The Pelecocrinidae

The latter part of the Palaeozoic represented a peak in crinoid diversity. More families of crinoid have been recognised from the Carboniferous and Permian than any period before or since. Among the various families of the Late Palaeozoic were representatives of the Pelecocrinidae.

Pelecocrinus insignis, from Moore & Teichert (1978).

The fossil record of the pelecocrinids was long-lasting but patchy. They are known from the early Carboniferous of North America (Pelecocrinus) and Great Britain (Forthocrinus), the late Carboniferous of North America (Exoriocrinus), and the late Permian of Italy (Tetrabrachiocrinus) and Indonesia (Drepaocrinus, Malaiocrinus). These locations largely correspond to what would have been a distribution along the northern coast of the Palaeotethys Ocean, possibly becoming extinct in the west as the gap between North America and the southern continent of Gondwana closed to form the Pangean supercontinent. Pelecocrinids seem to be so far unknown from the southern continents.

In life, pelecocrinids were characterised by a high crown arising from a low, bowl-shaped cup. The stem could be round or pentagonal. The base of the cup was flattened or shallowly concave, so the infrabasals (the lowest circle of plates above the stem) were barely or not visible where the cup to be observed from the side. The articulations between the upper plates of the cup and the bases of the arms were angled downwards and outwards with the articular facets being somewhat narrower than the plates they sat on. The arms themselves had a wedge-shaped cross-section and divided into equal branches two or more times along their length. Each arm bore one or two rows of pinnules. The anal sac, as described for Pelecocrinus, was relatively short and slender and summited by heavy, spinose plates.

The structure of the arm articulations and pinnules indicates that the arms would have been subject to muscular control with individual arms being able to be moved in more than one plane. This arrangement became increasingly common among crinoids from the Carboniferous onwards, allowing them to function in more high-current habitats. The position of the arms and pinnules could be adjusted to optimise filtration from the water column, while the current provided lift to the crown so it did not need to be mechanically supported by the stem alone. The effectiveness of this arrangement is attested to by the long history of the pelecocrinids. Nevertheless, the end-Permian extinction was to end their lineage along with that of so many of their contemporaries.

REFERENCE

Moore, R. C., & C. Teichert (eds) 1978. Treatise on Invertebrate Paleontology pt T. Echinodermata 2. Crinoidea vol. 2. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).