Triassic: the crucial period of post-Palaeozoic crinoid diversification
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Triassic: the crucial period of post-Palaeozoic crinoid diversification Hans Hagdorn
Received: 26 July 2010 / Accepted: 24 September 2010 / Published online: 7 December 2010 Ó Akademie der Naturwissenschaften Schweiz (SCNAT) 2010
Abstract After their near-extinction around the end of the Permian, crinoids recovered during the Triassic and reoccupied 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
H. Hagdorn (&) Muschelkalkmuseum Ingelfingen, Schloss-Str. 11, 74653 Ingelfingen, Germany e-mail: [email protected]
was added. Crinoidal limestones, as common in the Palaeozoic, had their last appearance in Middle Triassic times. Keywords Crinoids Triassic Diversity Functional morphology Evolutionary biology Palaeoecology Niche adaptation
Introduction Triassic crinoids gained scientific interest (Agricola 1546) long before their echinoderm nature was known (Rosinus 1719). Encrinus liliiformis, the stone lily of the eighteenth century naturalists, was a favorite object in art cabinets. Moreover, crinoidal limestones from the Muschelkalk were named by early stratigraphers using Agricola’s term trochites (wheelstones) for the cylindrical encrinid columnals. The Trochitenkalk (Schlotheim 1820) has nowadays the rank of a formation. Since then, Encrinus liliiformis has become one of the best known crinoids in morphological, palaeoecological and taphonomic respects, and the Trochitenkalk became a model for crinoidal limestone formation. Although several Triassic crinoid species and genera have been added subsequently (for an overview see Biese 1934 and Simms 1990b), their true diversity has long been disregarded, as many taxa
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