within the available sample of odontocete petrosals from the Purisima Formation. Ŋese isolated petrosals (UCMP 95963, 219480, 219486, etc.) share these characteristics with a petrosal associated with a partial skeleton from the Purisima Formation near Santa Cruz (UCMP 128285) which was identified as “ Phocoenidae species C” by Barnes (1977). Ŋis specimen and another cranium (UCMP 219504) exhibit a combination of delphinid and phocoenid characteristics, including an asymmetrical skull with a right premaxilla extending further posteriorly than the left and contacting the nasal (a delphinid synapomorphy), as well as premaxillary eminences, a frontal knob, and spatulate teeth (phocoenid synapomorphies). In addition to this undescribed taxon and Haborophocoena toyoshimai Ichishima & Kimura, 2005 , several additional delphinid-like phocoenids have been described recently from the Pliocene of Japan including Archaeophocaena teshioensis Murakami, Shimada, Hikida & Hirano, 2012 , Haborophocoena minutis Ichishima & Kimura, 2009 , and Miophocaena nishinoi Murakami, Shimada, Hikida & Hirano, 2012 ( Ichishima & Kimura 2009; Murakami et al. 2012b). Given this mixture of characteristics, it is interesting to note that an isolated petrosal of Phocoenidae unnamed genus 1 (UCMP 95963) was identified by Barnes (1977) as aff. Tursiops . Although similar in some regards to delphinid petrosals (e.g., Tursiops ), UCMP 219480 and other petrosals of this unnamed taxon (listed above) differ in having a much larger anterior process and smaller posterior bullar facet that lacks striations. Delphinoid features include relatively short anterior and posterior processes and the lack of an anterior bullar facet ( Fordyce 1994; delphinidan features according to Muizon 1988a). Identification of UCMP 219480 is based on skull features of crania of this unnamed taxon (UCMP 128285, 219504).

Petrosals of Phocoenidae unnamed genus 1 (including UCMP 219480) are most similar to the Early Pliocene H.toyoshimai and the Late Miocene M. nishinoi ; UCMP 219480 shares two features in common with Haborophocoena Ichishima & Kimura, 2005 to the exclusion of other phocoenids, notably an unusually robust and swollen anterior process (also shared with Miophocaena Murakami, Shimada, Hikida & Hirano, 2012 ), and a posterolaterally directed posterior process with a spine (present in UCMP 219486, SCMNH 21221, 21222, and 21224; also shared with Pterophocaena nishinoi Murakami, Shimada, Hikida & Hirano, 2012 ). UCMP 219480 differs from Pterophocaena Murakami, Shimada, Hikida & Hirano, 2012 in having a more inflated and bilobate anterior process, and differs from Miophocaena in having a smaller internal acoustic meatus. Interestingly, crania of this undescribed taxon from the Purisima Formation ( Phocoenidae unnamed genus 1) also share several features with Haborophocoena , including large size, an elongate rostrum, an asymmetrical facial region, a frontal knob, relatively flat facial plane and low premaxillary eminences. Consideration of the generic identity of this taxon and relationship with Haborophocoena awaits description of more complete cranial material from the Santa Cruz section of the Purisima Formation. Fossils of Phocoenidae unnamed genus 1 have thus far not been found outside the Purisima Formation in California.

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. 41. — Left petrosal of Phocoenidae unnamed genus 1 (UCMP 219840); A, ventral view; B, dorsal view; C, medial view. Scale bar: 20 mm.