Boessenecker Robert W, Churchill Morgan
Department of Geology, University of Otago, Dunedin 9010, New Zealand Museum of Paleontology, University of California, Berkeley, CA 94720, USA
Department of Geology and Geophysics, University of Wyoming, Laramie, WY, USA Program in Ecology, University of Wyoming, Laramie, WY, USA.
Biol Lett. 2015 Feb;11(2):20140835. doi: 10.1098/rsbl.2014.0835.
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 P2-4 metaconid cusps. Eotaria crypta eliminates the otariid ghost lineage and confirms that otariids evolved from an 'enaliarctine'-like ancestor.
海狗和海狮（海狮科）鲜为人知的化石记录并未反映出它们目前的多样性和广泛分布的丰富程度。这种有限的化石记录与其他鳍足类动物（如海象科）更完整的化石记录形成了对比。已知最古老的海狮科动物比最早的海象科动物晚出现500 - 600万年，而且海狮科显著特化的颅齿形态几乎没有为它们的早期进化历史以及在鳍足类动物中的系统发育亲缘关系提供线索。我们报道了一种来自加利福尼亚南部中新世中期下部“托潘加”组（1500 - 1710万年前）的新海狮科动物，隐秘始海狮，其化石为带有保存完好齿列的部分下颌骨。隐秘始海狮在地质年代上处于“海熊兽类”鳍足类形态祖先（1660 - 2700万年前）和先前描述的海狮科化石（730 - 1250万年前）之间，在形态上也处于中间状态，保留了M2和一个缩小的M1元尖齿尖，且没有P2 - 4元尖齿尖。隐秘始海狮消除了海狮科的幽灵世系，并证实海狮科是从类似“海熊兽类”的祖先进化而来的。
