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Were the synapsids primitively endotherms? A palaeohistological approach using phylogenetic eigenvector maps
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Edité par CCSD ; Royal Society, The -
International audience. The acquisition of mammalian endothermy is poorly constrained both phylogenetically and temporally. Here we inferred the resting metabolic rates (RMR) and the thermometabolic regimes (endothermy or ectothermy) of a sample of eight extinct synapsids using palaeohistology, phylogenetic eigenvector maps, and a sample of seventeen extant tetrapods of known RMR (quantified using respirometry). We inferred high RMR values and an endothermic metabolism for the anomodonts (Lystrosaurus sp., Oudenodon baini) and low RMR values and an ectothermic metabolism for Clepsydrops collettii, Dimetrodon sp., Edaphosaurus boanerges, Mycterosaurus sp., Ophiacodon uniformis and Sphenacodon sp. A maximum likelihood ancestral states reconstruction of resting metabolic rates performed using the values inferred using phylogenetic eigenvector maps in extinct synapsids, and the values measured using respirometry in extant tetrapods, shows that the nodes Anomodontia and Mammalia were primitively endotherms. Finally, we performed a parsimony optimisation of the presence of endothermy using the results obtained in the present study and those obtained in previous studies that used phylogenetic eigenvector maps. For this, we assigned to each extinct taxa a thermometabolic regime (ectothermy or endothermy) depending on whether the inferred values were significantly higher, lower or not significantly different from the RMR value separating ectotherms from endotherms (1.5 mL O 2 .h −1 .g −0.67). According to this optimisation, endothermy arose independently in Archosauromorpha, in Sauropterygia, and in Therapsida.
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