On Jurassic and Cretaceous disasterid deep-sea echinoids

In Jurassic times, the superorder Atelostomata was only represented by disasterid echinoids with a disjunct apical system. Groups with joint apical system, holasterids and toxasterids, appeared in the Early Cretaceous (Berriasian). They gave rise to numerous Mesozoic and Cenozoic atelostome groups. Pushing out of disasterid echinoids from marine habitats happened gradually during the Neocomian. Holasterids and toxasterids became dominant groups and spread globally, while the distribution areas of disasterid echinoids became much smaller in the Early Cretaceous. Three species of disasterid echinoids from a red Lower Barremian limestone from South-Western Crimea have been investigated - Tetraromania ovulum, Corthya ambigua, Tithonia exile. According to some authors this limestone is analogous to the "Ammonitico Rosso" facies, deposited at 500-600 m depth. This depth is also attested by the analysis of the strength index of ammonite shells, found in the same deposit. Some features of the echinoid species (thin test, simple nonpetaloid ambulacra, disjunct apical system with genital pores 1 and 4 located on the ocular plates II and IV, etc.) demonstrate strong convergent similarity with Recent deep-sea pourtalesiids. These features also imply living in relatively deep waters. This conclusion can be compared to recently published data on Tithonia oxfordiana from the Oxfordian of South-East France that are associated with deep water seeps deposits (Gaillard et al., 2011). Other species of this genus from Late Callovian and Tithonian apparently also lived in relatively deep water. Thus the migration of some groups of disasterid echinoids to deep water habitats might have happened as early as the late Middle Jurassic.