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

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.

KEYWORDS:

• Body form
• endothermy
• evolution
• geometric morphometrics
• phylogeny
• shark

Acknowledgments

Earlier versions of this work were reviewed by many people, including M. A. Becker, M. L. Griffiths, J. Kriwet, J. Liston, and H. M. Maisch, IV, as well as J. A. Cooper, C. Pimiento, H. G. Ferrón and M. J. Benton, and we thank their input. We thank Stephen Godfrey and an anonymous reviewer who greatly improved the quality of this manuscript. We also thank the following individuals for the loan of the specimens illustrated in Figure 7 to one of us (KS) many years ago: J. A. Seigel (LACM), D. W. Nelson (UMMZ), and J. Finan, L. Palmer, S. Raredon, S. Smith, E. Wilbur, D. Pitassy, and J. T. Williams (USNM). This work was partially supported by the National Science Foundation Sedimentary Geology and Paleobiology Award to KS (Award Number 1830858).

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the National Science Foundation Sedimentary Geology and Paleobiology Award [1830858].