Re-assessing the age of the type locality of Nythosaurus larvatus (Therapsida, Cynodontia) and implications on the evolutionary dynamics of cynodonts

IF 1.2 4区地球科学 Q2 Earth and Planetary Sciences

Proceedings of the Geologists Association Pub Date : 2024-10-01 DOI:10.1016/j.pgeola.2024.08.007

J. Benoit, S. Jirah, E.S. Lund, T. Lafferty, V. Buffa, L.A. Norton

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Abstract

Nythosaurus larvatus was one of the first cynodonts named and, for a long time, the only therapsid for which the endocranial cast was described. The holotype and only skull of this species is thus a landmark in the history of therapsid palaeontology. It is believed to have come from Commissie Drift 303 (Free State Province, South Africa), a locality that has been traditionally dated to the Lystrosaurus declivis Assemblage Zone. However, this age was mostly based on the synonymy between Nythosaurus larvatus and Thrinaxodon liorhinus, the former being the only fossil indisputably reported from this locality and the latter being an index taxon for the Lystrosaurus declivis Assemblage Zone. A recent revision of Nythosaurus larvatus also questioned the age of Commissie Drift. Here, we report the result of our fieldwork to re-assess the age of this locality. We find that the stratigraphy conforms to the lower Burgersdorp Formation and that the fossil tetrapod fauna does not match the Lystrosaurus declivis Assemblage Zone. The overall scarcity of fossil bones, absence of plants and dicynodonts, and relative dominance of amphibian remains all point to an assignation to the Langbergia–Garjainia Subzone of the Cynognathus Assemblage Zone, which is consistent with the stratigraphy. The discovery of new material attributable to cf. Nythosaurus bolsters the notion that this taxon is from the late Olenekian. Nythosaurus larvatus can safely be considered a valid taxon, and may in fact occur elsewhere in this subzone.

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重新评估秧鸡龙（Therapsida, Cynodontia）模式产地的年龄及其对秧鸡龙进化动态的影响

幼龙是最早被命名的犬齿龙之一，而且在很长一段时间里，它是唯一一个被描述了颅内铸模的兽脚类。因此，该物种的主模式和唯一的头骨是兽脚类古生物学史上的一个里程碑。据信，它来自Commissie漂流区303号（南非自由州省），该地点历来被定为Lystrosaurus declivis集合区。然而，这一年代主要是基于幼龙（Nythosaurus larvatus）和栗鼻龙（Thrinaxodon liorhinus）之间的同义关系，前者是该地点无可争议地报告的唯一化石，而后者则是Lystrosaurus declivis Assemblage Zone的索引类群。最近对幼龙（Nythosaurus larvatus）的修订也对康米西河漂流的年代提出了质疑。在此，我们报告了重新评估该地点年龄的实地考察结果。我们发现，这里的地层与下伯格斯多普层（Burgersdorp Formation）一致，而且四足动物化石与Lystrosaurus declivis集合带不一致。骨骼化石的总体稀缺性、植物和双齿兽的缺失以及两栖动物化石的相对优势都表明，该化石区应归属于Cynognathus集合带的Langbergia-Garjainia亚区，这与地层学是一致的。发现了可归属于尼特龙的新材料，这进一步证实了该类群来自奥利尼克晚期的观点。可以有把握地将幼龙（Nythosaurus larvatus）视为一个有效的类群，事实上它可能出现在该子区的其他地方。

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来源期刊

Proceedings of the Geologists Association 地学-地质学

CiteScore

2.70

自引率

8.30%

发文量

审稿时长

6-12 weeks

期刊介绍： The Proceedings of the Geologists'' Association is an international geoscience journal that was founded in 1859 and publishes research and review papers on all aspects of Earth Science. In particular, papers will focus on the geology of northwestern Europe and the Mediterranean, including both the onshore and offshore record. Following a long tradition, the PGA will focus on: i) a range of article types (see below) on topics of wide relevance to Earth Sciences ii) papers on aspects of Earth Science that have societal relevance including geoconservation and Earth management, iii) papers on palaeoenvironments and palaeontology of the Mesozoic and Cenozoic, iv) papers on aspects of Quaternary geology and climate change, and v) papers on the history of geology with particular reference to individuals that have shaped the subject. These topics will also steer the content of the themes of the Special Issues that are published in the PGA.