SeaCACHE - Seawater Chemistry And CHondrichthyan Evolution

Chondrichthyans (sharks, rays and holocephalans) have an extremely rich and long fossil record dominated by teeth and dental plates. These mineralized structures provide a deep-time, evolutionary, perspective of the group. Recently, a hypermineralized tissue called whitlockin, containing a biomineral rich in magnesium (whitlockite), has been discovered in holocephalan dental plates and in living shark body-denticles and teeth. This unusual tissue potentially provides a strengthening function, but when did whitlockin first appear in the evolutionary history of chondrichthyans? It is known that the ratio of magnesium:calcium (Mg/Ca) in seawater has shifted through Earth history, creating intervals of 'calcite' seas and 'aragonite' seas (lower and higher Mg/Ca ratios respectively). These changes, along with temperature, apparently influenced the uptake of particular elements and minerals into the carbonate skeletons of invertebrate animals. Have marine vertebrate skeletons been shaped in a similar way? Or is their skeletal mineralization independent of the surrounding environment?

The SeaCACHE project will test these possibilities in this critical group of marine animals. There are three main aims: (1) to analyze the dentitions of a broad range of living and, for the first time, fossil chondrichthyans from the Devonian to the present day, to test for the presence of whitlockin via a combination of histological and chemical analysis; (2) to examine the relationship between the Mg/Ca content of seawater, seawater temperature, and the presence of whitlockite in chondrichthyans in modern and ancient ecosystems; and (3) to update the phylogeny of chondrichthyans as a whole to elucidate rates of evolution of whitlockite. During this project, the research fellow will gain experience in chemical, palaeonvironmental and phylogenetic analyses, in open science practices, in natural history collection management, and in other transferable skills that will substantially improve their future career as an independent researcher.