Paleohistological inferences of thermometabolic regimes in Notosuchia

Notosuchia is a group of mostly terrestrial crocodyliforms. The presence of a prominent crest overhanging the acetabulum, slender straight-shafted long bones with muscular insertions close to the joints, and a stable knee joint, suggests that they had an erect posture. This stance has been proposed to be linked to endothermy because it is present in mammals and birds, and it contributes to the efficiency of their respiratory systems. However, a bone paleohistological study unexpectedly suggested that Notosuchia were ectothermic organisms. The thermophysiological status of Notosuchia deserves further analysis because the methodology of the quoted study can be improved. First, it was based on a relationship between red blood cell size and bone vascular canal diameter tested using 14 extant tetrapod species. Here we present evidence for this relationship using a more comprehensive sample of extant tetrapods (31 species). Moreover, contrary to previous results, bone cross-sectional area appears as a significant explanatory variable (in addition to bone vascular canal diameter). Second, red blood cell size estimations were performed using phylogenetic eigenvector maps, and this method excludes a fraction of phylogenetic information. This is because it generates a high number of eigenvectors requiring a selection procedure to compile a subset of them to avoid model overfitting. Here we inferred the thermophysiology of Notosuchia using phylogenetic logistic regressions, a method that overcomes this problem by including the whole phylogenetic information. These analyses suggest that Araripesuchus wegeneri, Armadillosuchus arrudai, Baurusuchus sp., Iberosuchus macrodon and Stratiotosuchus maxhechti were ectothermic organisms.Funding provided by: Sorbonne UniversitéCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100019125Award Number: 2019 N° 24337