Getting Warmer, Getting Colder: Reconstructing Crocodylomorph Physiology

Abstract

Seymour et al. (2004) deserve credit for a provocative article. The question they raise—why crocodilians have a four-chambered heart when other reptiles get along with just three—is valid and interesting, and it deserves far more attention than it has thus far received. However, their suggestion that crocodilians once had endothermic ancestors and reverted back to ectothermy when they invaded aquatic niches is less than compelling for a number of reasons. Seymour et al. actually deal with two separate questions: What was the metabolic status of the early crocodylomorphs? and What selective factors might account for the attributes of modern crocodilians and the transformation that distinguishes this lineage from the ancestral forms? With regard to the first question, they argue not merely that crocodilian biology was modified from that of their Triassic and Early Jurassic ancestors but explicitly that these ancestors had attained full endothermic status. Unfortunately, they provide no compelling evidence that endothermy was in fact present in the ancestral forms. The authors point to the agile body sizes, delicate bones, and erect, possibly bipedal gait of early crocodylomorphs as evidence of a terrestrial, cursorial lifestyle and then postulate that “upright stance and the capacity for highly active, terrestrial behavior are characteristic of endotherms” (p. 1054). However, as the authors themselves acknowledge, attributes such as posture, gait, and bone structure are not compelling evidence of endothermy (in addition to the reviews cited by the authors, see also Farlow 1990; Farlow et al. 1995; Ruben 1995; Chinsamy and Hillenius 2004; Hillenius and Ruben 2004). Early croco-