Abstract
Abstract. Chondrichthyan teeth (sharks, rays, and chimaeras) are mineralized in isotopic equilibrium with the surrounding water, and parameters such as water temperature and salinity can be inferred from the oxygen isotopic composition (δ18Op) of their bioapatite. We analysed a new chondrichthyan assemblage, as well as teeth from bony fish (Pycnodontiformes). All specimens are from Kimmeridgian coastal marine deposits of the Swiss Jura (vicinity of Porrentruy, Ajoie district, NW Switzerland). While the overall faunal composition and the isotopic composition of bony fish are generally consistent with marine conditions, unusually low δ18Op values were measured for the hybodont shark Asteracanthus. These values are also lower compared to previously published data from older European Jurassic localities. Additional analyses on material from Solothurn (Kimmeridgian, NW Switzerland) also have comparable, low-18O isotopic compositions for Asteracanthus. The data are hence interpreted to represent a so far unique, freshwater-influenced isotopic composition for this shark that is classically considered a marine genus. While reproduction in freshwater or brackish realms is established for other hybodonts, a similar behaviour for \\textit{Asteracanthus} is proposed here. Regular excursions into lower salinity waters can be linked to the age of the deposits and correspond to an ecological adaptation, most likely driven by the Kimmeridgian transgression and by the competition of the hybodont shark Asteracanthus with the rapidly diversifying neoselachians (modern sharks).
Highlights
Chondrichthyan teeth are mineralized in isotopic equilibrium with the surrounding water, and parameters such as water temperature and salinity can be inferred from the oxygen isotopic composition (δ18Op) of their bioapatite
While the overall faunal composition and the isotopic composition of bony fish are generally consistent with marine conditions, unusually low δ18Op values were measured for the hybodont shark Asteracanthus
Chondrichthyan remains are common in the Mesozoic fossil record of Western Europe, and in many different palaeoenvironmental settings (Duffin and Thies, 1997; Müller, 2011; Underwood, 2002)
Summary
Chondrichthyan remains are common in the Mesozoic fossil record of Western Europe, and in many different palaeoenvironmental settings (e.g. lagoonal, open marine, reduced salinity) (Duffin and Thies, 1997; Müller, 2011; Underwood, 2002) Their teeth are predominantly composed of fluorapatite, the most resistant variety of apatite (Vennemann et al, 2001) and are continuously shed and replaced, except in chimaeras (Cappetta, 2012; Stahl, 1999). In addition to their abundance, their mechanical and chemical resistance make them an ideal material for stable isotope analyses They mineralize in isotopic equilibrium with the surrounding water, their primary oxygen isotopic composition (δ18Op) reflects that of the ambient water at a given temperature when they formed (Kolodny et al, 1983; Longinelli and Nuti, 1973). Based on phosphate oxygen isotope analyses obtained from this Late Jurassic chondrichthyan fauna, this study proposes answers to the following questions: (1) is there any unexpected isotopic composition for the associated marine fauna recorded in Porrentruy? (2) Are the Porrentruy isotopic data unique so far, or comparable to other European localities? (3) What do we learn about the palaeoecology of the hybodont shark Asteracanthus based on the isotopic composition?
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
