Body forms of extant lamniform sharks (Elasmobranchii: Lamniformes), and comments on the morphology of the extinct megatooth shark, Otodus megalodon, and the evolution of lamniform thermophysiology

Abstract

ABSTRACT The megatooth shark, Otodus megalodon, is an iconic Neogene lamniform shark known only from its teeth and vertebrae. Its thermophysiology is previously inferred to have been regionally endothermic, like the extant lamnids that are active predatory lamniforms. By considering the entire Lamnidae as the ecological and physiological analogue to O. megalodon, a recent study proposed inferred body dimensions of O. megalodon based on morphometric analyses on body forms of extant lamnids. Here, we reassessed the recent study by testing whether a two-dimensional approach used in the study can actually tease out the difference between ectothermic and endothermic fishes in the first place. Morphometric comparisons of the whole body and different body parts (e.g. head, different fins, and precaudal body with and without fins) among the 15 extant species of Lamniformes were conducted using principal component analyses and simple clustering methods. Our study strongly indicates that, two-dimensionally, there is no relationship between thermophysiology and body form in lamniforms. The reality is that there are presently no scientific means to support or refute the accuracy of any of the previously published body forms of O. megalodon. We also clarify that regional endothermy likely evolved in multiple clades independently through lamniform phylogeny.