Paleohistological inferences of thermometabolic regimes in Notosuchia (Pseudosuchia: Crocodylomorpha) revisited

Abstract

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 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 previous 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 to be a significant explanatory variable (in addition to vascular canal diameter). Second, red blood cell size estimations were performed using phylogenetic eigenvector maps, and this method excludes a fraction of the 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 all of the phylogenetic information and a sample of 46 tetrapods. These analyses suggest that Araripesuchus wegeneri, Armadillosuchus arrudai, Baurusuchus sp., Iberosuchus macrodon, and Stratiotosuchus maxhechti were ectothermic organisms.

Phylogenetic structure of the extinction and biotic factors explaining differential survival of terrestrial notosuchians at the Cretaceous–Palaeogene crisis

“…Their probable ectothermic metabolism (Cubo et al . 2020, 2022) could have strongly restricted their possible dispersal to higher latitude and habitats. Lower latitude and habitats seem to be a limiting constraint too, maybe because the rainforest of tropical climates may have been problematic for terrestrial cursorial crocodyliforms.…”

“…Notosuchians were probably ectothermic (Cubo et al . 2020, 2022). Almost all of them were adapted to a terrestrial environment (Wu & Sues 1996; Gomani 1997; Pol 2005; Turner 2006; Sereno & Larsson 2009; Nascimento & Zaher 2010; Riff & Kellner 2011; Godoy et al .…”

On the origins of endothermy in amniotes

“…This kind of scenario probably occurred in the loss of endothermy or, at least, tachymetabolism, in the crocodilian clade. 12 , 15 , 32 , 43 …”

“…The relationship between the size of the erythrocytes and HMC is thus not always exact, despite the statistical relationship still being significant. 32 Moreover, Starck and Chinsamy 34 demonstrated that the size of vascular canals can vary accordingly to the availability of food during osteogenesis: less available food leads to smaller vascular canals and, consequently, to an overestimation of . Then, an ectothermic taxon can be wrongly associated with endothermy because of food limitation.…”

“…Such a cell size reduction is associated with a diminution of the capillary diameters, as the erythrocytes must be larger than the capillaries they are passing through in order to deform and increase the oxygen delivery rate. 30 , 31 Thus, the size of bone vascular canals can be used to infer the size of erythrocytes and, through them, the aerobic capacity 17 and the thermophysiological status 32 of extinct amniotes. As this feature was not studied by Grigg et al., 4 its analysis is an independent test of their ancestral amniote endothermy hypothesis.…”