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

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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.