New data on the early odobenid Neotherium mirum Kellogg, 1931, and other pinniped remains from the Sharktooth Hill Bonebed, California

Deposited over a short period of time during the Middle Miocene Climatic Optimum, the Sharktooth Hill Bonebed in California is one of the world’s densest marine vertebrate concentrations. Pinnipeds are represented here by odobenids and desmatophocids, but some are still poorly known. Herein, the mandibular morphology of the odobenid Neotherium mirum is described in detail. Its morphology resembles that of early pinnipedimorphs, with postcanine teeth consisting of three or four longitudinally aligned cusps and double-rooted p2–m1, but shows derived characteristics such as reduced metaconid and bilobed canine root. Additionally, Neotherium is represented by two distinct size classes, which is likely the earliest evidence of sexually dimorphic body size in odobenids. Two additional mandibles represent other pinnipeds. One represents an unknown odobenid, with a unique suite of morphological characteristics setting it apart from contemporaneous odobenids such as Neotherium and Pelagiarctos. The other reconfirms the presence of Allodesmus cf. A. sadoensis, a species otherwise known from the middle Miocene of Japan. This species is distinguished from Allodesmus kernensis, based on anteriorly crowded and reduced postcanine dentition. This record adds further support for the presence of this taxon in the middle Miocene of California and represents the earliest evidence of trans-Pacific dispersal in pinnipeds. The Sharktooth Hill pinniped assemblage now consists of five species: the odobenids Neotherium mirum, Pelagiarctos thomasi, an unknown odobenid, and the desmatophocids Allodesmus kernensis and Allodesmus cf. A. sadoensis. Niche partitioning within this sympatric pinniped community was possible due to differences in body size, and feeding and foraging preferences.

产于加利福尼亚州的鲨鱼齿山骨层（Sharktooth Hill Bonebed）形成于中中新世气候适宜期（Middle Miocene Climatic Optimum）的短时间沉积时段内，是全球已知密度最高的海洋脊椎动物化石聚集点之一。本地产出的鳍足类（Pinnipeds）化石以海象科（Odobenidae）和短面海豹科（Desmatophocidae）为主，但部分类群的研究程度仍较为有限。本文详细记述了海象科奇异新海象（Neotherium mirum）的下颌形态（mandibular morphology）：其形态与早期鳍足形类（pinnipedimorphs）相似，后犬齿（postcanine teeth）具3至4个纵向排列的齿尖（cusps），p2至m1均为双根齿；同时具有衍生特征，如下后尖（metaconid）退化、犬齿齿根（canine root）呈双叶状。此外，奇异新海象存在两个显著的体型类群，这可能是海象科中已知最早的身体大小性别二态性证据。另有两件下颌骨分别代表其他鳍足类类群：其一属于未知的海象科新类群，其独特的形态组合使其区别于同期的奇异新海象与远洋海象属（Pelagiarctos）；其二确认了海狮兽属（Allodesmus）比较种A. sadoensis（Allodesmus cf. A. sadoensis）的存在，该物种此前仅发现于日本中中新世地层。与克氏海狮兽（Allodesmus kernensis）相比，该物种的后犬齿列前部拥挤且齿列退化。这一记录进一步支持了加利福尼亚州中中新世地层存在该类群的观点，同时代表了鳍足类跨太平洋扩散的最早证据。目前鲨鱼齿山骨层的鳍足类组合（pinniped assemblage）共计5个物种：海象科的奇异新海象、汤氏远洋海象（Pelagiarctos thomasi）、未知海象科类群，以及短面海豹科的克氏海狮兽与海狮兽属比较种A. sadoensis。该同域分布的鳍足类群落可通过体型差异、摄食与觅食偏好的分化实现生态位分化（niche partitioning）。