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

四足动物至少进化了两次恒温性，一次是在通过恐龙进化为鸟类的谱系中，另一次是在进化为哺乳动物的谱系中，可能是在传统上被称为兽孔目的非哺乳类联会动物中。追踪温血动物进化的最好方法是通过研究骨骼的微观结构，通过组织类型进行定性，通过骨骼年代学进行定量，这在兽形目动物中已经做得相当详细。然而，哺乳动物谱系最基本的代表是盘龙类，或者更确切地说，是非兽脚类联会动物。这些几乎只在德克萨斯州中北部的下二叠统红层中发现，包括标志性的动物，如异齿龙和Edaphosaurus。Pelycosaurs可能不是endoscopic，但缺乏证据，因为他们的长骨组织学仍然基本上没有现代标准的研究。在这项研究中，我们打算改善这种情况，全面采样和分析长骨组织学从代表性的Pelycosaur类群。定量的生长标记记录和描述的骨组织学的个体发育变化将揭示相对和绝对的增长率，揭示最早的哺乳类爬行动物的热生理学，因为变温动物的增长速度比endo-temperature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.