Triassic: the crucial period of post-Palaeozoic crinoid diversification

Abstract

After their near-extinction around the end of the Permian, crinoids recovered during the Triassic and re-occupied almost all ecological niches they had held in Palaeozoic times. Triassic crinoids comprise 33 genera in 12 well-defined families and 5 orders of the subclass Articulata; the systematic position of 4 additional families is unknown. The highest diversity was before the Mid Carnian Wet Intermezzo that caused the extinction of the order Encrinida. Major morphologic changes were connected with the adaptation to various benthic habitats and to pseudoplanktonic and eleutherozoic modes of life. Convergently, the cups of Encrinida and Holocrinida–Isocrinida became cryptodicyclic with large muscular radial facets, arm numbers increased from 5 to more than 300, and the arms of Encrinida became gradually biserial. The Encrinida remained permanently fixed to hardgrounds and acted as frame builders in bioherms. By encrusting bivalve mudstickers some dadocrinids also became secondary soft bottom dwellers. The holocrinid stem evolved preformed rupture points at the lower nodal facets, allowing these crinoids to attach intermittently by cirri. The pseudoplanktonic traumatocrinids evolved extremely long, flexible stems with multiple pore systems and terminal root cirri. Paracomatulids and eocomatulids reduced their stems and adapted to an eleutherozoic mode of life. Somphocrinids miniaturized and remodeled their skeleton towards lightweight construction and adapted to a planktonic life style. After the Triassic no fundamentally novel adaptation was added. Crinoidal limestones, as common in the Palaeozoic, had their last appearance in Middle Triassic times.