The oldest known fur seal

Abstract

The poorly known fossil record of fur seals and sea lions (Otariidae) does not reflect their current diversity and widespread abundance. This limited fossil record contrasts with the more complete fossil records of other pinnipeds such as walruses (Odobenidae). The oldest known otariids appear 5–6 Ma after the earliest odobenids, and the remarkably derived craniodental morphology of otariids offers few clues to their early evolutionary history and phylogenetic affinities among pinnipeds. We report a new otariid, Eotaria crypta, from the lower middle Miocene ‘Topanga’ Formation (15–17.1 Ma) of southern California, represented by a partial mandible with well-preserved dentition. Eotaria crypta is geochronologically intermediate between ‘enaliarctine’ stem pinnipedimorphs (16.6–27 Ma) and previously described otariid fossils (7.3–12.5 Ma), as well as morphologically intermediate by retaining an M₂ and a reduced M₁ metaconid cusp and lacking P_2–4 metaconid cusps. Eotaria crypta eliminates the otariid ghost lineage and confirms that otariids evolved from an ‘enaliarctine’-like ancestor.

1. Introduction

Modern fur seals and sea lions (Otariidae) are a clade of pinnipeds characterized by shelf-like supraorbital processes on the skull, loss of M₂ and a simplified dentition [1,2]. Although most extant otariids (11 of 16 species) are restricted to the Southern Hemisphere, prior fossil evidence indicates a North Pacific centre of origin for the clade [2–4]. Other crown pinnipeds, including their sister group Odobenidae (walruses), are well-represented within middle Miocene assemblages worldwide [3,5]. By contrast, otariids first appear in the early late Miocene [4,6] and a 5–6 Ma gap exists between the oldest described otariids and the oldest odobenids [4,6,7], indicating a significant ghost lineage for Otariidae. A century of extensive sampling and study of middle Miocene marine vertebrate assemblages from the Pacific coast of North America has otherwise failed to unearth other fossils of true otariids, including the robustly sampled Sharktooth Hill Bonebed. Kohno [8] remarked upon the rarity of middle Miocene otariids and hypothesized that the earliest otariids may have been primarily pelagic in distribution, only rarely straying into shallow marine coastal regions where preservation potential is higher.

Otariids are diagnosed by few cranial and postcranial synapomorphies and many simplified (or lost) dental features, in concert with primitively retained postcranial features [9]. The morphological conservatism of fossil and extant otariids hinders interpretation of their early evolution. We report a new genus and species of stem otariid, Eotaria crypta, based on a partial mandible with well-preserved dentition (figure 1) from the lower middle Miocene ‘Topanga’ Formation of southern California. Eotaria is morphologically intermediate between previously described fossil otariids and stem pinnipedimorphs, thereby filling a morphological gap in our understanding of pinniped evolution and elucidating the early dental evolution of Otariidae. Eotaria is also geochronologically intermediate and eliminates the otariid ghost lineage.

Figure 1.

Holotype specimen of E. crypta, OCPC 5710 in (a) medial view, (b) lateral view and (c) dorsal view. (Online version in colour.)

2. Material and methods

The new fossil resides in collections of the John D. Cooper Archaeological and Paleontological Center (Orange County Paleontological Collection, OCPC). The holotype specimen (OCPC 5710) was collected from the ‘Topanga’ Formation in Mission Viejo, Orange County, California, which has produced marine birds, other pinnipeds (Allodesmus sp., Pelagiarctos sp.), and dolphins (Kentriodon sp., cf. Zarhinocetus errabundus) [10,11]. Age determinations for the ‘Topanga’ Formation include a 15.8 ± 1.3 Ma K : Ar date from andesite near the base of the Paulerino Member of the ‘Topanga’ Formation to the west in the San Joaquin Hills [12] and land mammals from Oso Reservoir [13] indicating a Late Hemingfordian (15.9–17.5 Ma) to Barstovian (12.5–15.9 Ma) Land Mammal Age [14]. The finest local age control is provided by foraminifera of the Relizian and Lower Luisian zones [15], which indicate an age of 14.9–17.1 Ma [16] for the ‘Topanga’ Formation in Orange County.

To determine phylogenetic placement of E. crypta, we performed a phylogenetic analysis of 115 morphological characters (modified from earlier studies [2,9,10,17] and including three novel characters) coded for 23 taxa representing all families and all contemporary pinnipeds (electronic supplementary material). Character sampling was focused on those characters most useful for resolving the phylogenetic relationships of stem otariids and early pinnipeds, and coding focused on male specimens to minimize effects of sexual dimorphism. A polymorphic character coding method was used to accommodate intra-taxon character variation [18]. We also employed polymorphic coding [18] to accommodate widespread individual variation in pinnipeds [2]. Phylogenetic analyses were carried out in TnT [19], using 10 000 replicates with sectorial and tree-fusing options and with equal and implied weighting (K = 2–6); analyses using both a constrained and unconstrained topology were executed (electronic supplementary material, appendix).

3. Systematic palaeontology

Eotaria crypta gen. et sp. nov.

Etymology. The generic name is from the Greek Otaria, the name of the type genus of the family Otariidae and referring to the diminutive external ear of sea lions, plus the Greek eos, meaning dawn and referring to the early age of this new genus. The species name is from the Greek kryptos, meaning hidden, and referring to the rarity of middle Miocene otariids.

Holotype. OCPC 5710, a partial right mandible including P_2–4, M₁ and M₂ alveolus.

Type locality and horizon. Lower middle Miocene (Burdigalian–Langhian) ‘Topanga’ Formation, Mission Viejo, Orange County, California.

Diagnosis. A diminutive otariid that differs from all other members of the family in combined possession of these features: M₁ anteroposteriorly longer than premolars, reduced metaconid cusp present and a reduced M₂ present.

4. Morphology and phylogeny

The ramus is transversely narrow and tabular in shape with a shallow masseteric fossa, a minute crest-like genial tuberosity positioned below P₂, and several small mental foramina; a distinct ventral crest for digastric insertion is absent. P₂–M₁ are double-rooted, while the M₂ bears a single small, circular alveolus. P₂–P₄ crowns are high-crowned, morphologically similar to one another, lanceolate, and dominated by the protoconid. The paraconid and hypoconid are developed as minute anterior and posterior accessory cusps (respectively). The hypoconid is almost absent in P₂. The M₁ is anteroposteriorly longer than P₂, but the protoconid is not as high; it bears a similar paraconid, and a more strongly developed and posterodorsally oriented hypoconid. A vestigial metaconid cusp is present on the basal heel of the posterior crista of the M₁ protoconid; this cusp is absent on premolars. All teeth bear a smooth crest-like lingual cingulum. The mandible of E. crypta is relatively small and comparable in size to mandibles of adult Arctocephalus philippii and the extinct otariid Pithanotaria starri. Eotaria crypta differs from all other otariids (extinct and extant) in primitively retaining an M₂ and reduced metaconid cusp on M₁.

Constrained and non-constrained phylogenetic analyses produced identical trees, with E. crypta recovered as the sister taxon to a clade comprising all other otariids (figure 2). One most parsimonious tree was recovered (retention index = 0.56, consistency index = 0.45, tree length = 47.69). A K-value of 3 for implied weighting was generally found to be the most optimal weighting scheme; variation in K generally did not affect the placement of E. crypta, but did influence odobenid monophyly. Weaker weighting schemes recovered Odobenidae as paraphyletic, with Odobenus embedded within crown Otariidae, and the remaining walruses reduced to an assemblage of stem taxa outside a clade comprising Phocidae, Desmatophocidae and Otariidae.

Figure 2.

Geochronologically calibrated cladogram depicting phylogenetic relationships of E. crypta. Support values include bootstrap support values derived from non-constraint analysis (left) and constraint analysis (right). Squares at nodes denote position of enforced clades defined in constraint analysis. For geochronologic ages of pinnipeds, see the electronic supplementary material. †Denotes extinct group.

A clade comprising E. crypta and all other otariids was supported by two unambiguous synapomorphies: postcanine crowns that are transversely narrow, equi-dimensional and anteroposteriorly shorter than high (character 74); and postcanine teeth with reduced metaconid cusp and concave posterior margin of the protoconid (character 94). All later diverging otariids were united by the absence of M₂ (character 98).

5. Discussion

Dentition of E. crypta is morphologically intermediate between ‘enaliarctine’ stem pinnipedimorphs (e.g. Enaliarctos and Pteronarctos) and extant otariids. Eotaria crypta lacks a trenchant paraconid cusp as in ‘enaliarctines’, but primitively retains a metaconid cusp (albeit reduced and positioned basally), limited heterodonty of the postcanines, and an M₂. As all later otariids (fossil and modern) are more dentally derived, E. crypta provides dental evidence for the derivation of otariids from an ‘enaliarctine’-like ancestor. Significantly, E. crypta is also geochronologically intermediate between late surviving ‘enaliarctines’ (e.g. Pteronarctos goedertae, Astoria Formation, OR, USA; 16.6–20.2 Ma) and the earliest published fossil otariids (Otariidae indet., Aoki Formation, Japan, 11.8–12.5 Ma [6]; Pithanotaria starri, Monterey Formation and Santa Margarita Sandstone, CA, USA, 7.3–10 Ma; see the electronic supplementary material). Discovery of E. crypta eliminates a 5–6 Ma ghost lineage for the Otariidae, formerly defined by the gap between Proneotherium repenningi (16.6–17.3 Ma [20]) and the Aoki Formation otariid (11.8–12.5 Ma [6]). Formerly, Barnes [7] hypothesized that the ‘enaliarctine’ Pteronarctos goedertae represented an early diverging otariid that filled the temporal gap (10–19.1 Ma) between Enaliarctos and Pithanotaria; however, cladistic analysis ([9,10,21]; this study) conclusively demonstrates that Pteronarctos lies outside crown Pinnipedia.

Although E. crypta convincingly demonstrates that otariids evolved within the North Pacific, it represents the entire published fossil record of middle Miocene otariids. Eotaria crypta was collected from the ‘Topanga’ Formation along the Pacific coast, while the more intensely sampled Sharktooth Hill Bonebed was deposited within a large restricted embayment in the San Joaquin Basin [22], provisionally supporting Kohno's [8] hypothesis that the earliest stem otariids were pelagic in distribution. Perhaps, adaptation to pelagic (rather than coastal) environments facilitated the early divergence (and later success) of otariids.

Author contributions

R.W.B. and M.C. designed the research, collected data and wrote the paper; M.C. conducted the phylogenetic analysis.

Funding statement

R.W.B. was supported by a University of Otago Doctoral Scholarship.

Conflict of interests

Authors have declared that no competing interests exist.