MORPHOMETRIC STUDY OF THE LATE CRETACEOUS SHARK FAUNA FROM THE ENGLISH CHALK AND ITS PALEOECOLOGIC IMPLICATIONS

The Upper Cretaceous Chalk of southern England spanning from the Cenomanian to the Campanian stages have been a focus of paleontological research for a few decades now. As a consequence, a large number of fossils have been documented from the various Chalk divisions. These Chalks have yielded diverse genera of sharks, represented mainly by isolated teeth. Morphometric analyses of these teeth demonstrate how their shape and size varied through time, which in turn provides information on the evolution of body size and feeding strategy in these genera of sharks. Studies involved a total of 875 lamniform shark from the Chalk deposits of England, representing 14 genera of extinct sharks. Assemblages were studied across the Cenomanian/Turonian boundary and from the Lower, Middle and Upper Chalk lithostratigraphic divisions. 12 different morphometric measurements were obtained from these shark teeth, following the approach of Belben et al. (2017, PLoS ONE 12(6): e0178294), and the data were analyzed using 2 sample tests, Analyses of Variances (ANOVA), and Principal Component Analysis (PCA). The Cenomanian/Turonian boundary marks a major crisis in marine ecosystems associated with an oceanic anoxic event and the analyses focused on determining whether any significant changes occurred in the shark fauna at that time. Results show that shark teeth became rarer in the Turonian (Middle Chalk), but this may be the result of sampling bias rather than reflecting smaller population sizes. No shark species became extinct across the Cenomanian/Turonian boundary, but the morphology of their teeth underwent measurable and significant change, which likely indicates that a shift in their feeding behavior and ecology occurred during that time. Upper Chalk (Coniacian-Campanian) teeth have very similar shapes to Lower Chalk specimens, indicating a return to pre-Turonian feeding behaviors, but they are significantly larger, indicating that the body sizes of lamniform sharks increased through the Upper Cretaceous.