High Blood Flow Into the Femur Indicates Elevated Aerobic Capacity in Synapsids Since the Synapsida-Sauropsida Split

Philipp L Knaus,Jacqueline K Lungmus,Anneke H Van Heteren,P Martin Sander

doi:10.3389/fevo.2021.751238

Abstract

Varanids are the only non-avian sauropsids that are known to approach the warm-blooded mammals in stamina. Furthermore, a much higher maximum metabolic rate (MMR) gives endotherms (including birds) higher stamina than crocodiles, turtles, and non-varanid lepidosaurs. This has led researchers to hypothesize that mammalian endothermy evolved as a second step after the acquisition of elevated MMR in non-mammalian therapsids from a plesiomorphic state of low metabolic rates. In recent amniotes, MMR correlates with the index of blood flow into the femur (Qi), which is calculated from femoral length and the cross-sectional area of the nutrient foramen. Thus,Qimay serve as an indicator of MMR range in extinct animals. Using theQiproxy and phylogenetic eigenvector maps, here we show that elevated MMRs evolved near the base of Synapsida. Non-mammalian synapsids, including caseids, edaphosaurids, sphenacodontids, dicynodonts, gorgonopsids, and non-mammalian cynodonts, showQivalues in the range of recent endotherms and varanids, suggesting that raised MMRs either evolved in synapsids shortly after the Synapsida-Sauropsida split in the Mississippian or that the low MMR of lepidosaurs and turtles is apomorphic, as has been postulated for crocodiles.

Highlights

Comparisons of endothermic mammals and birds with ectothermic sauropsids reveal a chasm in metabolic rates between the two functional groups of amniotes
The sample of non-mammalian synapsids spans two orders of body size magnitude, with the lightest animal estimated at 1.86 kg (Galesaurus) and the heaviest at 503 kg (Dinodontosaurus)
The search for the best-fitting evolutionary model found phylogenetic signal in all groups except varanids and nonmammalian synapsids, which were best described with the White Noise model

Summary

Introduction

Comparisons of endothermic mammals and birds with ectothermic sauropsids reveal a chasm in metabolic rates between the two functional groups of amniotes. MMR is much higher in endotherms than ectotherms, but varanids can exceed half the mass-specific MMR of endotherms under ideal temperatures (Taylor et al, 1981; Clemente et al, 2009), whereas varanid BMR is comparable to other nonavian sauropsids (Christian and Conley, 1994). When scaling metabolic rates of amniote ectotherms up to the body temperature of mammals (higher than the behaviorally optimized ideal temperature of most ectotherms, while bird body temperatures are even higher), their BMR can be compared to that of tenrecid mammals (Oelkrug et al, 2013). Recent experiments with black and white tegus (Salvator merianae) have revealed seasonal non-shivering thermogenesis in these squamates, if at a relatively low body temperature (Tattersall et al, 2016)

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