Tessellated calcified cartilage and placoid scales of the Neogene megatooth shark, Otodus megalodon (Lamniformes: Otodontidae), offer new insights into its biology and the evolution of regional endothermy and gigantism in the otodontid clade

ABSTRACT

The late Neogene megatooth shark, Otodus megalodon (Lamniformes: Otodontidae), is mostly known for its gigantic teeth and vertebrae. Re-examination of the rock matrix surrounding a previously described associated tooth set of O. megalodon from the upper Miocene of Japan resulted in the observation of numerous fragments of tessellated calcified cartilage and placoid scales. The morphology of each tessera and the arrangement of overall tessellated calcified cartilage are practically identical to those of extant chondrichthyans. Many placoid scales possess pronounced, rather broadly-spaced keels. A quantitative relationship between interkeel distances of keeled scales and reported cruising speeds across extant pelagic lamniforms and carcharhiniforms suggests that O. megalodon with a representative interkeel distance of ca. 100 µm was not a fast swimmer. We propose that O. megalodon was generally a slow cruising shark with occasional burst swimming for prey capture, where much of its metabolic heat through regional endothermy was possibly used to facilitate the digestion of large pieces of ingested meat as well as absorbing and processing nutrients. If so, the relative importance of the functional roles of regional endothermy possibly shifted from maintaining high cruising speeds to visceral food processing through the evolution towards gigantism in otodontids.

KEYWORDS:

• Dermal denticles
• fossil record
• Japan
• miocene
• swimming
• tesserae

Acknowledgments

The first author (KS) takes this opportunity to thank all authors of Uyeno et al. (1989) (T. Uyeno, O, Sakamoto, and H. Sekine) as well as those who were involved in the excavation of the original tooth set material (I. Okabe, S. Shinozaki, and T. Naito as noted by Uyeno et al. 1989), for their mentorship and friendship during his teenage years. Besides the second author (YY) who arranged KS’s examination of the rock samples for the purpose of this study, KS is also delighted to coauthor this work with the third (YK) and fourth (YT) authors, who were his ‘fossil hunting buddies’ in the mid-1980s. The serendipitous nature of this study is particularly notable as KS vividly recalls a personal phone call he received from H. Sekine in 1986 starting with “Guess what I found?” [in Japanese] soon after Sekine first discovered the original Otodus megalodon tooth set described by Uyeno et al. (1989). Logistical support provided by the Department of Environmental Science and Studies and the Department of Biological Sciences at DePaul University, Chicago, Illinois, USA, is also appreciated. We also thank J. A. Cooper and V. J. Perez for their review which greatly improved the quality of this paper.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was financially supported by a National Science Foundation Sedimentary Geology and Paleobiology Award to M.L.G. and M.A.B. (Award #1830581), R.A.E. (Award #1830638), and K.S. (Award #1830858).