Data from: Palaeohistological evidence for ancestral high metabolic rate in archosaurs

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 modelling using the new method of phylogenetic eigenvector maps on a set of bone histological features for a sample of extant and extinct vertebrates, in order to estimate metabolic rates of fossil archosauromorphs. This modelling procedure serves as a case study for eigenvector-based predictive modelling 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 an 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.

初龙类（archosaurs）的代谢产热在其中生代的演化辐射进程中发挥了关键作用，其祖先代谢状态长期以来一直是分类学与古生物学领域的争论焦点。对化石骨组织学的研究可提供骨生长速率的关键信息，此前学界曾借助该信息间接探究初龙类的体温代谢演化历程。然而，此前尚未有研究通过骨组织学特征对化石样本开展代谢率的定量估算；此外，截至目前，也尚无研究将系统发育信息以预测变量的形式纳入推断模型之中。本研究针对一组涵盖现生与已灭绝脊椎动物的骨组织学特征数据集，采用系统发育特征向量映射（phylogenetic eigenvector maps）这一新方法开展统计预测建模，以期估算已灭绝初龙形类（archosauromorphs）的代谢率。该建模流程既可为系统发育框架下基于特征向量的预测建模提供案例参考，也有助于探究初龙类代谢率演化中尚不明确的模式。研究结果显示：中生代兽脚类恐龙的代谢率与现代鸟类极为接近；初龙类的祖先代谢率显著高于现生变温动物；而这种衍生的高代谢率是在系统发育树上更为宽泛的支系层级中演化获得的，即非初龙类的初龙形类类群。此外，本研究结果还凸显了将特定产热策略（即内温性（endothermy）、外温性（ectothermy））对应至估算得到的代谢率数值的难点，并证实了此前研究的结论：内温-外温二分法的定义可能存在歧义。