Enaliarctos sp.

Poust, Ashley W. & Boessenecker, Robert W., 2018, Expanding the geographic and geochronologic range of early pinnipeds: New specimens of Enaliarctos from Northern California and Oregon, Acta Palaeontologica Polonica 63 (1), pp. 25-40 : 30-34

Enaliarctos sp. , cf. E. tedfordi Berta, 1991

Fig. 5 ; Tables 1, 2.

Material.— UCMP 253400 , associated right mandible, thoracic vertebra, and two ribs (collected by David G. Taylor, October 6, 1963); from Ona Beach, Lincoln County, Oregon ( UCMP locality V79036); collected from a loose concretion of the Yaquina Formation , late Oligocene (30.6–27.4 Ma , late Rupelian –early Chattian equivalent; Prothero et al. 2001b). Detailed locality information available on request to qualified researchers from UCMP or the authors. The block containing UCMP 253400 remained unprepared and undescribed for over fifty years ( Fig. 5A ). The bones are preserved out of articulation but in contact with one another along a single plane in the block. The matrix is a well-sorted, moderately fine-grained sandstone. Immediately surrounding the bones the sandstone is iron-stained and more friable than the remainder of the matrix. In addition to the mammalian bones, the sandstone block includes fish vertebrae, scales, and bone fragments and some lignitized plant material. In places, the sandstone is composed of masses of small peloids with a radius of about a millimetre .

Description.— Mandible: UCMP 253400 includes the anterior right mandible, missing the posterior end behind the p4 ( Fig. 5 ). The preserved portion of the horizontal ramus is subrectangular and relatively dorsoventrally shallow. A strongly expressed genial tuberosity occurs below p1–p2 and marks the deepest point on the mandible. Two large mental foramina are positioned below p2 (3.8 mm) and p3 (3.1 mm), similar to those of the other enaliarctine mandibles described. The symphyseal surface is poorly preserved but appears to have been broadly lanceolate.

The alveolar margin is linear, though the p2, p3, and p4 are consistently higher with more root exposed; it is unclear if these teeth were slightly displaced out of their alveoli during decomposition and prior to burial. Posterior to the p4 the mandible is not preserved, but the impression of the m1 (see Fig. 5 ) shows that this trend was not continued in the molars, suggesting postmortem displacement of the anterior teeth.

Dentition: No incisors are preserved in UCMP 253400. The canine is relatively small, dorsoventrally short, and curved posteriorly. The canine exhibits a posterior bulge on the base of the crown, the buccal side of which has a deep, oval wear facet from the upper canine that has penetrated the enamel. Above this bulge on the lingual side is an upward-tapering wear facet, formed from occlusion with I3. The c1 bears a double posterior crista. Towards the root, the ancillary ridge deflects medially, dividing the postero-medial half of the tooth crown into three triangular faces.

The alveolus of the p1 indicates that the tooth was single-rooted. The remaining premolars are double-rooted. The p2 has a large crown with a small conical paraconid, a very small metaconid on the mid-crown, and a low but well-defined hypoconid. The p3 crown bears a larger, more rounded paraconid, a high protoconid, and a metaconid elevated to mid-crown height; the hypoconid is a low cusp situated at the posterior terminus of the lingual cingulum which encircles a shallow talonid basin. The p4 has a larger conical paraconid, a high, sharp protoconid, and a well-developed metaconid, more distinct from the principal cusp than in p3; the hypoconid is broken, but the impression shows it was a very low, conical cusp. All premolars are labially smooth and lack labial cingulum, but possess well developed smooth lingual cingulum.

The m1 is lost, but an impression of the lingual surface remains in the sandstone block ( Fig. 5E ). The mold shows an anteroposteriorly elongate crown with a low protoconid cusp and a high trenchant paraconid. The protoconid has a similar

A1

A 2 c1 protoconid metaconid paraconid hypoconid i3 p1 p2 i2 p3 p 4 m 1 m 2 masseteric fossa mental foramina B m1 C m2 20 mm i3 i2 p1 c1 p4 p3 p2

height to p4, with a distinct, rounded metaconid; an impression of the hypoconid is not preserved. The diastema between p3 and p4 is particularly large at 5.5 mm, whereas no diastema in UCMP 114474 or UWBM 89114 exceeds 2 mm.

Posterior cervical vertebra: The convex, oval anterior articular surface of the centrum sits below a large dorsoventerally flattened neural canal ( Fig. 5A ). The laminae are flat, broad, and inclined slightly anteriorly. Gracile transverse processes extend from the anterior surface of the pedicles, tilting anterolaterally at approximately a 45° from the dorsal plane. The blades of the transverse processes bifurcate into a “T”-shape with a flat lateral edge. Prezygapophyses are not preserved; the postzygapophyses are broad, arranged at an angle greater than 45° from the sagittal plane, and have facets facing ventrolateral, more or less in line with the wings of the transverse processes. The neural spine is broken, but appears to have been low and posteriorly inclined. The posterior articular surface of the centrum is raised slightly dorsally relative to the anterior face to allow increased axial flexion.

Ribs: The two partial ribs of UCMP 253400 are short segments of the distal and mid-shafts respectively; positions are unclear. Rib A was preserved beneath the mandible (see Fig. 5A ) and extends from a somewhat flattened mid-shaft to a distal end more circular in cross-section with rough, porous cortical bone where the element met costal cartilage. Rib B is very incomplete, and is represented in part by a natural mold of the internal surface of the mid-shaft showing a very distinct, anterior costal groove.

Comparisons.— UCMP 253400 shares with Enaliarctos emlongi smooth labial crown surfaces further lacking labial cingula, relatively small paraconid cusps, and low metaconid cusps. UCMP 253400 is from the Oligocene Yaquina Formation (28.1–27.4 Ma) and therefore much older than Enaliarctos emlongi from uppermost Nye Mudstone or lowermost Astoria Formation (20.2–19.1 Ma); owing to this age difference, UCMP 253400 is probably not referable to E. emlongi . However, E. tedfordi , which is not known from a mandible or lower dentition, shares loss of the labial cingulum and reduction of the lingual cingulum of the upper dentition with E. emlongi . Enaliarctos tedfordi originates from the Yaquina Formation, and assuming that the lower dentition parallels the upper dentition as in E. emlongi, UCMP 253400 is tentatively referable to E. tedfordi .

Figures

Fig. 5. Elements of pinniped mammal Enaliarctos sp., cf. E. tedfordi Berta, 1991 (UCMP 253400) from Ona Beach, Lincoln County, Oregon, Yaquina Formation, early late Oligocene; overview of original block (A), lingual (B), occlusal (C), and labial (D) views; matrix preserving impression of lingual side of p4 and m1 (E).