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 : 34-35

Enaliarctos sp.

Figs. 6 , 7; Tables 1, 2.

Material.— UWBM 89114 , a partial skeleton exposed in relief in a small concretion including left and right mandibles with i2–i3, c1, p1–p4, m1, atlas, axis, CV6, CV7, TV1, several other thoracic vertebrae, one rib, and parts of three sternebrae (Collected by Jason Love in March 2004, from UWBM locality C1024); from Moloch Beach , Lincoln County, Oregon, USA, Iron Mountain Bed , Astoria Formation , late early Miocene (17.3–16.6 Ma; Burdigalian equivalent). Detailed locality information available on request to qualified researchers from UWBM or the authors.

Description.—Mandible: UWBM 89114 includes both mandibles, with the right mandible missing the posterior end and worn down below the alveolar margin posterior to the p2 ( Figs. 6A , 7). Minimal wear of the teeth argues against advanced age, but the postcanine teeth are not crowded as in juveniles, suggesting subadult or possibly adult status; the canine is relatively large and similar in proportion to the presumed male E. emlongi holotype, suggesting that UWBM 89114 is a male. The left mandible is nearly complete and is missing only the coronoid process, mandibular condyle, and angular process. The medial surface of the right mandible and lateral surface of the left mandible are exposed. The horizontal ramus of the mandible is subrectangular, relatively shallow dorsoventrally, and is deepest posteriorly at the level of the p3. Three mental foramina are positioned on the middle of the mandible below p2–p3. The symphyseal surface is rugose and is lenticular in shape; no genial tuberosity is developed, but a delicate ventral crest is present posteroventral to the symphysis.

The alveolar margin is undulatory and descends ventrally at the level of the p3, giving the toothrow a somewhat dorsally concave profile in lateral view. the alveolar margin rises toward the m1 and descends again to give the margin anterior to the coronoid process a dorsally concave profile. The masseteric fossa is shallow and does not extend ventral to the toothrow.

Dentition: Both i2 and i3 are preserved and exhibit transversely narrow roots with slightly wider crowns bearing apical transverse grooves ( Fig. 7A 2 , C). The canine is relatively small, transversely wide but dorsoventrally short and posteriorly curving. It exhibits a posterior bulge on the base of the crown, and bears a posterior crista, mesial crista, and lingual cingulum. A small wear facet from the upper canine is present labially on the base of the crown as in Enaliarctos sp. , cf. E. tedfordi (UCMP 253400, see above).

The p1 is single-rooted and bears a small crown with an indistinct paraconid (= anterior accessory cusp), a low protoconid (= principal cusp), and a low hypoconid (= posterior accessory cusp); the metaconid is absent. As in other enaliarctines the p2–p4 are double-rooted. The p2 bears a somewhat larger crown with an indistinct paraconid, a small metaconid, and a small hypoconid. The p3 crown is lower than the p2 owing to the “dip” in the alveolar margin of the mandible and bears a small conical paraconid, a high protoconid, and a metaconid elevated to mid-crown height; the hypoconid is obscured by matrix. The p4 has a larger conical paraconid, a high but apically worn protoconid, and a well-developed metaconid like p3; the hypoconid is low and conical. All premolar crowns are labially smooth and lack a labial cingulum.

The m1 is double-rooted and bears a low, anteroposteriorly elongate crown with a low, trenchant paraconid, a low protoconid, an indistinct metaconid, and a conical and low hypoconid. The m2 is absent but a small partially prepared alveolus indicates a single circular root. All lower postcanines are tightly spaced and no diastemata are apparent.

Atlas: The anterior condylar facets are concave, transversely narrow ventrally and broad and widely separated dorsally ( Fig. 6A ). A low neural spine is present. The transverse process is subrectangular and posteroventrally oriented in lateral view, approximately 15–20° from vertical. The transverse process is pierced by a lateral vertebral canal positioned medially within a fossa on the anterior surface.

Axis: The axis bears a well developed conical odontoid process with a convex and transversely narrow atlantal facet. The centrum bears a longitudinal ridge dorsally within the neural foramen and a deep ridge ventrally, flanked by deep ventrolateral fossae. A secondary fossa is present on the posterolateral surface below the pedicle of the neural arch, separated by the ventrolateral fossa by a thin ridge. The neural spine is large, falcate, and extends anteriorly to the level of the odontoid apex in lateral view; the neural spine rises posteriorly towards its dorsal apex. A small tongue-shaped postzygapophysis is present. A small, posterolaterally projecting transverse process is developed.

Third? or fourth? cervical vertebra: One incompletely preserved vertebra exhibits a circular lateral vertebral canal larger than in CV6; it bears a low subrectangular neural spine with an apical tubercle and a small fan-shaped transverse process that is less elongate than in CV6. Owing to the short transverse process this vertebra represents an anterior cervical.

Sixth cervical vertebra: A small, rectangular anterior articular surface of the centrum is flanked by small oval lateral vertebral canals ( Fig. 6B ). The transverse process is elongate, ventrolaterally projecting and bears a sharp anterior crest. Small triangular prongs are present anteriorly at the level of the ventral margin of the centrum; the ventrolateral apex of the transverse process is bifurcated. A low neural spine is present; spoon-like prezygapophyses are present and have dorsomedially facing articular surfaces approximately 45° from the sagittal plane, whereas the postzygapophyses have subhorizontal facets with small medial tubercles.

Seventh cervical vertebra: The CV7 differs from the CV 6 in having a taller neural spine and a short, transversely oriented transverse process with a laterally deepening, fanlike shape. The posterior articular surface of the centrum is oval-shaped. The neural spine bears an apical tubercle for the nuchal ligament.

Thoracic vertebrae: The TV1 is similar to the CV7 but has a further dorsally positioned transverse process with a concave facet for the first rib and a dorsal spur, and a slightly taller neural spine. Two other thoracic vertebrae of uncertain position are preserved. One exhibits a more cylindrical centrum than the other vertebrae, while the second consists only of a neural arch and spine. Both show a posteriorly inclined neural spine lacking an apical tubercle, and horizontal postzygapophyses on the posterior part of the neural arch.

Ribs: The proximal end of one rib is preserved and includes a bulbous, spherical capitulum, a short neck, and a flattened tubercle. A low lateral ridge is present anteriorly.

Sternebrae: Parts of three sternebrae are present ( Fig. 6A, B ). One is complete and subcylindrical in overall form and exhibits a rugose and quadrate articular surface for the next sternebra; it is transversely and dorsoventrally narrow or “pinched” in the middle.

Remarks. — UWBM 89114 differs from all other enaliarctines for which a mandible is known ( Enaliarctos barnesi , Enaliarctos emlongi , Enaliarctos mealsi , Pteronarctos goedertae ) in its smaller absolute size, mandible with sinuous alveolar margin, and a masseteric fossa not extending ventral to the toothrow, and further differs from E. barnesi and E. mealsi in lacking labial cingulum (which it shares with E. emlongi ). UWBM 89114 is identifiable as Enaliarctos based on the presence of a lingual bulge on the base of the canine crown (shared with Potamotherium , Puijila , and other arctoid “fissipeds”), differing from all pinnipediformes; it shares this feature with all Enaliarctos spp. for which a lower canine is known. UWBM 89114 further differs from E. emlongi in possessing somewhat more inflated protoconid cusps on p2–p4, more basally positioned metaconids on p2–m1, and a lower protoconid on m1. UWBM 89114 shares similar small size with E. mitchelli , and has a postcanine toothrow length of 45 mm, which is somewhat smaller than the upper postcanine toothrow length of E. mitchelli (56–59 mm); in E. emlongi , it is approximately 70 mm, and in E. barnesi it measures 59 mm (measurements approximated from Berta 1991: tables 2 and 3). Within Enaliarctos , loss of upper postcanine labial cingulum is shared by E. emlongi , E. mitchelli , and E. tedfordi , with E. emlongi showing loss of the labial cingulum in both the upper and lower dentition ( Berta 1991). With the exception of Enaliarctos mealsi ( Berta and Ray 1990) , postcrania are sporadically reported amongst enaliarctine pinnipeds. The transverse process of the atlas is subrectangular, unlike the rounded margin in Pteronarctos ( Berta 1994b) . The axis shares an anteroposteriorly elongate, fan-shaped neural spine with E. mealsi . Thoracic vertebrae, ribs, and sternebrae do not differ from E. mealsi except in size. Body length estimation using mandibular equations from Churchill et al. (2015) consistently overestimated the body size (2–3 m) of Enaliarctos sp. (UWBM 89114). Mandibular measurements are approximately 67% the size of E. emlongi , which Churchill et al. (2015) estimated at 1.7 m in length; simple scaling suggests that Enaliarctos sp. was 1.1 m in length, similar in length to the extant Baikal seal ( Pusa sibirica ) and rivalling Eotaria spp. as the smallest pinnipedimorph of all time. Though UWBM 89114 is much younger chronologically than all reported species of Enaliarctos , its small size may suggest affinities with E. mitchelli , the youngest named species of Enaliarctos , recorded from the approximate level of the Nye Mudstone/Astoria Formation contact ( Berta 1994b), and dated to 24–19.1 Ma ( Prothero et al. 2001a). Owing to the stratigraphic separation between UWBM 89114 and the referred cranium of E. mitchelli from the Nye/Astoria contact, we conservatively identify this specimen as Enaliarctos sp.

Figures

Fig. 6. Mandibles and vertebrae of pinniped mammal Enaliarctos sp. (UWBM 89114) from Moolack Beach, Lincoln County, Oregon, Astoria Formation, late early Miocene. Two sides of the block (A, B). Photographs (A1, B1), explanatory drawings (A2, B2). Abbreviations: CV, cervical vertebra; TV, thoracic vertebra.

Fig. 2. Stratigraphic columns of Oregon (A) and California (B) localities. Columns based on Snavely et al. (1964) and Phillips et al. (1976); paleomagnetic and K/Ar dates from Prothero et al. (2001a, b) and Phillips et al. (1976). In the interest of clarity non-enaliarctine marine carnivores are omitted. Modified from Moore (1963) and Phillips et al. (1976). Abbreviations: FAD, first appearance datum; LAD, last appearance datum; t, type locality/horizon.