Quantitative analysis of elasmobranch assemblages from two successive Ypresian facies at Marke, western Belgium

Temporal patterns in biodiversity are affected largely by changes in environmental conditions. Sea level fluctuations rank amongst the major factors that affect marine biodiversity or community structure on a local or regional scale, as confirmed by numerous case studies relating lithology with fossil assemblages in order to reconstruct palaeoenvironmental conditions. However up to now, few studies quantified selachian and batoid faunas (Elasmobranchii) in such a context. In the present study, we compare elasmobranch teeth from two successive facies of Ypresian (Early Eocene) age, as exposed at the Marke clay pit in the southern part of the Belgian Basin. We present a significant link between the difference in lithology of these levels and elasmobranch community structure. In general, selachians are notably more common in clayey levels, while batoids predominate in sandy levels. Following the principle of uniformitarianism, such a link indicates that the recognised patterns in elasmobranch diversity depend mainly on the preferred sea level and/or habitat requirements by a species or a species group, in analogy to what is seen in modern communities. Additional notes on the palaeoenvironment are presented, as well as a list of 36 elasmobranch taxa from Marke, including a number of new recorded taxa for the Ypresian of Belgium.