A new tuskless walrus from the Miocene of Orange County, California, with comments on the diversity and taxonomy of odobenids

Material examined

Aivukus cedrosensis Repenning & Tedford, 1977 (LACM 154671, cast of type); Dusignathus santacruzensis Kellogg, 1927 (LACM 1527, cast of type; LACM 3011, 4342); Dusignathus seftoni Deméré, 1994a (LACM 155310, cast of type; LACM 135545, cast of SDNHM 20801); Gomphotaria pugnax Barnes & Raschke, 1991 (LACM 105151, 121508; LC 7750); Imagotaria downsi Mitchell, 1968 (LACM 144453, cast of type; USNM 23858, 184060); Neotherium mirum Kellogg, 1931 (LACM 4319, 52172, 98147, 123000, 123002, 127697; UCMP 81665); Odobenidae Allen, 1880 gen. et sp. indet. (LACM 135920); Odobenus rosmarus (LACM 31336, 52376, 52423, 72561); Odobenus mandanoensis (NMNS 18911); Ontocetus Leidy 1859 sp. (LACM 150001, cast of SFMCV-0001); Pelagiarctos Barnes, 1988 sp. (SDNHM 131041); Pelagiarctos thomasi Barnes, 1988 (LACM 121501); Pliopedia pacifica Kellogg, 1921 (LACM 57055, cast of type); Pontolis magnus True, 1905 (LACM 101151, cast of type); Proneotherium Barnes in Kohno, Barnes & Hirota 1995 sp. (LACM 128412); Protodobenus japonicus Horikawa, 1995 (LACM 140726, cast of type); Prototaria planicephala Kohno, 1994 (LACM 134826, cast of type); Prototaria primigena Takeyama & Ozawa, 1984 (LACM 130432, cast of type); Valenictus imperialensis Mitchell, 1961 (LACM 3926).

Phylogenetic methods

We added Titanotaria orangensis to the 23 taxon, 90 character, data matrix of Boessenecker & Churchill (2013) with the addition of Archaeodobenus akamatsui Tanaka & Kohno, 2015 and character 91 from Tanaka & Kohno (2015). We analyzed the data set with all unordered characters as well as with some characters ordered. For the analysis with ordered characters, we identified 21 characters that have more than two-character states, form a plausible transformation series, and are phylogenetically informative for the Odobenidae. The ordered characters are 5, 9, 11, 33, 37, 51–53, 55, 56, 61, 62, 70, 71, 74, 76, 77, 80, 82, 83, and 86. For three characters (52, 71, 86) we changed the number applied to character states so that they could be easily analyzed as ordered (switched 0s and 1s). For character 5 we combine states 1 and 2 into state 1 “1 = foramina coalesced, 2 = trilobed pits” (in this case state 3 became state 2). The coding for character 4 matches that of the (Boessenecker & Churchill, 2013) matrix, which is different from that described by their character descriptions (in which the descriptions for states for 2 and 1 are reversed). Also, following their character guide text we change the coding of I. downsi from 0 to ? for this character 4. Character 17 included a state 3 for Erignathus and Monachus, but there is no state 3 described; we change this character to state 1 for these taxa. For I. downsi, we updated character 48 (from 0 to 1) based on Repenning & Tedford (1977; USNM 23858). For N. mirum we updated character 59 (from 0 to 1) based on Velez-Juarbe (in press). Character 61 was updated for G. pugnax (from 1 to 0) based on the type. Character 79 was coded as state 0 for Callorhinus ursinus (Linnaeus, 1758) and Allodesmus kernensis Kellogg, 1922 by Boessenecker & Churchill (2013), but is corrected here to 1. We include the character codings for P. magnus that were included in Boessenecker & Churchill (2013) and Tanaka & Kohno (2015) while noting that these character states have not yet been described.

We analyzed the data matrix (Data S1) in PAUP* 4.0a build 159 (Swofford, 2002) using a heuristic search algorithm with 1,000 stepwise additions. We assessed support for each node with a bootstrap analysis of 1,000 replicates with 100 addition sequence replicates and Bremer decay indices.

Taxonomy

The electronic version of this article in portable document format will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: LSIDurn:lsid:zoobank.org:pub:7826C97C-3994-4519-89F0-603E6135F3DB The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central, and CLOCKSS.

Richness curve methods

A time constrained richness curve was constructed to show the minimum, maximum count of known and inferred odobenid lineages (richness) present throughout their history, taking into account chronostratigraphic uncertainty and ghost lineages. For each taxon, we estimated a minimum and maximum possible age of occurrence by performing a literature review of chronostratigraphic data to build on estimates presented by recent studies (Boessenecker & Churchill, 2015; Barboza et al., 2017; Kloess & Parham, 2017; Velez-Juarbe, 2017; Boessenecker & Churchill, 2018). We exclude the recently described Nanodobenus arandai Velez-Juarbe & Salinas-Márquez, 2018, because it appeared late in the review process for this paper and is the least chronostratigraphically constrained odobenid taxon (15.7 and 9.2 Ma, 6.5 Ma range is more than double that of any other taxon known from just one site). Detailed explanations for the age ranges of all taxa are included in Appendix 1. We used these data and the phylogenetic trees from our preferred hypothesis to generate a stratigraphically-constrained cladogram which was used to count the minimum and maximum number of lineages at 0.1 Ma time slices (0.0–17.3 Ma, 173 time slices).

The maximum lineage count was determined by assuming that each taxon spanned from the oldest to the youngest possible age of the rock units that include referred specimens. Six taxa are known from multiple formations (I. downsi, D. santacruzensis, Ontocetus emmonsi Leidy, 1859, Pelagiarctos [incl. P. thomasi], P. pacifica, V. imperialensis) and so the maximum age range of both units was used for their broadest possible age range. The broadest possible age range of four presumably valid taxa that are not included in the cladistic analysis (Odobenus mandanoensis Tomida, 1989, P. pacifica, P. planicephala, V. imperialensis) were included in the maximum count. We also extended the maximum age range of the Valenictus lineage based on Boessenecker (2017).

The minimum lineage count was generated by assuming originations and extinctions within the possible range that gave the lowest possible number of lineages at any given time. For the most part this involved counting ghost lineages since, at any time within the actual estimate range, a taxon can be presumed to be extinct or unoriginated. However, two taxa are known from more than one formation that do not overlap in time (I. downsi, O. emmonsi) providing a “known age range” for those taxa that was always included in the minimum count.

Maximum and minimum lineage counts from the nine trees from the ordered character analysis that varied among the ingroup were compared, and the maximum and minimum age estimates for each 0.1 Ma bin among the trees was included in the curves. We did not include undescribed or insufficiently characterized specimens that have been mentioned in recent papers (Appendix 2). By excluding these specimens we bias our richness estimates downward and so comparisons with other studies must be evaluated in this context. Nevertheless, this exercise is useful as an exploration of other factors that influence estimates of odobenid richness, as well as an estimate from the more established part of the record that can be used as a baseline for future studies.

In addition to the maximum and minimum global curves, we also generated a curve based on maximum lineage counts from an analysis where all taxa from outside the east Pacific were pruned from the trees. The purpose of this curve is to demonstrate the relative contribution of the East Pacific sites to the fossil record of odobenids.

Cranium

The cranium (Figs. 3–7) is nearly complete and preserves diagnostic characters of the rostrum, frontal, parietal, zygomatic arches, palate, basicranium, and dentition. The skull also exhibits asymmetry, with the right side being more robust. This asymmetry is obvious when comparing the outline of the orbits and the relative dorsoventral thickness of the zygomatic arches. We attribute this asymmetry to a pathology on the left side of the skull, likely from a healed injury; a similar case was described in a modern pinniped by Howell (1925). Unless stated otherwise, the description of the cranium is based on the right side of the skull.

Rostrum—The rostrum (Figs. 3–7; Table 1) is anteroposteriorly elongate and expanded anterolaterally at about the level of the C1 roots. In lateral view, the premaxilla has a triangular outline, with its anterior end recurved and elevated slightly above the maxillary alveolar row. In dorsal view, the ascending processes of the premaxilla extend posteriorly creating short overlap between the nasal and maxilla (c. 3[1]). A prominent, knob-like, prenarial process is present on the anterodorsal tip of the premaxilla (c. 1[1]). The surface posterolateral to this process forms a shallow, oval concavity, dorsal to the space between the canine and incisors, and is interpreted as the area of origin for the m. lateralis nasi (Kastelein, Gerrits & Dubbeldam, 1991). The anterior narial opening is transversely compressed and dorsoventrally elliptical with thick lateral and ventral margins (c. 2[1]) (Fig. 3). Dorsally, the nasals are long (∼60% of the rostrum length), and their lateral edges are roughly parallel to each other with a posterior termination that forms a broad V-shaped frontal/nasal suture (c. 4[1]).

The maxilla is well preserved in dorsal and lateral aspect. In lateral view, the maxillary alveolar row is sinuous and swollen near the roots of the canines. The maxilla forms the anterior portion of the orbital wall and lacks a fossa muscularis (c. 14[1]); together with the frontal, the maxilla forms the antorbital process (c. 15[1]). In ventral view, the palatal margins of the maxilla are nearly parallel anteriorly, becoming more sinuous at the level of m1–2 (c. 10[0]). The palate is widest near the canines and transversely arched (c. 9[1]). The palatal width index (70) is greater than that of Proneotherium repenningi Barnes in Kohno, Barnes & Hirota 1995 (Deméré & Berta, 2001: 52–60), owing to its more parallel toothrows. The transverse palatal arch index of T. orangensis is greater than that of P. repenningi and N. mirum, becoming higher between P3–4 and again smaller at the level of M1–2 (Table 2; Deméré & Berta, 2001). The incisive foramina are largely obscured by sediment so their morphology is not discernible, but are located at about the level of the posterior half of the canines (c. 5[?]). The maxillo-palatine suture is fused; the palatines are elongated, fused along the midline, and posterolaterally expanded into the orbital region (c. 11[1]); their posterior margin is rounded.

Zygomatic Arch—The zygomatic arch is dorsoventrally broad, (Figs. 3–6) with a low, oval ventral tuberosity on the maxillary root (c. 7[1]) (Fig. 4). The dorsal and ventral struts of the maxillary root give the infraorbital foramen an elliptical outline. The infraorbital foramen is relatively small (c. 6[0]) with a maximum diameter of ∼14 mm, which is about the same diameter as in the much smaller skulls referred to I. downsi (USNM 23858 and USNM 184060; Repenning & Tedford, 1977). The jugal is dorsoventrally thick and has a relatively small triangular postorbital process (c. 18[0]), similar to that of D. seftoni and unlike the much larger process of Valenictus chulavistensis Deméré, 1994a. Anteriorly, the jugal-maxilla articulation is largely fused with the maxilla with an anterodorsal and anteroventral process, the latter not approaching the toothrow (c. 8[0]). Posteriorly, the zygomatic process of the jugal extends to the level just anterior to the preglenoid process on the right side, while is much shorter on the left (100 vs. 59 mm long). The zygomatic process of the squamosal is dorsally curved, relatively short and dorsoventrally thin relative to the jugal (c. 19[1]). The preglenoid process is relatively long, and the glenoid fossa is mediolaterally broad, deep, and is oriented anteroventrally (c. 26[0]). The squamosal fossa on the dorsal surface of the zygomatic root is relatively broad relative to the proportionately narrow fossa of I. downsi (e.g., USNM 184060), slopes anteroventrally (c. 25 [0]), and there is no indication of the presence of a transverse ridge (c. 27[0]).

Frontal—In dorsal view (Fig. 3), the interorbital bar is transversely narrow with parallel lateral margins that diverge anteriorly making the frontal widest anteriorly (c. 17[1]). Anteriorly, the frontals have a transversely concavo-convex contact with the maxilla, that continues along the lateral surface of the skull toward the antorbital region. The frontal lacks a supraorbital process (c. 16[1]). The temporal crests arise just posterior to the contact with the nasals, these then merge posteriorly form the anterior portion of the sagittal crest. In the orbito-temporal region the ventral margin of the frontal is dorsally convex and contacts the maxilla anteriorly and parietal posteriorly.

Orbit—The orbit (Figs. 3, 5 and 6) has an anterodorsal orbital wall that preserves a frontal-maxillary suture that bisects the antorbital process (c. 15[1]). A low ridge extends posteriorly from the antorbital process, forming the ventral edge of the orbit. A sediment filled orbital vacuity is located anteriorly in the orbital wall (c. 20[1]). The posterior margin of the orbital wall is formed by the orbitosphenoid and frontal. The orbitosphenoid is plate-like and extends anteriorly forming part of the optic foramen (c. 21[0]). The ventral half of the orbito-temporal region is dorsally convex and continues posteriorly to the pterygoid strut. The pterygoid strut is dorsoventrally thickened and transversely broad (c. 13[2]; Fig. 4). The hamular process of the pterygoid is small, transversely compressed, anteroposteriorly elongated, and projects posteroventrally (c. 12[0]).

Parietal—The parietal (Figs. 3 and 5) is fused with the frontals anteriorly and the squamosals laterally. The temporal surface is smoothly convex, with no visible pseudosylvian sulcus (c, 28[1]). In dorsal view, the parietals form most of the broad, boxlike braincase, which is roughly as wide transversely as long anteroposteriorly. The sagittal crest is prominent and elongate, extending from the frontal anteriorly and connecting with the nuchal crest posteriorly. The sagittal crest has a sinuous profile in lateral view, unlike the low posterodorsally sloping crest seen in other odobenids (e.g., I. downsi, N. mirum; Repenning & Tedford, 1977; Kohno, Barnes & Hirota, 1995). The nuchal crest is transversely wide and projects posterodorsally obscuring the occipital condyles (c. 32[0]). From the dorsal midline, the nuchal crest curves posterolaterally, then anteroventrally, reaching the mastoid process.

Basicranium—The basioccipital (Fig. 4) is broad and pentagonal in ventral view with anteriorly converging lateral margins and a prominent medial margin (c. 30[1]). The hypoglossal foramen is rounded and located anterior to the occipital condyle. The posterior lacerate foramen is rounded and located anterolateral to the hypoglossal foramen (c. 31[0]); anteromedially it merges with the carotid canal. The carotid canal is partially underlapped anteromedially by the tympanic bulla; both anterior and posterior openings are of similar diameter (∼9 mm) (c. 24[0]). The hyoid fossa is shallow (<10 mm) and relatively small, facing mainly posteroventrally; it is separated from the smaller (∼6 mm) stylomastoid foramen by a low ridge. The mastoid processes are large and broad (c. 33[1]), in contrast to the more delicate processes of N. mirum and early pinnipeds, while the paroccipital process is plate-like, posteriorly directed, and laterally continuous with the mastoid process (c. 34[3]). The tympanic bullae are well preserved, their ventral surface has a triangular outline and their ventral surface is shallowly convex, without any ornamentation. The ecto and entotympanic portions are indistinguishable from each other. There is no distinct posterior bullar process, as in the type of I. downsi (Mitchell, 1968). The stylomastoid foramen opens anterolaterally. The foramen ovale and alisphenoid canal are relatively small (∼14 mm diameter).

Occipital region—Posteriorly, the occipital surface is broadly concave, with a pentagonal outline. A thick external occipital protuberance extends ventrally from the convergence of the sagittal and nuchal crests to about 45 mm dorsal to the foramen magnum. The occipital condyles are large, robust, and form the margins of the circular foramen magnum. As with most of the skull, the occipital condyles are asymmetric, with the left condyle being noticeably smaller, and less prominent than the right one.

Mandible and dentition

OCPC 11141 preserves both left and right mandibles while LACM 160199 is the anterior part of a right mandible (Figs. 8 and 9). In dorsal view, the articulated mandibles form a nearly parallel arch (c. 44[0]) and lack a mandibular furrow and edentulous mandibular terminus (c. 40[0], 41[0]). The right mandible lacks preserved dentition with the exception of a fragment of the canine. The left mandible preserves the canine as well as p2–4. The horizontal ramus has a straight dorsal and a moderately sinuous ventral margins (c. 39[0], 45[1]). The genial tuberosity is located at the posteroventral margin of the symphysis at a level between p1–2, and extends well below the ventral margin of the horizontal ramus forming the deepest part of the horizontal ramus (c. 37[2], 42[0], 43[1]). The mandibular symphysis is unfused with a rugose symphyseal surface, and occupies less than 50% of the length of the horizontal ramus (c. 35[0], 36[0]). The anterior surface of the symphysis is smooth, and curves anterodorsally from the genial tuberosity (c. 38[0], 40[0]). Several small, anteriorly oriented, mental foramina are present between p1–3 and the ventral border of the ramus. The anterior margin of the mandibular foramen is directed anteroventrally (c. 48[0]).

The coronoid process has a triangular outline and is anteroposteriorly narrow relative to the length of the mandible (c. 50[0]), its anterior edge ascends posterodorsally at about 52° from the horizontal ramus. The masseteric fossa is shallow, as in I. downsi (USNM 23858), and extends to a point near the anterior end of the coronoid process. Medially, the coronoid process has a shallow mandibular temporal fossa. The mandibular condyle is elevated only slightly above the level of the tooth row (c. 47[0]). The digastric insertion is moderately enlarged but not to the degree observed in P. magnus (c. 46[0]; Boessenecker & Churchill, 2013). The angular process is reduced, forming a small medial shelf (c. 49[1]).

Upper dentition—The preserved upper dentition (Fig. 7) and alveoli indicate a dental formula of I1–3, C1, P1–4, M1–2. The teeth preserved include left and right I3, C1 and P3, and right P2; while the rest of the teeth are represented by their respective alveoli. The alveolar morphology of I1–2 indicate anteroventrally directed single-rooted teeth. Both left and right I3 are present, but heavily worn, they are single-rooted, long and slender (c. 51[0], 52[0]). In occlusal view, I3 are laterally compressed and elliptical in cross section. The left I3 retains a small portion of the crown on its mesial side that begins 25 mm below the alveolar margin and has a distal wear facet; the crown of the right I3 is completely worn.

The canines are conical, ventrally directed, robust, and proportionate in size with respect to c1 (c. 55[0]). The base of the crown is oval in cross section. The enamel is smooth, and thick; the distal wear facet is large and elliptical on the left canine, and relatively smaller on the right.

Only the right P2 and left and right P3 are preserved. The alveolar morphology of P1 indicates a single-rooted tooth. P2 has a single, bi-lobed root, (c. 72[0]) and the crown is bulbous, and heavily worn on its distal surface. The paracone is partially preserved but extremely worn and broken at the apex; the occlusal wear has obliterated all other cusps. P2 also preserves a reduced crenulated lingual cingulum and lacks a labial cingulum (c. 71[0]).

P3 is double-rooted with cylindrical anterior and posterior root lobes that are of equal width as the crowns (c. 64[0], 74[0]). The crowns on both left and right P3 are poorly preserved, however, the right P3 preserves relatively more enamel with visible cusps. As in P2, the crown bulbous and the paracone forms the apex of the crown. Tooth wear on the distal surface has obliterated all other cusps. The lingual cingulum is partially worn, but it still preserves some of the crenations; there is no labial cingulum.

Although P4 and M1 are missing, the alveolar morphology indicates they were double-rooted teeth with cylindrical anterior and posterior root lobes (c. 76[1], 77[1]). M2 was apparently much smaller than M1; the preserved alveoli indicate it was a double-rooted tooth (c. 80[0]).

Lower dentition—The left and right mandibles of OCPC 11141 (Fig. 10; Table 3) preserve c1 and p2–4, and c1, respectively; alveoli for p1, m1–2, for a tooth count of i?, c, p1–4, m1–2 (c. 53[?], 61[0]). The tooth row is relatively short making up less than 40% of the mandible length (c. 65[1]). The crowns have well-developed enamel (c. 62[0]), with wear surfaces on the apex and distal and mesial surfaces (c. 82[1]). The incisors are missing in both mandibles, and their alveolar morphology is obscured by sediment.

The lower canines are nearly as large as the upper ones, vertically oriented, and posteriorly curved (c. 56[0], 60[0]). The crown is conical, transversely compressed, with an oval cross section; the enamel is smooth and thick with a posterior crista (c. 57[0], 58[0]), and posterior wear facet formed by occlusion with the upper canines. The lingual and labial surfaces of the canine roots exhibits longitudinal sulci that terminate below the base of the crown, giving it a bilobed cross section (c. 59[1]) as in I. downsi, P. thomasi, and N. mirum (Mitchell, 1968; Barnes, 1988; Velez-Juarbe, in press).

The first premolar (p1) is missing on the type and referred mandibles, however, the alveoli indicate a single, cylindrical root. The second through fourth premolars (p2–4) are double-rooted or bilobed. Their crowns are bulbous with oval outline (c. 63[1]) and are worn on their mesial and distal surfaces. Premolar 1 has a prominent protoconid cusp; mesiolingual wear preclude us from determining if there were other cusps (c. 68[?]); distally, the crown is worn but there are remnants of a small metaconid (c. 70[0]). Lingually, a crenulated cingulum is present (c. 67[1]), while distolabially there is an incipient cingulum.

The left p3 is double-rooted with divergent and bulbous anterior and posterior root lobes that result in a root as wide as the crown (Character 64[0], 73[0]). The crown is bulbous and as in p2, the crown has a prominent protoconid cusp, the wear facets have nearly obliterated other cusps. The lingual cingulum is worn mesially and distally, but it was prominent with small crenulations.

The left p4 resembles p3 and is double-rooted with cylindrical anterior and posterior root lobes, but which are not as wide as the crown. Although the crown is damaged on its labial side, the lingual surface is well preserved; the crown is bulbous with small mesial and distal wear facets; with a prominent, crenulated lingual cingulum that borders a small talonid basin (c. 69[1]), prominent protoconid and paraconid cusps [c. 66 [0]).

The molars are not preserved in any of the mandibles. But, the alveolar morphology suggests m1 has double-rooted cylindrical roots while m2 had a single, oval root (c. 78[0], 79[0], 81[0]).

Postcrania

The holotype includes an associated, mostly complete postcranial skeleton. However, here we describe the baculum, atlas, and phylogenetically informative postcranial elements (Figs. 10–12; Tables 4–7), which include the atlas, axis, humerus, radius, scapholunar, metacarpal 1, astragalus, calcaneum, and entocuneiform.

Atlas—The atlas (Figs. 11A–11D) is virtually complete, missing only a fragment of the right transverse process. The anterior articular surface is shallowly concave transversely and dorsoventrally, forming a nearly continuous surface ventrally. The dorsal arch has a low midline crest dorsally. The neural canal is round in outline. The vertebrarterial foramina are round (diam. ∼10 mm) and open laterally. The transverse processes are arcuate, flange-like and oriented posteroventrally. The transverse foramina open anteriorly within a shallow fossa at the base of the transverse processes; posteriorly they open anteromedially on the transverse process. The posterior articular surfaces are shallowly concave dorsoventrally and are smaller than their anterior counterparts. The articular surface for the odontoid process is shallowly convex and faces dorsally.

Axis—The axis (Figs. 11E–11H) is nearly complete, missing only the distal end of the left transverse process. The odontoid process is cylindrical and forms a continuous surface with the anterior articular surfaces, which are shallowly convex. Proximally, the dorsal surface of the odontoid forms a low keel, which continues posteriorly into the floor of the neural canal. The neural canal is oval to subtriangular in outline. The transverse processes are oriented posteroventrally and are pierced proximally by the transverse foramina. The postzygapophyses are oriented posteriorly with relatively large mammilary processes. The neural spine is tall and slopes posterodorsally with a blade-like outline in lateral view (c. 91[1]), similar to the condition of A. akamatsui (Tanaka & Kohno, 2015). The dorsal surface of the neural spine is flat and transversely expanded. The posterior surface of the centrum is shallowly convex with a rounded outline.

Baculum—The baculum (Figs. 13J–13L) is nearly cylindrical, dorsally arched, and elliptical in cross-section. The ventral surface is nearly flat, while dorsally it is rounded. The proximal end is bulbous with a rough surface, while distally its diameter becomes smaller toward the distal end. The distal end is anteriorly convex and dorsoventrally expanded in the form of dorsal and ventral knobs. The baculum of T. orangensis is 10% of the estimated body length (34.2 cm/est body length 331 cm). This is considerably smaller than that of O. rosmarus, which has the relatively largest baculum among pinnipeds, with its length being equivalent to around 18% of the body length (Fay, 1982; Miller, 2018).

Humerus—The left humerus is laterally bowed (Figs. 12A–12C; Tables 5 and 6), and relatively robust, with length/least diameter and proximal width/greatest length ratios between those of Valenictus spp. (more robust/stocky) and those of the more slender D. seftoni and I. downsi (Repenning & Tedford, 1977; Deméré, 1994a; Table 6). The greater tuberosity is prominent extending proximally beyond the level of the head as in I. downsi, G. pugnax, and D. seftoni (Mitchell, 1968; Repenning & Tedford, 1977; Barnes & Raschke, 1991; Deméré, 1994a). The humeral head is large and rounded. The lesser tuberosity is knob-like and does not extend proximally beyond the level of the head, thus differing from the condition of I. downsi and being more similar to D. seftoni (Mitchell, 1968; Repenning & Tedford, 1977; Deméré, 1994a). In lateral or medial view the deltopectoral crest is moderately prominent, resembling the condition observed in P. pacifica and Valenictus spp., and differing from the more prominent crest of I. downsi (Mitchell, 1961; Repenning & Tedford, 1977; Deméré, 1994a). The deltopectoral crest gradually tapers distally approaching the radial fossa, and curves toward the medial lip of the trochlea. The deltoid tubercle is on the pectoral crest (c. 83[0]). The distal articular surface has a saddle-like outline in posterior or anterior views (Figs. 12A and 12B). The medial lip of the trochlea is prominent with a diameter greater than that of the capitulum (c. 84[1]). The coronoid and radial fossae are present, with the former being deeper and wider. The medial epicondyle is prominent, with an oval outline as in I. downsi and D. seftoni, and unlike the more prominent medial epicondyle that distinguishes Valenictus spp. (Mitchell, 1961, 1968; Repenning & Tedford, 1977; Deméré, 1994a).

Radius—The radius (Figs. 12D and 12E) is stout with its distal end anteroposteriorly expanded with a large radial process (c. 85[2]); it is similar in size to one of the referred radii of I. downsi (Repenning & Tedford, 1977: table 9). Proximally, the head of the radius is rounded and with a shallowly concave articular surface. The proximal end of the diaphysis is transversely narrow relative to the distal end, and oval in cross section. The pronator ridge is prominent and located about midlength on the shaft. Distally, the radial crest is rounded in lateral view, and together with the radial process, it forms the widest part of the radius (Table 7). The radial process is not as displaced medially as that of Imagotaria. Distally, the anterolateral surface is marked by relatively deep grooves for the M. extensor metacarpi pollicis and M. extensor digitorum.

Scapholunar—Proximally (Fig. 11E), the radial articular surface is anteroposteriorly convex with a subrectangular outline. Distally (Fig. 11F), the articular surface for the trapezium and trapezoid is oval and deeply concave transversely. The unciform articular surface is elongated and shallowly concave and is continuous posteriorly with the cuneiform articular surface. The magnum articular surface is deeply concave, forming a distinct pocket (c. 87[1]) as in other odobenids (Repenning & Tedford, 1977; Deméré, 1994b).

Metacarpal 1—The first metacarpal (Fig. 11) is elongated, curved laterally, and slender, resembling that of I. downsi (Repenning & Tedford, 1977). The proximal surface of the bone is smoothly concave transversely and dorsoventrally convex. Proximally, the dorsal surface has a shallow oval depression for the insertion of M. extensor pollicis (c. 86[1]). The distal articular trochlea has a rounded outline.

Astragalus—The head of the astragalus (Fig. 12) is prominent and dorsoventrally flattened; the neck is well defined and the articular surface is dorsoventrally and transversely convex. Ventrally, the navicular facet is continuous with a broad, oval (longer than wide) sustentacular facet, markedly differing from the relatively narrow navicular facet of Valenictus (Deméré, 1994a; Boessenecker, 2017). The sustentacular facet is separated from the ectal facet by a narrow astragalar sulcus. The ectal facet is concave and lunate, and separated from the oval, fibular facet by a low ridge. Dorsally, the navicular facet bears a median notch, and is separated from the trochlea by a groove. The trochlea is rectangular in outline with a short, shallow intertrochlear pit along its distal edge; the articular surface is convex and divided by a shallow anteroposteriorly-oriented groove. The lateral trochlear ridge is longer than the medial trochlear ridge. The calcaneal process is short and rounded (c. 88[1]). The medial plantar tuberosity is rounded and more robust and prominent than the nearly inconspicuous lateral plantar tuberosity.

Calcaneum—The calcaneal tuber is elongated with a prominent medial process (= internal tuberosity of Repenning & Tedford (1977)) (c. 89[1]; Figs. 13D and 13E), as in most odobenids and Allodesmus spp. (Mitchell, 1966; Deméré, 1994b; Tonomori et al., 2018). The posterior articular surface is convex and broad. The sustentaculum tali is robust, extending as far medially as the medial process of the calcaneal tuber. The ectal face is anteroposteriorly convex, with an oval outline, and oriented obliquely to the long axis of the bone. The sustentacular facet is flat posteriorly, becoming concave distally; a secondary shelf in the sustentaculum is missing, as in I. downsi and V. chulavistensis (Repenning & Tedford, 1977; Deméré, 1994a). The groove for the peroneus longus tendon extends along the distoventral surface of the sustentaculum tali. Distally, the calcaneum is mediolaterally broader than proximally, similar to the condition in I. downsi, but unlike the proportionately broader distal end of the calcanei of V. chulavistensis (Repenning & Tedford, 1977; Deméré, 1994a). On the distal surface, the cuboid articular surface is broad, subrounded, and makes up most of the distal surface of the calcaneum.

Entocuneiform—The entocuneiform (Figs. 13F–13H) has an irregularly pentagonal outline, and is mediolaterally wide and dorsoventrally flattened (Table 4). In proximal (posterior) view the entocuneiform is divided into two facets. The medial facet for the navicular is shallowly convex, L-shaped and oriented proximally, while the mesocuneiform facet is oriented proximolaterally, both forming a broad angle (c. 90[1]). Proximally, the medial surface has a suboval pisiform facet. The anterolateral surface of the bone has a ventrolaterally oriented oval facet for metatarsal II. Ventrally, there is a rounded, prominent, plantar process on the proximomedial corner. Anteriorly, the articular surface for metatarsal I is irregularly oval, with a dorsomedial depression; the surface is transversely concave and dorsoventrally convex.

Phylogenetic results

Our phylogenetic analysis without ordered characters recovered 27 equally parsimonious trees of 265 steps. An analysis with ordered characters, recovered 27 equally parsimonious trees of 274 steps (Fig. 13). Both analyses recover nine trees that vary within the ingroup (Odobenidae). In the strict consensus trees for both analyses, the topology for the ingroup is almost entirely pectinate except for three polytomies that can be resolved to form exclusive sister taxa: (1) A polytomy that includes N. mirum, Kamtschatarctos sinelnikovae Dubrovo, 1981, and odobenids more closely related to O. rosmarus. The shortest trees include N. mirum and K. sinelnikovae as sister taxa or paraphyletic with either taxon as more basal; (2) A polytomy that includes D. seftoni, D. santacruzensis, and odobenids more closely related to O. rosmarus. The shortest trees include a monophyletic Dusignathus Kellogg, 1927 and a paraphyletic Dusignathus, with D. santacruzensis as sister to the Odobeninae. In the unordered analysis, G. pugnax is always sister to the Dusignathus polytomy, but the shortest trees from the ordered analysis also include G. pugnax as more closely related to O. rosmarus than a monophyletic Dusignathus. (3) A polytomy that includes P. japonicus, O. emmonsi, and odobenids more closely related to O. rosmarus. The shortest trees include P. japonicus as either the sister taxon of O. emmonsi or the sister taxon of a clade that includes O. emmonsi and odobenids more closely related to O. rosmarus. Titanotaria orangensis is found to be sister to a clade that includes I. downsi and all other more crownward odobenids, albeit with low support. Titanotaria orangensis, Pelagiarctos sp., and I. downsi represent a grade of phenetically similar taxa at the base of the clade that includes P. magnus with Dusignathus, G. pugnax, and Odobeninae.

Phylogenetic taxonomy

The first study to provide phylogenetic definitions for Odobenidae and its subclades was Deméré (1994b), which demonstrated the paraphyly of the “Imagotariinae” Mitchell, 1968, and provided phylogenetic definitions for Dusignathinae Mitchell, 1968, Odobenidae Allen, 1880, Odobeninae Mitchell, 1968, and the new taxon Odobenini Deméré, 1994b. Since that time, all workers have accepted the non-monophyly of the “Imagotariinae,” although this term is used as an informal grade by some authors (Boessenecker & Churchill, 2013). Given that I. dowsni is more closely related to P. magnus and other late Miocene odobenids, the continued referral of the most basal middle Miocene forms (with more plesiomorphic characters) as imagotariines is misleading. Some studies have chosen to refer to more basal forms as “archaic odobenids,” although the boundary of this informal grouping can differ even within a single paper (see Tanaka & Kohno, 2015: fig. 11 vs. text).

The definition of Dusignathinae Mitchell, 1968, proposed by Deméré (1994b) is more exclusive than that used by Repenning & Tedford (1977) who included all non-odobenines in this subfamily. Deméré’s (1994b: 103) definition was based on “the most recent ancestor of Pontolis and Dusignathus,” a grouping that has been found to be paraphyletic by all later analyses (Kohno, 2006; Boessenecker & Churchill, 2013; Tanaka & Kohno, 2015; this study). Kohno (2006) proposed an amended phylogenetic definition for the Dusignathinae as “a clade containing the most recent common ancestor of Dusignathus and Gomphotaria and all of its descendants.” Since that time the concept of the Dusignathinae has been restricted to those two genera, but even the amended definition becomes problematic in light of recent analyses that show that it is paraphyletic and even question the monophyly of the genus Dusignathus (Boessenecker & Churchill, 2013; Tanaka & Kohno, 2015; this study). In light of this history, we do not propose a phylogenetic definition for Dusignathinae and question the further use of this name as the only way to ensure its monphyly is to restrict it to D. santacruzensis.

Setting aside “imagotariines” and “dusignathines,” there are three groupings (Odobenidae, Odobeninae, and Odobenini) that have crystallized in their usage over the past 14–50 years and are consistent with the phylogenetic definitions proposed by Deméré (1994b). We propose slightly amended definitions for two of these names by making them branch-based instead of node-based. We also propose a new name for an often-discussed and well-supported clade of odobenids. The phylogenetic names used in this study are listed below.

Neodobenia, new clade name

Definition—The least inclusive clade that includes Odobenus rosmarus (Linnaeus, 1758), Dusignathus seftoni Deméré, 1994a, and Gomphotaria pugnax Barnes & Raschke, 1991.

Reference phylogeny—Figure 14 of this study.

Etymology—Neo- for “new;” -odobenia for Odobenidae.

Comments—One of the best supported nodes of our analysis unites the clade Odobeninae (see below) with the “dusignathines” (in the recent usage sense of a grade or clade that includes G. pugnax and Dusignathus spp., but not P. magnus). We do not use D. santacruzensis to define Neodobenia because it is poorly known, and in any case is always either sister to D. seftoni or else more closely related to O. rosmarus than D. seftoni in all analyses. Because this clade unites two important groupings of odobenids, and is evidently worth talking about (see mentions in Deméré, 1994b; Kohno, 2006; Boessenecker & Churchill, 2013), we feel it is useful to name. Neodobenia is diagnosed by single-rooted, simplified teeth. Most neodobenians are thought to have tusks except for P. japonicus and possibly A. cedrosensis (broken canines). We refer to all taxa on the stem of Neodobenia, informally, as “basal odobenids.”

Odobenidae Allen, 1880, converted clade name

Definition—All taxa more closely related to Odobenus rosmarus (Linnaeus, 1758) than to Phoca vitulina Linnaeus, 1758 or Otaria byronia (De Blainville, 1820).

Reference phylogeny—Figure 14 of this study.

Comments—The first phylogenetic definition for Odobenidae was provided by Deméré (1994b: 102): “The Odobenidae are defined as the monophyletic group containing the most recent common ancestor of Neotherium and Odobenus and all of its descendants.” Later, Berta, Sumich & Kovacs (2006: 496) provided an amended definition for the Odobenidae: “Monophyletic group containing the common ancestor of stem odobenids Imagotaria, Kamtschatarctos, Neotherium, Proneotherium, Pseudotaria, Prototaria, Pelagiarctos, and crown odobenids and all of their descendants including Aivukus, Dusignathus, Gomphotaria, Ontocetus, Pliopedia, Pontolis, Protodobenus, and Valenictus and Odobenus.” This definition is problematic because it misapplies the crown clade concept (meant to be defined by most recent common ancestor of extant species) to apply to a clade name that is defined by extinct taxa. Both the Deméré (1994b) and Berta, Sumich & Kovacs (2006) definitions have the same content as a total clade name (Cantino & De Queiroz, 2014) for the extant Odobenus rosmarus. Therefore, we propose a branch-based Odobenidae, which is consistent with recent usage and complements the construction of its sister taxon, the branch-based Pan-Otariidae Gill, 1866 (Velez-Juarbe, 2017).

Odobeninae Mitchell, 1968, converted clade name

Definition—All taxa more closely related to Odobenus rosmarus (Linnaeus, 1758) than to Dusignathus santacruzensis Kellogg, 1927, Dusignathus seftoni Deméré, 1994a, or Gomphotaria pugnax Barnes & Raschke, 1991.

Reference phylogeny—Figure 14 of this study.

Comments—The first phylogenetic definition for Odobeninae was provided by Deméré (1994b: 103): “The Odobeninae are here defined as the monophyletic group containing the most recent common ancestor of Aivukus and Odobenus and all of its descendants.” This definition includes all those species more closely related to O. rosmarus than to “dusignathines” and so we propose a branch-based Odobeninae.

Odobenini Deméré, 1994b

Definition—”The monophyletic group containing the most recent common ancestor of Alachtherium [Ontocetus emmonsi] and Odobenus” (Deméré, 1994b: 104). The type species of Alachtherium is Alachtherium cretsii Du Bus, 1867, which was later referred to Ontocetus emmonsi Leidy, 1859, by Kohno & Ray (2008).

Reference phylogeny—Deméré (1994b: fig. 1).

Comments—Deméré (1994b) included all odobenines except Aivukus cedrosensis Repenning & Tedford, 1977 in his node-based definition of Odobenini. We do not propose a branch-based or otherwise emended phylogenetic definition for this name because it is less clear that this would capture the original intent (at the time it was coined, Odobenini was diagnosed by 14 unequivocal synapomorphies) and usage. Shortly after Deméré (1994b) was published, Protodobenus japonicus Horikawa, 1995, was described from the early Pliocene of Japan (published in parallel, neither paper cited each other). Protodobenus japonicus has been placed in the Odobenini by some later authors (Deméré, Berta & Adam, 2003) and not others (Boessenecker & Churchill, 2013). Our phylogenetic analysis alternatively resolves this taxon as the sister to O. emmonsi within Odobenini or as the sister taxon to Odobenini.

Richness curve results

The results of the maximum and minimum lineage count are shown in Fig. 15 (top). The maximum lineage count shows peaks near the base of Odobenidae, Neodobenia, and Odobenini. The lineage count is as high as nine in the Langhian and eight in the Messinian and as low as three and four in the intervening stages (Serravallian and Tortonian); the period of low lineage counts is between ∼15 and 8 Ma. After the Miocene, the maximum linage count decreases with a precipitous drop at the end of the Pliocene.

The minimum lineage count reaches a high of four briefly in the Langhian and throughout the Zanclean. The difference between the maximum and minimum curves is almost entirely determined by stratigraphic uncertainty because maximum lineage counts among the 27 trees were identical for 107 of the 173 time slices (61%) and within one for the other 67 (39%).

The results of the maximum lineages count when all taxa from outside the East Pacific were pruned from the trees is shown in Fig. 15 (bottom). When compared to the maximum global curve, the difference between the two curves is the relative input of lineages from outside the East Pacific. After 10 Ma, the richness curve is dominated by East Pacific lineages as shown by the close tracking of the two curves.

Capistrano Formation, California, USA

In addition to G. pugnax and T. orangensis, at least two undescribed taxa of odobenids have been mentioned from the Capistrano Formation (Barboza et al., 2017; Velez-Juarbe, 2017; Boessenecker & Churchill, 2018). Barboza et al. (2017) and Velez-Juarbe (2017) both list LACM 118967, and the latter lists LACM 150922, as representative specimens of these undescribed taxa. Boessenecker & Churchill (2018: suppl. data) indicate that Deméré, Berta & Adam (2003) list P. magnus from the Capistrano Formation, but those data are not given in that paper.

Deguynos Formation, California, USA

Boessenecker & Churchill (2018: suppl. data) cite an abstract (Atterholt, Jefferson & Schachner, 2007) for the presence of a potential new species of Valenictus in the Deguynos Formation, the type strata of V. imperialensis.

Empire Formation, Oregon, USA

Besides P. magnus, the Empire Formation is known to have additional taxa, although they are currently undescribed. Boessenecker & Churchill (2018: suppl. data) cite Deméré (1994b), who lists USNM 335594 and 335599 as “Imagotaria sp. cf. I. downsi,” an abstract (Sha[h]bazi et al., 2016), and personal observations to account for two undescribed taxa.

Monterey Formation, California, USA

In addition to I. downsi, Velez-Juarbe (2017) lists two taxa from the Monterey Formation and provides two specimen numbers (LACM 123282 and 122444) as their representatives. Velez-Juarbe (2017) also mentions two specimens (LACM 4324, 17588) from the Valmonte Diatomite Member that he hypothesizes could be a new taxon. LACM 4324 was described by Lyon (1941) and referred to P. magnus.

Purisima Formation, California, USA

Boessenecker, Perry & Schmitt (2014) and Boessenecker (2017) list four species of odobenids from the basal Purisima Formation. Currently Dusignathus is the only one that is described (Kellogg, 1927; Boessenecker, 2013b, 2017).

San Diego Formation, California, USA

In addition to D. seftoni and V. chulavistensis, Boessenecker & Churchill (2018: suppl. data) cite Boessenecker (2013b) which provides a specimen (LACM 28432) and a preliminary diagnosis for a new taxon.

Santa Margarita Sandstone, California, USA

In addition to I. downsi, Boessenecker & Churchill (2018: suppl. data) list two taxa “Imagotaria n. sp. 1 and 2.” One of these new species is based on “Desmatophocine A” of Barnes (1972), UCMP 85197. This taxon is incorrectly listed as coming from the Round Mountain Silt by Boessenecker & Churchill (2018: table 9). The other undescribed taxon is based on material described by Repenning & Tedford (1977) and is most likely USNM 23858.

“Topanga” Formation, California, USA

In addition to Pelagiarctos sp., Velez-Juarbe (2017) mentions “Neotherium sp.” from the “Topanga” Formation of Orange County. The specimen number for this material is LACM 123713. Boessenecker & Churchill (2018: suppl. data) lists cf. Imagotaria citing personal observations.

Towsley Formation, California, USA

Boessenecker & Churchill (2018: suppl. data) cite an abstract (Berkoff & Barnes, 1988) that diagnoses a new species of Gomphotaria. Although Berkoff & Barnes (1988) report the age of this specimen as being 10–9 Ma, which was later expanded upon by Whitmore & Barnes (2008), parts of the Towsley are considered Late Miocene or even Pliocene by other authors (Squires, 1991; Yeats, Huftile & Stitt, 1994; 6.5–5.0 Ma).

Other Japanese Formations

Miyazaki et al. (1995, fig. 2) lists two odobenids in the late Messinian: “Imagotaria cf. downsi” and “Imagotariinae,” but only cites a reference for one of them (Imagotariinae, Nakagawa, 1994) which is missing from the literature cited section of that manuscript. We were given the full citation for the Nakagawa (1994) paper, as well as another (Aizu Fossil Research Group, 1985), during review, but were unable to evaluate them before resubmission.