Cenozoic megatooth sharks occupied extremely high trophic positions

Area of Science:

• Paleontology
• Paleoecology
• Biogeochemistry

Background:

• Trophic position is crucial for understanding animal ecology but is often unknown for extinct species.
• Nitrogen isotope ratios (δ¹⁵N) in bone collagen are established proxies for trophic level.
• Enameloid-bound organic matter (δ¹⁵N_EB) offers a potential alternative for fossilized teeth.

Purpose of the Study:

• To validate the use of δ¹⁵N_EB as a reliable proxy for trophic position in extinct marine vertebrates.
• To reconstruct the trophic ecology of the extinct megatooth shark genus, *Otodus*, including *Otodus megalodon*.

Main Methods:

• Ground-truthing the δ¹⁵N_EB proxy by comparing it with dentin collagen δ¹⁵N values in extant and fossil taxa.
• Applying the validated δ¹⁵N_EB method to fossil teeth of *Otodus* sharks from the Cenozoic.
• Analyzing nitrogen isotope ratios to infer the diet and trophic level of these extinct predators.

Main Results:

• The δ¹⁵N_EB proxy demonstrated a strong correlation with established trophic level indicators.
• Fossil *Otodus megalodon* exhibited exceptionally high δ¹⁵N_EB values (22.9 ± 4.4‰), indicating a top-tier trophic position.
• Analysis revealed a dietary shift within the *Otodus* lineage, with the highest trophic level potentially preceding peak body size.

Conclusions:

• δ¹⁵N_EB is a robust and applicable proxy for reconstructing the trophic position of extinct marine vertebrates, especially when collagen is absent.
• *Otodus megalodon* occupied a higher trophic level than any previously known marine species, extinct or extant.
• The evolution of the megatooth shark lineage involved significant dietary adaptations and a dynamic relationship between trophic level and body size.