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. 2023 Jul 4;120(27):e2218153120.
doi: 10.1073/pnas.2218153120. Epub 2023 Jun 26.

Endothermic physiology of extinct megatooth sharks

Michael L Griffiths  1 , Robert A Eagle  2   3 , Sora L Kim  4 , Randon J Flores  2   3 , Martin A Becker  1 , Harry M Maisch 4th  5 , Robin B Trayler  4 , Rachel L Chan  4 , Jeremy McCormack  6 , Alliya A Akhtar  1   7 , Aradhna K Tripati  2   3 , Kenshu Shimada  8   9   10
Affiliations

Endothermic physiology of extinct megatooth sharks

Michael L Griffiths et al. Proc Natl Acad Sci U S A. .

Abstract

The evolution of the extinct megatooth shark, Otodus megalodon, and its close phylogenetic relatives remains enigmatic. A central question persists regarding the thermophysiological origins of these large predatory sharks through geologic time, including whether O. megalodon was ectothermic or endothermic (including regional endothermy), and whether its thermophysiology could help to explain the iconic shark's gigantism and eventual demise during the Pliocene. To address these uncertainties, we present unique geochemical evidence for thermoregulation in O. megalodon from both clumped isotope paleothermometry and phosphate oxygen isotopes. Our results show that O. megalodon had an overall warmer body temperature compared with its ambient environment and other coexisting shark species, providing quantitative and experimental support for recent biophysical modeling studies that suggest endothermy was one of the key drivers for gigantism in O. megalodon and other lamniform sharks. The gigantic body size with high metabolic costs of having high body temperatures may have contributed to the vulnerability of Otodus species to extinction when compared to other sympatric sharks that survived the Pliocene epoch.

Keywords: Otodus megalodon; clumped isotopes; extinction; fossil; regional endothermy.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Shark teeth from various Miocene (triangle) and Pliocene (circle) localities provide phosphate oxygen isotope compositions (δ18Op) that reveal thermal physiology with temperature differences among sampled taxa. (A) The five localities provide context for Otodus (red) with comparisons to ectothermic (blue) sharks, predicted or known (yellow and orange based on extant Isurus oxyrinchus and Carcharodon carcharias, respectively) regionally endothermic (r. endothermic) sharks, and endothermic marine mammals (dark red). (B) Based on these δ18Op values, a Bayesian model correctly predicts body temperatures of endothermic marine mammals (dark red), distinguishes thermal differences among ectothermic (blue) and regionally endothermic (yellow and orange) sharks, and indicates elevated body temperatures in Otodus (red) similar to or beyond the extant C. carcharias. The Bayesian δ18Op–based temperature model is described in the Materials and Methods section briefly with greater detail in SI Appendix, Text.
Fig. 2.
Fig. 2.
Modern Δ47–temperature calibration from wild-caught and aquarium-reared elasmobranch teeth, along with wild-caught bony fish and mammals. Lines indicate linear regressions calculated from our data (orange line and colored symbols) along with that of Anderson et al. (47) (blue line). Shaded regions surrounding regression lines indicate 95% CI. Error bars in the x and y directions on symbols indicate uncertainty in temperature and one external SE of the average Δ47, respectively. Uncertainty in regression parameters indicates one SE.
Fig. 3.
Fig. 3.
Body temperature reconstructions of O. megalodon, Mysticeti, and the Carcharodon lineage (C. hastalis and C. carcharias) from the eastern United States (North Carolina) and Japan from Δ47 (n = 53) and δ18Op (n = 39) for the Pliocene. Shark illustrations by Christina Spence Morgan, copyright 2021.
See this image and copyright information in PMC

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