Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, United States of America.
División Paleontologia de Vertebrados, Instituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan, San Juan, Argentina.
PLoS One. 2019 Aug 7;14(8):e0218791. doi: 10.1371/journal.pone.0218791. eCollection 2019.
We describe a new probainognathian cynodont, Pseudotherium argentinus, from the early Late Triassic Ischigualasto Formation of Argentina. Pseudotherium adds to a growing assemblage of small Triassic cynodonts that offers new insight into events leading up to the origin of crown Mammalia and the successively more inclusive Mammaliaformes and Mammaliamorpha. Using high-resolution X-ray computed tomography, we illustrate and describe the holotype and only known specimen, which consists of a well-preserved isolated skull. It preserves apomorphic features of the orbit and braincase. Prefrontal and vestigial postorbital bones are present, despite the absence of an ossified postorbital bar. As in Brasilitherium riograndensis, thin turbinal-like bones are present in the nasopharyngeal passage, and we discuss impediments to establishing their identity and function. Compared to more basal cynodonts, the cochlea is elongated but uncoiled and in this and other features it resembles basal mammaliamorphs. Our analysis found weak support for Pseudotherium as the sister taxon of Tritylodontidae. However, a broader assessment of its relationships in light of additional character data from the literature and unpublished computed tomography data suggest that it may be more realistic to view the relationships of Pseudotherium as an unresolved polytomy with tritylodontids, and the taxa referred to as tritheledontids and brasilodontids (groups of variable membership and questionable monophyly). Thus, Pseudotherium may lie just inside or just outside of Mammaliamorpha, and there is also weak character support for its sister taxon relationship with Brasilitherium. Our results amplify previous conclusions that phylogenetic relationships in this adaptive radiation of small cynodonts will remain somewhat uncertain until more complete specimens are recovered, and until high-resolution CT scans of existing specimens become available to the larger community. Toward that goal, we make the CT dataset for the holotype of Pseudotherium argentinus publically available under a Creative Commons license at www.DigiMorph.org.
我们描述了一种来自阿根廷早三叠世伊希瓜拉斯托组的新的始新兽形亚目兽类，假兽兽。假兽兽增加了一个不断增长的小型三叠纪兽形亚目动物组合，为研究导致冠哺乳动物起源以及相继更具包容性的哺乳动物形类和哺乳动物类的事件提供了新的见解。使用高分辨率 X 射线计算机断层扫描，我们展示并描述了正型标本和唯一已知的标本，该标本由一个保存完好的孤立头骨组成。它保留了眶和脑颅的特化特征。尽管没有骨化的眶后骨，但存在额骨和退化的眶后骨。与更基础的兽形亚目动物相比，在鼻咽道中存在薄的鼻甲样骨头，我们讨论了确定其身份和功能的障碍。与更基础的兽形亚目动物相比，假兽兽的耳蜗较长但未卷曲，在这方面和其他方面，它类似于基干哺乳动物类。我们的分析发现，假兽兽作为三瘤齿兽科的姊妹分类群得到了微弱的支持。然而，根据文献中的附加特征数据和未发表的计算机断层扫描数据对其关系进行更广泛的评估表明，将假兽兽的关系视为一个未解决的多系群，与三瘤齿兽科以及被称为三齿兽科和巴西齿兽科的分类群（成员可变且单系性可疑的分类群），可能更现实。因此，假兽兽可能位于哺乳动物类内部或外部，其与巴西齿兽的姊妹分类群关系也得到了微弱的特征支持。我们的结果放大了先前的结论，即在这个小型兽形亚目动物的适应性辐射中，系统发育关系在获得更多完整标本之前以及高分辨率 CT 扫描现有标本可供更大的社区使用之前，仍将有些不确定。为此，我们在 Creative Commons 许可下将假兽兽正型标本的 CT 数据集在 www.DigiMorph.org 上公开提供。
