Morphology, ontogeny and phenotypic plasticity of the microcrinoid Treocrinus from the Třebotov Limestone (Devonian, Daleje-Třebotov Formation; Barrandian area, Czech Republic)

Abstract

New data on morphology, ontogeny, phenotypic plasticity and mode of life of the microcrinoid Treocrinus ericius from the Třebotov Limestone are discussed. The aboral cup shape variations from the barrel- to the bowl-shaped and the ontogenetic sequence of radial facets are presented. Some specimens were likely stemless to the adult stage, and their cup bases were directly fitted to a holdfast. The irregularity of cup shape is explained by different habitats. Individuals in the restricted, likely cryptic habitat lacked the stem. They had low barrel-like cups, whereby while those in more spaced microhabitat formed higher bowl-shaped cups. Examination of 160 specimens indicate that bowl-shaped cup represents a prevalent form of a theca. The aboral articular facet of bowl-shaped specimens keep a distinct lumen, a narrow crenularium and a shallow areola that confirm a development of a column in part of the population. The species achieved the maximum occurrence in the upper part of the Třebotov Limestone (Daleje-Třebotov Formation) near the Emsian-Eifelian boundary. Abundance of the Treocrinus and other microcrinoids, and abundance of associated invertebrates in the Třebotov Limestone clearly contrast with the invertebrate diversity after the Choteč Event near the base of the Choteč Formation.