Cenozoic megatooth sharks occupied extremely high trophic positions - PubMed (original) (raw)

. 2022 Jun 24;8(25):eabl6529.

doi: 10.1126/sciadv.abl6529. Epub 2022 Jun 22.

Michael L Griffiths 3, Sora L Kim 4, Zixuan C Rao 1, Kenshu Shimada 5 6 7, Martin A Becker 3, Harry M Maisch 8, Robert A Eagle 9, Chelesia A Clarke 3, Allison N Neumann 3, Molly E Karnes 4, Tina Lüdecke 10 11, Jennifer N Leichliter 10 12, Alfredo Martínez-García 13, Alliya A Akhtar 1, Xingchen T Wang 14, Gerald H Haug 13 15, Daniel M Sigman 1

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Cenozoic megatooth sharks occupied extremely high trophic positions

Emma R Kast et al. Sci Adv. 2022.

Abstract

Trophic position is a fundamental characteristic of animals, yet it is unknown in many extinct species. In this study, we ground-truth the 15N/14N ratio of enameloid-bound organic matter (δ15NEB) as a trophic level proxy by comparison to dentin collagen δ15N and apply this method to the fossil record to reconstruct the trophic level of the megatooth sharks (genus Otodus). These sharks evolved in the Cenozoic, culminating in Otodus megalodon, a shark with a maximum body size of more than 15 m, which went extinct 3.5 million years ago. Very high δ15NEB values (22.9 ± 4.4‰) of O. megalodon from the Miocene and Pliocene show that it occupied a higher trophic level than is known for any marine species, extinct or extant. δ15NEB also indicates a dietary shift in sharks of the megatooth lineage as they evolved toward the gigantic O. megalodon, with the highest trophic level apparently reached earlier than peak size.

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Figures

Fig. 1.

Fig. 1.. Comparison between δ15NEB and dentin collagen δ15N.

(A) δ15NEB versus dentin collagen δ15N for 13 modern C. taurus teeth. Open circles show each measured sample, the solid line is a Deming regression with a bootstrapped 95% CI shown by the shaded gray region, and the dashed line is 1:1. (B) Difference between δ15NEB and dentin collagen δ15N for the same measured samples (open circles) with a box plot of the distribution.

Fig. 2.

Fig. 2.. Shark enameloid-bound δ15N.

(A) Shark δ15NEB for each studied epoch, by taxon. Piscivorous shark teeth are plotted as gray circles. Otherwise, symbol shapes show the genus (triangle, Cretalamna; diamond, Otodus; square, Carcharodon) and colors show the species. Black symbols with error bars show the mean δ15NEB ± 1 SD for each genus. Numbers indicate the number of teeth measured. Overlaid tooth diagrams are scaled to the estimated total length of each species (22). The map shows locations of sampled shark teeth; white symbols are collecting localities, and larger gray circles group the teeth into broad locations, with the size according to number of teeth. (B) Species-averaged δ15NEB difference from contemporaneous piscivorous shark δ15NEB ± 1 SD. The trophic level offset (right axis) is calculated from this δ15NEB difference using a TDF of 2.5‰. Asterisks indicate species significantly different from contemporaneous piscivorous sharks (table S1).

Fig. 3.

Fig. 3.. Estimated diet δ15N of O. megalodon compared to modern shark and marine mammal δ15N.

(A) Gray bars show the estimated diet δ15N, calculated by subtracting the 1.7‰ offset between δ15NEB and dentin collagen δ15N and a TDF of 2.5‰ from each O. megalodon δ15NEB value. Curves show the distribution of modern shark (blue) and marine mammal (yellow) δ15N from the literature (data files S2 and S3). (B) Modern shark and (C) marine mammal δ15N plotted by family and sized by the number of individuals in each observation. Solid vertical line shows the average estimated diet δ15N of O. megalodon, and dashed vertical lines are the minimum and maximum estimated diet δ15N. Black x symbols in (B) are δ15NEB measurements of modern C. taurus (family Carchariidae) and C. carcharias (family Lamnidae). In (B), C. carcharias are highlighted with white filled symbols, and in (C), polar bears (family Ursidae) and transient orcas (family Delphinidae) are highlighted with the same symbols.

Fig. 4.

Fig. 4.. Trends in body size and trophic level of megatooth sharks (Otodus) through time, starting with their ancestor Cretalamna sp.

While body forms of depicted sharks are hypothetical, they are sized relative to their estimated conservative maximum body size (Cretalamna sp. 3.5 m, O. obliquus 8 m, O. auriculatus 9.5 m, O. angustidens 11.5 m, O. chubutensis 13.5 m, O. megalodon 15 m) (22) and positioned vertically by their average δ15NEB difference from contemporaneous piscivorous sharks. Ages are the boundaries between the geological time intervals (“Ma ago”, million years ago). Illustration by Christina Spence Morgan, copyright 2021.

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