Cenozoic megatooth sharks occupied extremely high trophic positions

Abstract

Trophic position is a fundamental characteristic of animals, yet it is unknown in many extinct species. In this study, we ground-truth the 15 N/ 14 N ratio of enameloid-bound organic matter (δ 15 N EB ) as a trophic level proxy by comparison to dentin collagen δ 15 N 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 δ 15 N EB 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. δ 15 N EB 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.

Nitrogen isotopic composition of tooth enamel organic matter records trophic position in modern and fossil ecosystems

“…These observations are consistent with results of laboratory degradation experiments 37 , and with measurements of million-year-old marine microfossils 38,86,87 , which suggest that biomineral structures act as an effective physical barrier that protect organic matter from degradation. Although it is a small dataset, the paired δ 15 Nenamel and δ 15 Ndentin-collagen values for the four fossil specimens are also positively correlated, with a relationship similar to that between δ 15 Nenamel and δ 15 Nbone-collagen values in modern mammals (see Fig.…”

“…Taken together, these observations suggest that enamel-bound organic matter is highly protected and therefore has strong potential to preserve an in vivo nitrogen isotope signal even in diagenetically altered settings. Further evidence for this comes from the work of Kast et al 38 , who successfully reconstructed the trophic behavior of extinct megatooth sharks across the Cenozoic (~66 to 3.5 Ma) using δ 15 N values obtained from analysis of the enameloid of fossil shark teeth. It remains an open question, however, if fossil mammalian enamel preserves diet and trophic level information in a similar manner to the enameloid of shark teeth, given both the differences in mineral structures between mammalian enamel (hydroxylapatite) and shark enameloid (fluorapatite), as well as the differences in depositional environments.…”

Laboratory Assessment of the Impact of Chemical Oxidation, Mineral Dissolution, and Heating on the Nitrogen Isotopic Composition of Fossil‐Bound Organic Matter

“…The stable isotopes of nitrogen ( 14 N and 15 N) can offer important insights into present and past changes in the cycling of this key element through organisms, food webs, and environments (Casciotti, 2016; Deniro & Epstein, 1981; Fripiat et al., 2021; Sigman & Fripiat, 2019; Straub et al., 2021; Wolf et al., 2009). Their use in paleo‐reconstructions requires the development of faithful geochemical archives that are unaffected by diagenetic alteration and/or contamination by exogenous N. In recent years, the analysis of the N isotopic composition of the organic matter bound within the mineral structure of fossil skeletons (e.g., foraminifera, corals, diatoms, otoliths, and tooth enamel) has emerged as a promising archive of the original isotopic signature of the organism that is protected from degradation for thousands to millions of years (Ai et al., 2020; Altabet & Curry, 1989; Duprey et al., 2020; Erler et al., 2016, 2020; Farmer et al., 2021; Kast et al., 2019, 2022; Leichliter et al., 2021; Lueders‐Dumont et al., 2018; Martinez‐Garcia et al., 2014; Ren et al., 2009, 2017; Robinson et al., 2004, 2005; Shemesh et al., 1993; Sigman et al., 1999, 2021; Straub et al., 2013; Studer et al., 2015, 2018, 2021; Wang et al., 2014, 2016, 2017).…”

Exploring diet shifts and ecology in modern sharks using calcium isotopes and trace metal records of their teeth

“…opportunity to investigate the diet of extinct elasmobranch species (Kast et al, 2022;McCormack et al, 2022). All authors drafted the manuscript and gave final approval for publication.…”

“…In recent decades, the use of stable isotopic analyses (SIA) on soft and hard tissues using nitrogen (δ 15 N) and carbon (δ 13 C) isotopes has been proposed to investigate the ecology and feeding behaviours of shark species (Barría et al, 2018;Cree et al, 1999;Estrada et al, 2006;Kast et al, 2016Kast et al, , 2022Tierney et al, 2008;Vennemann et al, 2001).…”