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1 This is a postprint that has been peer reviewed and published in Biology Letters. The 2 final, published version of this article is available online. Please check the final 3 publication record for the latest revisions to this article. 4 5 Boessenecker, R.W., and M. Churchill, and J.H. Geisler. 2015. The oldest known fur 6 seal. Biology Letters 11:2:20140835 doi: 10.1098/rsbl.2014.0835 1 7 The oldest known fur seal 8 9 Robert W. Boessenecker 1,2 10 Morgan Churchill 3,4 11 1Department of Geology, University of Otago, Dunedin, New Zealand 9010 12 2Museum of Paleontology, University of California, Berkeley, California, U.S.A. 13 94720 14 3Department of Geology and Geophysics, University of Wyoming, Laramie, 15 Wyoming, U.S.A. 16 4Program in Ecology, University of Wyoming, Laramie, Wyoming, U.S.A. 17 18 Abstract 19 The poorly known fossil record of fur seals and sea lions (Otariidae) does not reflect 20 their current diversity and widespread abundance. This limited fossil record contrasts 21 with the more complete fossil records of other pinnipeds such as walruses 22 (Odobenidae). The oldest known otariids appear 5-6 Ma after the earliest odobenids, 23 and the remarkably derived craniodental morphology of otariids offers few clues to 24 their early evolutionary history and phylogenetic affinities among pinnipeds. We 25 report a new otariid, Eotaria crypta , from the early middle Miocene “Topanga” 26 Formation (15-17.5 Ma) of southern California, represented by a partial mandible 27 with well-preserved dentition. Eotaria crypta is geochronologically intermediate 28 between “enaliarctine” stem pinnipedimorphs (16.6-27 Ma) and previously described 29 otariid fossils (7.3-12.5 Ma), as well as morphologically intermediate by retaining an 30 M2 and a reduced M 1 metaconid cusp and lacking P 2-4 metaconid cusps. Eotaria 2 31 crypta eliminates the otariid ghost lineage and confirms that otariids evolved from an 32 “enaliarctine”-like ancestor. 33 34 Keywords: Otariidae, Miocene, Pinnipedia, North Pacific 35 36 1. Introduction 37 Modern fur seals and sea lions (Otariidae) are a clade of pinnipeds characterized by 38 shelf-like supraorbital processes on the skull, loss of M 2, and a simplified dentition [1, 39 2]. Although most extant otariids (11 of 16 species) are restricted to the southern 40 hemisphere, prior fossil evidence indicates a North Pacific center of origin for the 41 clade [2-4]. Other crown pinnipeds, including their sister group Odobenidae 42 (walruses), are well-represented within middle Miocene assemblages worldwide [3, 5]. 43 In contrast, otariids first appear in the early late Miocene [4, 6] and a 5-6 Ma gap 44 exists between the oldest described otariids and the oldest odobenids [4, 6, 7], 45 indicating a significant ghost lineage for Otariidae. A century of extensive sampling 46 and study of middle Miocene marine vertebrate assemblages from the Pacific coast of 47 North America has otherwise failed to unearth other fossils of true otariids, including 48 the robustly sampled Sharktooth Hill Bonebed. Kohno [8] remarked upon the rarity of 49 middle Miocene otariids, and hypothesized that the earliest otariids may have been 50 primarily pelagic in distribution, only rarely straying into shallow marine coastal 51 regions where preservation potential is higher. 52 Otariids are diagnosed by few cranial and postcranial synapomorphies and 53 many simplified (or lost) dental features, in concert with primitively retained 54 postcranial features [9]. The morphological conservatism of fossil and extant otariids 55 hinders interpretation of their early evolution. We report a new genus and species of 3 56 stem otariid, Eotaria crypta , based on a partial mandible with well-preserved 57 dentition (Fig. 1) from the early middle Miocene “Topanga” Formation of southern 58 California. Eotaria is morphologically intermediate between previously described 59 fossil otariids and stem pinnipedimorphs, thereby filling a morphological gap in our 60 understanding of pinniped evolution and elucidating the early dental evolution of 61 Otariidae. Eotaria is also geochronologically intermediate and eliminates the otariid 62 ghost lineage. 63 64 2. Material and Methods 65 The new fossil resides in collections of the John D. Cooper Archaeological and 66 Paleontological Center (Orange County Paleontological Collection; OCPC). The 67 holotype specimen (OCPC 5710) was collected from the “Topanga” Formation in 68 Mission Viejo, Orange County, California, which has produced marine birds, other 69 pinnipeds (Allodesmus sp., Pelagiarctos sp.), and dolphins (Kentriodon sp., cf. 70 Zarhinocetus errabundus ) [10, 11]. Age determinations for the “Topanga” Formation 71 include a 15.8 ± 1.3 Ma K/Ar date from andesite near the base of the Paulerino 72 Member of the “Topanga” Formation to the west in the San Joaquin Hills [12] and 73 land mammals from Oso Reservoir [13] indicating a late Hemingfordian (15.9-17.5 74 Ma) to Barstovian (12.5-15.9 Ma) Land Mammal Age [14]. The finest local age 75 control is provided by foraminifera of the Relizian and lower Luisian zones (Raschke 76 1984) which indicate an age of 14.9-17.1 Ma [15] for the “Topanga” Formation in 77 Orange County. 78 To determine phylogenetic placement of Eotaria crypta , we performed a 79 phylogenetic analysis of 115 morphological characters (modified from earlier studies 80 [2, 9, 10, 16] and including 3 novel characters) coded for 23 taxa representing all 4 81 families and all contemporary pinnipeds (Supplementary Information). Character 82 sampling was focused on those characters most useful for resolving the phylogenetic 83 relationships of stem otariids and early pinnipeds, and coding focused on male 84 specimens to minimize effects of sexual dimorphism. A polymorphic character coding 85 method was used to accommodate intra-taxon character variation [17]. We also 86 employed polymorphic coding [17] to accommodate widespread individual variation 87 in pinnipeds [2]. Phylogenetic analyses were carried out in TnT [18], using 10,000 88 replicates with sectorial and tree-fusing options and with equal and implied weighting 89 (K=2-6); analyses utilizing both a constrained and unconstrained topology were 90 executed (Appendix). 91 92 3. Systematic Palaeontology 93 94 Eotaria crypta gen. et sp. nov. 95 Etymology. The generic name is from the Greek Otaria , the name of the type genus 96 of the family Otariidae and referring to the diminutive external ear of sea lions, plus 97 the Greek eos, meaning dawn and referring to the early age of this new genus. The 98 species name is from the Greek kryptos , meaning hidden, and referring to the rarity of 99 middle Miocene otariids. 100 Holotype. OCPC 5710, a partial right mandible including P2-4, M 1, and M 2 alveolus. 101 Type Locality and Horizon. Early middle Miocene (Burdigalian-Langhian) 102 “Topanga” Formation, Mission Viejo, Orange County, California. 103 Diagnosis. A diminutive otariid that differs from all other members of the family in 104 combined possession of these features: M 1 anteroposteriorly longer than premolars, 105 reduced metaconid cusp present, and a reduced M 2 present. 5 106 107 4. Morphology and Phylogeny 108 The ramus is transversely narrow and tabular in shape with a shallow masseteric fossa, 109 a minute crest-like genial tuberosity positioned below P 2, and several small mental 110 foramina; a distinct ventral crest for digastric insertion is absent. P 2-M1 are double- 111 rooted, while the M 2 bears a single small, circular alveolus. P 2-P4 crowns are high- 112 crowned, morphologically similar to one another, lanceolate, and dominated by the 113 protoconid. The paraconid and hypoconid are developed as minute anterior and 114 posterior accessory cusps (respectively). The hypoconid is almost absent in P 2. The 115 M1 is anteroposteriorly longer than P2, but the protoconid is not as high; it bears a 116 similar paraconid, and a more strongly developed and posterodorsally oriented 117 hypoconid. A vestigial metaconid cusp is present on the basal heel of the posterior 118 crista of the M 1 protoconid; this cusp is absent on premolars. All teeth bear a smooth 119 crest-like lingual cingulum. The mandible of Eotaria crypta is relatively small and 120 comparable in size to mandibles of adult Arctocephalus philippii and the extinct 121 otariid Pithanotaria starri . Eotaria crypta differs from all other otariids (extinct and 122 extant) in primitively retaining a M2 and reduced metaconid cusp on M 1 . 123 Constrained and non-constrained phylogenetic analyses produced identical 124 trees, with Eotaria crypta recovered as the sister taxon to a clade comprising all other 125 otariids. (Fig. 2). One most parsimonious tree was recovered (Retention index (RI) = 126 0.56, Consistency Index (CI) = 0.45, tree length = 47.69). A K value of 3 for implied 127 weighting was generally found to be the most optimal weighting scheme; variation in 128 K generally didn’t affect the placement of Eotaria crypta , but did influence odobenid 129 monophyly. Weaker weighting schemes recovered Odobenidae as paraphyletic, with 130 Odobenus embedded within crown Otariidae, and the remaining walruses reduced to 6 131 an assemblage of stem taxa outside of a clade comprising Phocidae, Desmatophocidae, 132 and Otariidae. 133 A clade comprising Eotaria crypta and all other otariids was supported by two 134 unambiguous synapomorphies: postcanine crowns which are transversely narrow, 135 equidimensional, and anteroposteriorly shorter than high (character 74); and 136 postcanine teeth with reduced metaconid cusp and concave posterior margin of the 137 protoconid (character 94) All later diverging otariids were united by the absence of 138 M2 (character 98). 139 140 4. Discussion 141 Dentition of Eotaria crypta is morphologically intermediate between “enaliarctine” 142 stem pinnipedimorphs (e.g. Enaliarctos , Pteronarctos ) and extant otariids. Eotaria 143 crypta lacks a trenchant paraconid cusp as in “enaliarctines,” but primitively retains a 144 metaconid cusp (albeit reduced and positioned basally), limited heterodonty of the 145 postcanines, and an M 2.