Origins of Endothermy in the Mammalian Lineage

Abstract

This study reports on long bone histology of the pelycosaurs, and proposes the hypothesis that the presence of fibro-lamellar bone (FLB) and high growth rates were already present in the basal most synapsids during the Upper Carboniferous and Lower Permian which was due to the hyperoxic environment. This is contrary to the long held views that pelycosaurs were the plesiomorphic condition being poikilothermic sluggish reptiles and that mammalian endothermy originated within Therapsida. This was accomplished by the consumptive sampling of various pelycosaur taxa throughout the clade from every group, excluding eothyrids. evidence was taken from sphenacodontids, ophiacodontids, edaphosaurids, caseids, and varanopids. For the first time consumptive sampling and analysis of ontogenetic material was also made available from the following taxa: Ophiacodon, Dimetrodon, and Cotylorhynchus. Results show inter- and intraspecific histological variation among this clade. Basically, carnivores grew much faster than herbivorous pelycosaurs who appear to have secondarily reduced their growth rates during the Lower to Middle Permian despite having evolved to record sizes. Histological differences seem to be mostly due to diet and how it was acquired. Further consumptive sampling of the earliest insectivorous forms is required to fully understand this phenomenon and to finally resolve what the true plesiomorphic condition is for tetrapods. A complete list of all material sampled and measured for this research is given in the appendix. Unfortunately, there was not enough time to include all the material acquired over the last five years in this study.