Body, jaw, and dentition lengths of macrophagous lamniform sharks, and body size evolution in Lamniformes with special reference to ‘off-the-scale’ gigantism of the megatooth shark, Otodus megalodon

Abstract

ABSTRACT Extinct lamniform sharks (Elasmobranchii: Lamniformes) are well represented in the late Mesozoic‒Cenozoic fossil record, yet their biology is poorly understood because they are mostly represented only by their teeth. Here, we present measurements taken from specimens of all 13 species of extant macrophagous lamniforms to generate functions that would allow estimations of body, jaw, and dentition lengths of extinct macrophagous lamniforms from their teeth. These quantitative functions enable us to examine the body size distribution of all known macrophagous lamniform genera over geologic time. Our study reveals that small body size is plesiomorphic for Lamniformes. There are four genera that included at least one member that reached >6 m during both the Mesozoic and Cenozoic, most of which are endothermic. The largest form of the genus Otodus, O. megalodon (‘megatooth shark’) that reached at least 14 m, is truly an outlier considering that all other known macrophagous lamniforms have a general size limit of 7 m. Endothermy has previously been proposed to be the evolutionary driver for gigantism in Lamniformes. However, we contend that ovoviviparous reproduction involving intrauterine cannibalism, a possible synapomorphy of Lamniformes, to be another plausible driver for the evolution of endothermy achieved by certain lamniform taxa.