Palaeohistological Evidence for Ancestral High Metabolic Rate in Archosaurs

Abstract

Metabolic heat production in archosaurs has played an important role in their evolutionary radiation during the Mesozoic, and their ancestral metabolic condition has long been a matter of debate in systematics and palaeontology. The study of fossil bone histology provides crucial information on bone growth rate, which has been used to indirectly investigate the evolution of thermometabolism in archosaurs. However, no quantitative estimation of metabolic rate has ever been performed on fossils using bone histological features. Moreover, to date, no inference model has included phylogenetic information in the form of predictive variables. Here we performed statistical predictive modeling using the new method of phylogenetic eigenvector maps on a set of bone histological features for a sample of extant and extinct vertebrates, to estimate metabolic rates of fossil archosauromorphs. This modeling procedure serves as a case study for eigenvector-based predictive modeling in a phylogenetic context, as well as an investigation of the poorly known evolutionary patterns of metabolic rate in archosaurs. Our results show that Mesozoic theropod dinosaurs exhibit metabolic rates very close to those found in modern birds, that archosaurs share a higher ancestral metabolic rate than that of extant ectotherms, and that this derived high metabolic rate was acquired at a much more inclusive level of the phylogenetic tree, among non-archosaurian archosauromorphs. These results also highlight the difficulties of assigning a given heat production strategy (i.e., endothermy, ectothermy) to an estimated metabolic rate value, and confirm findings of previous studies that the definition of the endotherm/ectotherm dichotomy may be ambiguous.

Bone histology of Iberosuchus macrodon (Sebecosuchia, Crocodylomorpha)

“…Recently, resting metabolic rates of extinct archosaurs and non‐archosaurian Archosauromorpha were inferred using bone palaeohistology (Legendre et al . ), providing more direct support for that hypothesis.…”

Preliminary observations on the bone histology of the Middle Triassic pseudosuchian archosaurBatrachotomus kupferzellensisreveal fast growth with laminar fibrolamellar bone tissue

“…Vascular density is low or tissue may be even avascular. FLB consists largely of woven and centripetally deposited lamellar bone (primary osteons), and the matrix shows a high vascular density (Francillon-Vieillot et al, 1990;Cubo et al, 2005;Legendre et al, 2016). Like LZB, FLB can be stratified by growth marks, as was described for some dinosaurs (Horner et al, 2000;Horner and Padian, 2004;Klein and Sander, 2007) and mammals (e.g., Sander and Andr assy, 2006;K€ ohler et al, 2012).…”

Development and evolution of the metazoan heart

“…Arguments based on the characteristics of the cardiovascular architecture as early as in the egg likely precede other theories about the evolutionary onset of endothermy, such as change in juvenile and adult growth parameters, adult metabolic rate, or oxygen handling and mitochondrial activity . A further argument from embryo‐related studies came from analysis of fossil egg shells.…”