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

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.

KEYWORDS:

• Chondrichthyes
• Elasmobranchii
• fossil record
• intrauterine cannibalism
• tooth

Acknowledgements

We thank H. Maisch (William Paterson University), E. Kast, and A. Akhtar (Princeton University) for their discussion with us on extinct and extant sharks, as well as two anonymous reviewers whose constructive comments and suggestions greatly improved the quality of this paper. The first author (KS) thanks the following individuals who provided access to the specimens used in this study over the past 20+ years: B. Brown, R.A. Arrindell (AMNH); A. Y. Suzumoto (BPBM); D. Catania, T. Iwamoto (CAS); C McMahan, M. A. Rogers, W. Simpson, K. Swagel, M. W. Westneat, P. Willink (FMNH); K. Nakaya (HUMZ); R. Feeney, J. A. Seigel (LACM); K. E. Hartel, A. Williston (MCZ); B. Frable, C. Klepadlo, P. A. Hastings, H. J. Walker (SIO); L. M. Page, R. H. Robins (UF); J. Finan, L. Palmer, S. Raredon, S. Smith; E. Wilbur, D. Pitassy, J. T. Williams (NMNH); N. Kohno, and C. Sakata (NSM). This research was funded in part by a National Science Foundation Sedimentary Geology and Paleobiology Award to Kenshu Shimada (Award Number 1830858), and Michael Griffiths and Martin Becker (Award Number 1830581). In addition, various support provided by the Department of Environmental Science and Studies and Department of Biological Sciences at DePaul University is appreciated.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes added at proof

After this present manuscript went into production, a new paper written by Cooper et al. (2020) hypothesizing the body form in lateral view and lifestyle of Otodus megalodon appeared. In their paper, 16 m TL was given as a ‘conservative maximum body size’ by citing Shimada’s (2019) paper. However, for the sake of scientific accuracy, we must emphasize that the range of 14.1–15.3 m TL is the scientifically justifiable maximum TL estimates for O. megalodon at the present time based on voucher specimens in museum collections under public trust (Shimada, 2019; this present paper). This does not mean that individuals of O. megalodon exceeding the size range did not exist (Shimada, 2019), but unnecessarily inflated size estimates without justifiable evidence must be avoided in scientific literature.

Additional information

Funding

This work was supported in part by the U.S. National Science Foundation Sedimentary Geology and Paleobiology Award [Award Number 1830858]; U.S. National Science Foundation Sedimentary Geology and Paleobiology Award [Award Number 1830581].