Table_1_Was Mesosaurus a Fully Aquatic Reptile?.xlsx

Mesosaurs have been considered strictly aquatic animals. Their adaptations to the aquatic environment are well known and include putative viviparity, along with the presence of several skeletal characters such as a long, laterally compressed tail, long limbs, the foot larger than the manus, and presence of pachyosteosclerotic bones. They were also described as possessing non-coossified girdle bones and incompletely ossified epiphyses, although there could be an early fusion of the front girdle bones to form the scapulocoracoid in some specimens. Some of these features, however, are shared by most basal tetrapods that are considered semiaquatic and even some terrestrial ones. The study of vertebral columns and limbs provides essential clues about the locomotor system and the lifestyle of early amniotes. In this study, we have found that the variation of the vertebral centrum length along the axial skeleton of Mesosaurus tenuidens fits better with a semi-aquatic morphometric pattern, as shown by comparisons with other extinct and extant taxa. The present study allows us to suggest that whereas well-preserved mesosaur skeletons are mostly represented by juveniles and young adults that inhabited aquatic environments, more mature individuals might hypothetically have spent time on land. This is also supported, to an extent, by taphonomic factors such as the scarce representation and poor preservation of remains of mature individuals in the fossiliferous levels, and also by anatomy of the appendicular bones, and particularly the strongly ossified epiphyses and tarsus.

中龙（Mesosaurs）曾被认定为专性水生动物。它们对水生环境的适应性已有充分研究，包括疑似胎生习性，以及多项骨骼特征：侧扁修长的尾部、修长的四肢、后足大于前足（manus），以及厚骨硬化骨骼（pachyosteosclerotic bones）。此前研究还指出，中龙具有未骨化愈合的带骨与骨化不完全的骨骺（epiphyses），不过部分标本的前带骨可能会早期愈合形成肩胛喙骨（scapulocoracoid）。但其中多数特征，在多数被划分为半水生甚至陆生的基干四足类（basal tetrapods）中也普遍存在。对脊柱与四肢的研究，可为早期羊膜动物（amniotes）的运动系统与生活习性提供关键参考依据。本研究发现，细齿中龙（Mesosaurus tenuidens）中轴骨骼上的椎体（vertebral centrum）长度变化模式，与半水生类群的形态计量特征更为契合，该结论通过与其他已灭绝及现生类群的对比得到验证。本研究据此提出假说：尽管保存完好的中龙骨骼标本多来自栖息于水生环境的幼体与亚成体，但成熟个体可能曾有陆地活动经历。该假说在一定程度上得到了埋藏学因素（taphonomic factors）的支持：化石产出层位中成熟个体的化石遗存稀缺且保存状态较差；同时附肢骨骼（appendicular bones）的解剖结构，尤其是高度骨化的骨骺与跗骨（tarsus），也进一步佐证了这一观点。