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.2020 Mar 2;375(1793):20190138.
doi: 10.1098/rstb.2019.0138. Epub 2020 Jan 13.

Were the synapsids primitively endotherms? A palaeohistological approach using phylogenetic eigenvector maps

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Were the synapsids primitively endotherms? A palaeohistological approach using phylogenetic eigenvector maps

Mathieu G Faure-Brac et al. Philos Trans R Soc Lond B Biol Sci..

Abstract

The acquisition of mammalian endothermy is poorly constrained both phylogenetically and temporally. Here, we inferred the resting metabolic rates (RMRs) and the thermometabolic regimes (endothermy or ectothermy) of a sample of eight extinct synapsids using palaeohistology, phylogenetic eigenvector maps (PEMs), and a sample of 17 extant tetrapods of known RMR (quantified using respirometry). We inferred high RMR values and an endothermic metabolism for the anomodonts (Lystrosaurus sp.,Oudenodon bainii) and low RMR values and an ectothermic metabolism forClepsydrops collettii, Dimetrodon sp.,Edaphosaurus boanerges, Mycterosaurus sp.,Ophiacodon uniformis andSphenacodon sp. A maximum-likelihood ancestral states reconstruction of RMRs performed using the values inferred for 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 optimization of the presence of endothermy using the results obtained in the present study and those obtained in previous studies that used PEMs. For this, we assigned to each extinct taxon 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 O2 h-1 g-0.67). According to this optimization, endothermy arose independently in Archosauromorpha, Sauropterygia and Therapsida. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Keywords: endothermy; palaeophysiology; quantitative palaeohistology; synapsids.

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Conflict of interest statement

We declare we have no competing interests

Figures

Figure 1.
Figure 1.
Resting metabolic rates inferred using palaeohistology and phylogenyetic eigenvector maps. We used a model that includes the phylogeny plus relative primary osteon area as explanatory factors. Shapes of animals are from the PhyloPic database (www.phylopic.org). Blue squares indicate ectothermy and red squares endothermy. The broken line represents the lowest RMR value found in extant endotherms. For extinct taxa, segments represent the 95% confidence intervals of the inferences. For extant taxa, crosses represent values inferred in leave-one-out cross-validation tests. AICc, corrected Akaike information criterion. (Online version in colour.)
Figure 2.
Figure 2.
Maximum-likelihood (ML) ancestral states reconstruction of resting metabolic rates performed using the values inferred using phylogenetic eigenvector maps (PEMs) for the eight extinct synapsids, and the values measured using respirometry in the 17 extant tetrapods. Values are in ml O2 h−1 g−0.67. Within Synapsida, the nodes Anomodontia (node 35) and Mammalia (node 36) were primitively endotherms.
Figure 3.
Figure 3.
Parsimony optimization of the presence of endothermy using the results obtained in the present study and those obtained in previous studies using phylogenetic eigenvector maps [,–15]. For this, we assigned to each extinct taxon 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 O2 h−1 g−0.67). According to this optimization, endothermy arose independently in Archosauromorpha (green), in Sauropterygia (yellow) and in Neotherapsida (orange). Blue circles indicate ectothermy and red circles endothermy. (Online version in colour.)
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