Crowell Jordan W, Beard K Christopher, Chester Stephen G B
Ph.D. Program in Anthropology, The Graduate Center, City University of New York, 365 Fifth Avenue, New York, NY 10016, USA; New York Consortium in Evolutionary Primatology, New York, NY 10024, USA; Department of Anthropology, Brooklyn College, City University of New York, 2900 Bedford Avenue, Brooklyn, NY 11210, USA.
Biodiversity Institute and Department of Ecology and Evolutionary Biology, University of Kansas, 1345 Jayhawk Boulevard, Lawrence, KS 66045, USA.
J Hum Evol. 2025 Apr;201:103655. doi: 10.1016/j.jhevol.2025.103655. Epub 2025 Mar 12.
Palaechthonids are a likely paraphyletic or polyphyletic assemblage of dentally plesiomorphic plesiadapiforms known from the Paleocene of North America. This family is known primarily from isolated dental fossils, but one partial cranium of the palaechthonid Plesiolestes nacimienti (Division of Vertebrate Paleontology, Biodiversity Institute, University of Kansas [KUVP] 9557) exists and was studied a half-century ago to infer aspects of the paleobiology of basal or stem primates. Since then, additional plesiadapiform crania representing several families have been described, but KUVP 9557 remains the best preserved for a palaechthonid and is the geologically oldest known cranial fossil of any plesiadapiform or euarchontan mammal (primates + colugos + treeshrews). Here, for the first time, we scanned the partial cranium of P. nacimienti using micro-computed tomography (μCT) to assess previously described morphology, document novel morphology, and make comparisons with crania of other plesiadapiforms and euarchontans. While several previous cranial descriptions are reaffirmed here (e.g., caudal expansion of the nasals, an intraorbital lacrimal foramen), μCT scan data have clarified that certain previously identified structures (e.g., postorbital process) are not present and have unveiled previously unknown structures (e.g., foramen ovale, optic foramen). Comparisons indicate that the cranial anatomy of P. nacimienti is most like that of non-microsyopid plesiadapiforms and that unambiguous synapomorphies with an extant euarchontan clade are absent. Paleobiological inferences presented here suggest that P. nacimienti was broadly similar to the extant treeshrew Ptilocercus, albeit less insectivorous, which is in line with evolutionary scenarios proposed for the ancestral primatomorphan or the ancestral primate (sensu lato) that emphasize the importance of arboreality and angiosperm products.
古灵长类是一类可能为并系或多系的、具有似原猴类牙齿特征的近猴形类动物，发现于北美洲古新世。该类动物主要以零散的牙齿化石为人们所知，但有一种古灵长类动物纳氏近狐猴（Plesiolestes nacimienti）的部分颅骨（堪萨斯大学[KUVP]生物多样性研究所脊椎动物古生物学部9557号标本）保存了下来，半个世纪前曾对其进行研究，以推断基干或始祖灵长类动物的古生物学特征。从那时起，又有几个类群的近猴形类动物的颅骨被描述出来，但KUVP 9557号标本仍是保存最完好的古灵长类动物颅骨，也是已知的地质年代最古老的近猴形类或真灵长总目哺乳动物（灵长类+鼯猴+树鼩）颅骨化石。在此，我们首次使用微计算机断层扫描（μCT）技术对纳氏近狐猴的部分颅骨进行扫描，以评估之前描述的形态特征、记录新发现的形态特征，并与其他近猴形类动物和真灵长总目动物的颅骨进行比较。虽然这里再次确认了之前的一些颅骨描述（如鼻骨向后扩展、眶内泪孔），但μCT扫描数据表明，之前确定的某些结构（如眶后突）并不存在，同时还揭示了一些之前未知的结构（如卵圆孔、视神经孔）。比较结果表明，纳氏近狐猴的颅骨解剖结构与非微型近猴类近猴形类动物最为相似，与现存真灵长总目类群没有明确的共有衍征。这里提出的古生物学推断表明，纳氏近狐猴与现存的笔尾树鼩（Ptilocercus）大致相似，只是食虫性较弱，这与为始祖灵长类或始祖灵长类（广义）提出的进化假说一致，这些假说强调了树栖性和被子植物产物的重要性。
