Callorhinus Gray 1859

Callorhinus sp., cf. C. gilmorei Berta & Deméré, 1986

Thalassoleon sp. – Boessenecker 2006: 43 A.

REFERRED MATERIAL. — UCMP 219003, anterior fragment of lower canine; UCMP 219147, upper third incisor; UCMP 86297, partial left humerus; UCMP 219139 and 219138, two right radii; UCMP 219122,

right calcaneum; and UCMP 219149, right astragalus. Collected by R.W. Boessenecker and L. Olivera from UCMP localities V7085, V99833, V99838, V99839, and V99859.

STRATIGRAPHIC OCCURRENCE. — Middle and upper parts of the San Gregorio section of the Purisima Formation, Early to Late Pliocene (c. 5-2.5 Ma, Zanclean-Gelasian equivalent; Fig. 2).

DESCRIPTION

Ŋe right upper third incisor (UCMP 219147) is small (14.55mm crown length), caniniform, strongly recurved, and bears well-developed posterior and anterior cristae (Fig. 7 A-D). Ŋe crown measures 6.05 mm in transverse width, and 8.66 mm anteroposterior width. A posteriorly curving longitudinal crista occurs on the lingual side of the tooth, and is posterobasally convergent with a slight posterolingual cingulum. Faint longitudinal wrinkles occur in the enamel. Ŋe crown and root of UCMP 219147 both have an oval cross section. Ŋe root is slightly damaged, and a thin layer of the outer surface of the root is missing; this is interpreted to be taphonomic, and this preservation is also seen in shark, pinniped, and cetacean teeth from other localities in the Purisima Formation.

A fragment (UCMP 219003) of the anterior portion of a canine represents a lower canine because of its curvature (Fig. 7E, F); upper canines of early diverging otariids (Callorhinus, hvalassoleon) are relatively straight, while lower canines are more posteriorly curved. Ŋe crown is conical, posteriorly curved, and exhibits longitudinally wrinkled enamel and an open pulp cavity is exposed on the broken surface.

Ŋe partial left humerus (UCMP 86297) is relatively small (108.52 mm total length; Table 2), slender, has a hemispherical humeral head, and is missing most of the deltopectoral crest and the lesser and greater trochanters (Fig. 7 G-J). Ŋe preserved portion of the deltopectoral crest (partially reconstructed) is transversely thin. A triangular, knob-like medial entepicondyle is positioned along the distomedial margin. Ŋe lateral entepicondyle is anteroposteriorly thin and bladelike with an arcuate margin in anterior aspect. Ŋe trochlea is saddleshaped and concave posteriorly and distally. Ŋe radial capitulum is convex, and exhibits the same anteroposterior diameter as the medial lip of the trochlea. A shallow olecranon fossa is present on the posterior side, proximal to the trochlea.

Ŋe two right radii (UCMP 219139 and 219138) are relatively small (111.31 and 133.75 mm total length, respectively; Table 3) and slender and missing the distal epiphyses (Fig. 8), indicating young ontogenetic age. UCMP 219138 is slightly larger than UCMP 219139 and is damaged anteriorly.Ŋe proximal surface of the radial head is oval in proximal aspect, slightly concave, and the articular surface has a “stepped” morphology so that the lateral surface is less proximally elevated. Ŋe radial tuberosity is positioned distal to the head on the medial surface, forming a raised, rugose, and circular plateau. Ŋe shaft of both specimens is transversely thin and flattened. Ŋe shaft widens anteroposteriorly toward the pronator teres process, which is preserved in UCMP 219139. When accounting for the missing distal epiphysis, the pronator teres process is positioned about ⅓ of the element’s length from the proximal end. Ŋe medial surface is gently concave, and the lateral surfaces are slightly convex and smooth. Ŋe shaft is widest distally.

Ŋe right astragalus (UCMP 219149) is small (34.62mm total length; Table 4) and slightly damaged on the plantar side (Fig. 9D, E). Ŋe dorsal tibial articulation is divided into two gently convex ridges on the trochlear surface, separated by a shallow median furrow. Ŋe head of the astragalus has an anteriorly convex articular facet for the navicular (Fig. 9D); the neck is slightly constricted relative to the head. Ŋe sustentacular facet is small, smooth, and slightly convex, and located on the plantar side of the neck. Ŋe sustentacular facet is bordered anteriorly and laterally by a shallow groove. Ŋe astragalar sulcus is transversely oriented and separates the sustentacular facet from the calcaneo-astragalar facet. Ŋe calcaneo-astragalar facet is oval-shaped and concave. Ŋe posterior margin of the astragalus is convex, and slight indentations occur at the posterior ends of the lateral and medial processes. Ŋe lateral process is a small, laterally projecting knob, and the medial process is a small rectangular knob on the posteromedial surface.

Ŋe well-preserved right calcaneum (UCMP 219122) is small (59.34 mm total length; Table 4)

A

and roughly rectangular, with a robust calcaneal process (Fig. 9 A-C). Ŋe proximal surface of the calcaneal process is smooth and gently convex, with a shallow vertical groove for the gastrocnemius and soleus tendons. Ŋe medial calcaneal tubercle is small and situated on the medial surface of the calcaneal process. Ŋe calcaneal process is cylindrical and proximally rugose. Ŋe calcaneoastragalar facet is oriented posteromedially and is lunate in shape, dorsally convex, and slopes anteromedially. Ŋe lateral calcaneal tubercle is a slight ridge. Ŋe medial margin of the calcaneal process is concave, while the sustentacular facet is dorsally concave and projects medially as a triangular process. Ŋe peroneal tubercle is abraded away. Ŋe secondary shelf of the sustentaculum is developed as a narrow groove along the anteromedial margin of the sustentacular facet, and widens anteriorly. A slight groove occurs between the calcaneo-astragalar facet and the sustentacular facet, and a slight fossa lies lateral to the sustentacular facet. Ŋe cuboid facet occupies the slightly concave and oval shaped anterior surface of the calcaneum.

COMPARISONS

Specimens referred to Callorhinus sp., cf. C. gilmorei all exhibit otariid characteristics and small size, and constitute all otariid specimens from the San Gregorio section of the Purisima Formation, and are considered to represent a single otariid taxon. Some of these elements (incisor, radius, astragalus, calcaneum) exhibit features diagnostic of Callorhinus gilmorei Berta & Deméré, 1986 from the San Diego Formation (including new material within SDNHM collections), but several elements (canine, humerus) only possess features characteristic of the family. Although potentially belonging to a separate otariid taxon, they are tentatively included in the same taxon based on their small size and similarity with C. gilmorei material from the San Diego Formation. Furthermore, C. gilmorei is the only otariid present in the California Pliocene (Miyazaki et al. 1995; Boessenecker 2011c), with the exception of hvalassoleon macnallyae from the latest Miocene and earliest Pliocene of the Purisima Formation (Point Reyes and Santa Cruz sections), and cf. hvalassoleon from the earliest Pliocene Salada and San Mateo Formations (Barnes 1998; Deméré et al. 2003), to which these specimens cannot be referred. T. macnallyae is the only other otariid known from the Purisima Formation, and along with other specimens of hvalassoleon, consistently exhibit more plesiomorphic characteristics than these new specimens, in addition to larger overall size (approximately 150-200% larger). Additional diagnostic material is necessary to refine this preliminary identification.

Ŋe upper third incisor is relatively small and close in its morphology to C. gilmorei and Callorhinus ursinus. It differs from Arctocephalus pusillus Schreber, 1775, Eumetopias jubatus Schreber, 1775, Hydrarctos lomasiensis Muizon, 1978, Neophoca Gill, 1866, Otaria Peron, 1816, Phocarctos Peters, 1866, Proterozetes ulysses Barnes, Ray & Koretsly, 2006, hvalassoleon mexicanus Repenning&Tedford, 1977, and Zalophus, in having a small, gracile crown with an oval cross-section (Fig. 7D; Repenning et al. 1971). An isolated upper third incisor (UCMP 219433) referable to T. macnallyae from locality V- 6875 in the Purisima Formation is similar in size and cross-section, but differs from this specimen in possessing a lingual bulge at the base of the crown.

Ŋe humerus (UCMP 86297) exhibits typical otariid characteristics including a relatively straight shaft in lateral/medial aspect (as opposed to sinuous as in phocids), the medial lip of the distal trochlea having the same diameter as the radial capitulum (the medial lip is of a greater diameter in odobenids; Deméré 1994a), and a small, triangular medial entepicondyle (which is knob-like in most Late Neogene odobenids). Despite its small size, it has fully fused proximal and distal epiphyses, indicating it is an adult; it is here interpreted as an adult female. UCMP 86297 compares well with modern Callorhinus and Arctophoca Gill, 1866 humeri, as well as C. gilmorei humeri from the San Diego Formation (SDNHM 25398 and 38296). Ŋis specimen

exhibits fully fused epiphyses, while similarly sized humeri of T. mexicanus and T. macnallyae remain unfused, and adult humeri of hvalassoleon spp. are substantially larger than those of adult C. ursinus and C. gilmorei (SDNHM 38296). Ŋe small adult size of this humerus indicates referral to C. gilmorei.

Ŋe radii (UCMP 219138 and 219139) exhibit several otariid features, including a relatively straight and transversely compressed shaft, exhibiting a radial process that does not extend further distally than the distal scapholunar articulation facet, and having a pronator teres process that is positioned relatively far proximally (Fig. 8). Ŋese specimens are too small to represent an odobenid, and also lack the well developed bony ridges and processes on the distal end associated with extensor tendons, which typify odobenid radii. Ridges on the lateral side of the radius are also typical for many extant otariids as well as T. mexicanus (Repenning & Tedford 1977; Deméré & Berta 2005). UCMP 219138 and 219139 differ from T. mexicanus, T. macnallyae, and Eumetopias in having a more proximally placed pronator teres process, and their much smaller size. Extant Callorhinus and Arctophoca exhibit pronator teres processes that are more proximally positioned than in UCMP 219138; the pronator teres process is positioned similarly in Zalophus. Ŋese specimens compare best with fossil radii of C. gilmorei (i.e. SDNHM 25549, 35268) in their size and the proximal position of the pronator teres process.

Ŋe astragalus is relatively small (Fig. 9D, E), and too small to represent an odobenid or a sea lion (Otariinae), but approximately the same size and nearly identical to extant Callorhinus. Ŋis specimen differs from the otariine Eumetopias in its smaller size, having a relatively larger calcaneo-astragalar facet, a more circular sustentacular facet, a transversely narrower head, a smaller medial process, and tibial trochleae that are more medially rotated. Ŋis specimen differs from Zalophus in its slightly smaller size, a smaller and more convex navicular facet, lacking a medial spur off the head, more equally sized medial and lateral processes, and a larger calcaneo-astragalar facet. UCMP 219149 differs from T. mexicanus in having a narrower neck, smaller head, and in lacking a distinct anterior extension of the dorsal articular surface onto the neck of the astragalus. It compares relatively well with UCMP 219482, a recently collected disarticulated hindlimb of T. macnallyae from the Purisima Formation near Santa Cruz, but differs in having a wider neck, narrower astragalar sulcus, larger lateral and medial processes; it also differs from UCMP 219482 in lacking an anterior extension of the dorsal articular surface. Ŋe lack of an anterior extension of the dorsal articular surface precludes UCMP 21949 from referral to hvalassoleon; an isolated astragalus of C.gilmorei (SDNHM 25570) from the San Diego Formation is similarly small, lacks an anterior extension of the dorsal articular surface, and does not appreciably differ from the Purisima specimen. Ŋe preclusion of referral to hvalassoleon or any extant otariid, small size, and similarity with C. gilmorei and C. ursinus suggest referral to the former.

Ŋe calcaneum differs from odobenids in its tiny size and lack of a medial tuber of the calcaneal process, while it differs from phocid calcanea in being transversely broader and having large cuboid and calcaneo-astragalar facets (Fig. 9A, B). UCMP 219122 differs from T. mexicanus in having a slight sustentacular shelf (Fig. 9A; completely lacking in hvalassoleon spp., and present in all extant otariids; Robinette & Stains 1970; Deméré & Berta 2005), and a shorter calcaneo-astragalar facet. Eumetopias is much larger, exhibits a wider secondary sustentacular shelf, a narrower neck, and a smaller cuboid facet than UCMP 219122 (Robinette & Stains 1970). Zalophus has a slightly wider secondary sustentacular shelf, a longer calcaneo-astragalar facet, and a more robust calcaneal process than the fossil (Robinette & Stains 1970). Otaria differs from UCMP 219122 in having a bilobate calcaneo-astragalar facet, a narrower secondary sustentacular facet, and a broader neck (Robinette & Stains 1970). Arctophoca and Arctocephalus have similar proportions to UCMP 219122, but have a smaller cuboid facet, and like all other extant otariids, have a wider secondary sustentacular facet (Robinette & Stains 1970). Ŋis specimen differs from C. ursinus in having a more robust calcaneal process, a smaller secondary sustentacular shelf, and a shorter calcaneo-astragalar facet that does not extend as far proximally. UCMP 219122 shares with an isolated calcaneum of C. gilmorei from the San Diego Formation (SDNHM 65334) a secondary sustentacular shelf that is narrow, posteriorly tapering, and groove-like, to the exclusion of other fossil otariids.

REMARKS

Ŋese isolated bones and teeth consistently exhibit features that are more derived than the fossil otariids T. mexicanus and T. macnallyae, but are slightly plesiomorphic relative to extant otariids. Several features preclude referral to hvalassoleon, including the morphology of the radius, astragalus, and calcaneum. Additionally, all specimens are nearly identical in size and morphology to corresponding elements of C. gilmorei (from the San Diego Formation) and nearly identical in morphology to the extant C. ursinus (which is somewhat larger than the Purisima Formation fossils), suggesting identification as the former. In summary, these specimens possess features – among fossil otariids from the eastern North Pacific – which are known only in C. gilmorei. However, some specimens preserve features characteristic of the family only, and this collection of isolated elements is only tentatively identified as this taxon. Callorhinus gilmorei was originally described from the Pliocene San Diego Formation of southern California and Baja California (Berta & Deméré 1986), and has also been reported from the Upper Pliocene Rio Dell Formation of Northern California (Boessenecker 2011c), and the Upper Pliocene of Japan (Kohno & Yanagisawa 1997). Ŋese tentatively identified specimens of C. gilmorei now record this fur seal from the Pliocene of central California as hypothesized by Boessenecker (2011c), suggesting that it inhabited most of the California coastline during the Pliocene in addition to Southern California and Baja California (Berta & Deméré 1986) and Northern California (Boessenecker 2011c), and document the presence of at least two pinnipeds in the San Gregorio assemblage.