Rapid evolution in Turonian microcrinoids (Crinoidea, Roveacrinidae) and its significance for Late Cretaceous stratigraphy

Pelagic microcrinoids from the lower and middle Turonian chalks of the Anglo-Paris Basin (southern England, northern France) have distinctive morphologies that demonstrate rapid evolution among three genera, Dentatocrinus Gale, 2019a, Drepanocrinus Jækel, 1918 and Striacrinus gen. nov., and their constituent species and subspecific formae. These form the basis for a refined biostratigraphic zonation that can be applied across the basin. The genus Striacrinus is described as new (type species Drepanocrinus striatulus Gale, 2019a), with a new species S. ornatus described from the middle Turonian. A succession of stratigraphically restricted formae are recognised within Dentatocrinus dentatus Gale, 2019a, described herein as D. dentatus forma inflatus nov., D. dentatus forma subspinosus nov., D. dentatus forma spinosus nov., and D. dentatus forma conicus nov. These formae had ranges with a mean duration of 100–200 kyr as determined from orbital precession cycles.