Gaudin Timothy J, Scaife Thomas
Department of Biology, Geology & Environmental Science, University of Tennessee at Chattanooga, Chattanooga, Tennessee, USA.
Department of Geosciences, East Tennessee State University, Johnson City, Tennessee, USA.
Anat Rec (Hoboken). 2023 Mar;306(3):607-637. doi: 10.1002/ar.25062. Epub 2022 Sep 10.
The present study comprises a description of the skull and jaw anatomy of a juvenile specimen of the Antillean sloth Acratocnus ye, from the Holocene of Haiti. Detailed descriptions and illustrations are provided of the skull bones and their sutural connections, which normally fuse in adults. Descriptions are also provided for the mandible and ear ossicles, as well as endocranial surfaces and sinuses exposed by breaks. The anatomy of our juvenile A. ye is compared to that of adult A. ye to assess ontogenetic changes in the skull. Several of these ontogenetic features are significant new observations that impact the relationships within Xenarthra as a whole, or between Xenarthrans and other placental mammals, most notably, the presence of a separate mesethmoid element, the presence of alveoli for a lower deciduous canine and anterior incisor, and the presence of separate rostral and caudal entotympanic elements. A full list of such changes are provided. In addition, the specimen provides information on phylogenetically relevant characters, including features unique to the genus Acratocnus, and features of the clade Choloepodini, including Acratocnus, the smaller extinct Antillean sloth Neocnus, and the extant two-toed sloth Choloepus. Contrary to previous studies, Acratocnus shares as many features with Choloepus as it does with its fellow Antillean form Neocnus in the present study, which is consistent with current morphology-based phylogenetic hypotheses regarding the relationships within Choloepodini. The current study highlights the need for further anatomical and phylogenetic investigations of Antillean sloths (Megalocnidae/Megalonychidae), and juvenile sloths in general.
本研究描述了来自海地全新世的安的列斯树懒Acratocnus ye幼年标本的头骨和颌骨解剖结构。提供了对头骨骨骼及其缝合连接的详细描述和图示，这些在成年个体中通常会融合。还描述了下颌骨和听小骨，以及因骨折暴露的颅内膜表面和鼻窦。将我们的幼年A. ye的解剖结构与成年A. ye的进行比较，以评估头骨的个体发育变化。其中一些个体发育特征是重要的新观察结果，影响了整个贫齿总目内部的关系，或贫齿类与其他胎盘哺乳动物之间的关系，最显著的是，存在一个单独的中筛骨元素、存在下乳犬齿和前切牙的牙槽，以及存在单独的吻侧和尾侧内鼓骨元素。提供了此类变化的完整列表。此外，该标本提供了有关系统发育相关特征的信息，包括Acratocnus属独有的特征，以及包括Acratocnus、已灭绝的较小的安的列斯树懒Neocnus和现存的二趾树懒Choloepus在内的树懒科分支的特征。与先前的研究相反，在本研究中，Acratocnus与Choloepus共享的特征与其与同属安的列斯形态的Neocnus共享的特征一样多，这与当前基于形态学的关于树懒科内部关系的系统发育假设一致。当前的研究强调了对安的列斯树懒（大地懒科/大懒兽科）以及一般幼年树懒进行进一步解剖学和系统发育研究的必要性。
