Long bone histology of Lower Permian pelycosaurs, implications for the evolution of endothermy at the beginning of the mammal lineage

恒温动物在四足动物中至少进化了两次，一次是在通过恐龙进化到鸟类的谱系中，一次是在进化到哺乳动物的谱系中，可能是在传统上被称为兽孔类的非哺乳动物的衍生纲中。追踪温血动物进化的最好方法是通过对骨骼微观结构的研究，定性地通过组织类型，定量地通过骨骼年代学，这在兽脚类中已经做了一些详细的研究。然而，哺乳动物谱系中最基础的代表是盘龙，或者更确切地说，是非兽孔类。这些几乎只在德克萨斯州中北部的下二叠纪红层中发现，其中包括标志性的动物，如异齿龙和Edaphosaurus。盘龙可能不是恒温动物，但缺乏证据，因为它们的长骨组织学在现代标准下基本上没有被研究过。在这项研究中，我们打算通过全面取样和分析具有代表性的长鼻龙类群的长骨组织学来改善这一状况。对生长标记记录的量化和对骨组织个体发生变化的描述将揭示相对和绝对生长速率，从而揭示最早的类哺乳动物的热生理，因为恒温动物的生长速度比恒温动物慢一个数量级。

Endothermy evolved at least twice in tetrapods, once in the lineage that lead to birds via the dinosaurs and once in the lineage that led to mammals, probably in the derived non-mammalian synapsids traditionally know as therapsids. The best way to trace the evolution of warm-bloodedness is through the study of bone microstructure, qualitatively through tissue types and quanitatively through skeletochronology, which has been done in some detail in therapsids. However, the most basal representatives of the mammal lineage are pelycosaurs or, more properly, the non-therapsid synapsids. These are almost exclusively known from the Lower Permian red beds of North-Central Texas, and include iconic animals such as Dimetrodon and Edaphosaurus. Pelycosaurs probably were not endothermie, but evidence is lacking because their long bone histology remains essentially unstudied by modern standards. In this study we intend to improve this situation by comprehensively sampling and analyzing long bone histology from representative pelycosaur taxa. Quantification of the growth mark record and description of the ontogenetic variation of bone histology will reveal relative and absolute growth rates, shedding light on the thermophysiology of the earliest mammal-like reptiles, because ectothermic animals grow an order of magnitude slower than endothermie animals.