Dynamics of Selachian (Shark) Dental Morphology During the Early Mesozoic

Abstract: The ancestors of all modern day sharks and rays (Neoselachii) may have appeared during the Late Palaeozoic, but their major diversification happened sometime during the Early Mesozoic. Taxonomic evidence places the first neoselachian diversification in the Early Jurassic. Taxonomic diversity analyses, however, are often affected by incompleteness of the fossil record and sampling biases. On the other hand, the range of morphological variation (disparity) offers a different perspective for studying evolutionary patterns across time. Disparity analyses are much more resilient to sampling biases than diversity analyses, and disparity has the potential to provide a more ecologically-relevant context. This analysis focuses on the morphology of selachian teeth. Selachii include all neoselachian and stem-neoselachian sharks, but not batoids. In order to primarily test the hypothesis that the first selachian radiation took place in the Late Triassic, not the Early Jurassic, and secondarily to depict the dental evolution of sharks across time, 424 selachian teeth ranging from the Middle Triassic to the Late Jurassic were quantified, using a landmark and semi-landmark geometric morphometric method, in order to analyze patterns of selachian tooth disparity and morphospace occupation across the Early Mesozoic. The results of the analysis indicate a two-pulse radiation of the selachians. The first radiation took place in the Rhaetian (Late Triassic). The Rhaetian Transgression saw the rise of a large, shallow epicontinental sea that covered the area which is now Western Europe. The transgressive event opened new niches for the selachians, as indicated by the appearance of almost all known shark dentition types during the Rhaetian. The second radiation spans the Callovian – Oxfordian – Kimmeridgian interval. Transgressive events during the Callovian and the Oxfordian introduced new cephalopod and actinopterygian faunas to the Central European Basin, thus playing an important role in selachian dental disparity and morphospace patterns. The main drive in shark evolution across the Early Mesozoic seems to have been the breakup of Pangaea. Transgressive/regressive events tied to tectonic activity affect tooth disparity and, indirectly, influence shark dentition patterns, by directly affecting the diversity of other faunas which, in turn, are preyed upon by selachians of the Early Mesozoic.