Zinc isotopes as a novel dietary proxy for unravelling the ecology of extant and fossil sharks

• Applicant: Dr. Jeremy Michael McCormack
• Subject Area: Palaeontology
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 505905610

An animal’s diet defines its lifestyle and ecological niche thereby controlling the evolution of a species. For instance, the adaptation to specific prey is hypothesised to be the reason why Otodus megalodon became the largest carnivorous shark that ever existed, but also why it became extinct. Yet, identifying the diet of long extinct animals is problematic and commonly relies on anatomical evidence, or snapshots of prehistoric interactions such as identifiable remains in fossilised stomach content and faeces or bite marks. These methods, however, do not necessarily reflect a species regular diet. The nitrogen isotope composition of bone collagen is a well-established method indicating a species trophic level but is limited by the degree of protein preservation and typically not applicable for fossils older than 100,000 years. Only recently, the isotopic composition of zinc (δ66Zn) in bones and teeth of terrestrial and marine mammals was shown to record dietary and trophic level information. Because zinc is incorporated into the mineral phase of skeletal elements, it should exhibit a better long-term preservation potential compared to collagen-bound nitrogen. Indeed, preliminary data demonstrate, that the δ66Zn values of extant teleost fish and shark teeth are highly sensible recorders of the animal’s diet and that pristine dietary δ66Zn values can be preserved in fossil shark teeth (the most common vertebrate fossils) over millions of years. Building upon these initial results, this project proposes applying δ66Zn to assess the diet and ecology of modern and ancient sharks, aiming to better understand their ecological niches, evolution, and role as apex predators. Teeth and diet δ66Zn values of modern sharks from aquariums and teleost fish from a pisciculture will be analysed. Further, the project aims to understand δ66Zn discrimination within the full range of trophic levels of a modern marine food web from phytoplankton to apex predators. This will allow determining diet-tissue Zn discrimination factors and identifying ingested food-items based on tooth δ66Zn values and can be applied to the fossil record. The δ66Zn variability among Cenozoic megatoothed sharks from various localities and ages will be investigated with a special focus on O. megalodon. The goal of this project is to trace dietary and ecological changes through time among the megatooth lineage and sympatric shark species as well as investigate factors contributing to the rise and fall of some of the largest marine predators of Earth’s history. This project will establish the first deep-time application of δ66Zn on fossil vertebrates, thereby providing a much-needed geochemical tool to investigate the diet and trophic ecology of extinct animals.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Wolfgang Müller