The fossil document of the pelecocrinids was long-lasting however patchy. They’re identified from the early Carboniferous of North America (Pelecocrinus) and Nice Britain (Forthocrinus), the late Carboniferous of North America (Exoriocrinus), and the late Permian of Italy (Tetrabrachiocrinus) and Indonesia (Drepaocrinus, Malaiocrinus). These areas largely correspond to what would have been a distribution alongside the northern coast of the Palaeotethys Ocean, presumably turning into extinct within the west because the hole between North America and the southern continent of Gondwana closed to type the Pangean supercontinent. Pelecocrinids appear to be to date unknown from the southern continents.
In life, pelecocrinids have been characterised by a excessive crown arising from a low, bowl-shaped cup. The stem might be spherical or pentagonal. The bottom of the cup was flattened or shallowly concave, so the infrabasals (the bottom circle of plates above the stem) have been barely or not seen the place the cup to be noticed from the aspect. The articulations between the higher plates of the cup and the bases of the arms have been angled downwards and outwards with the articular sides being considerably narrower than the plates they sat on. The arms themselves had a wedge-shaped cross-section and divided into equal branches two or extra occasions alongside their size. Every arm bore one or two rows of pinnules. The anal sac, as described for Pelecocrinus, was comparatively brief and slender and summited by heavy, spinose plates.
The construction of the arm articulations and pinnules signifies that the arms would have been topic to muscular management with particular person arms with the ability to be moved in a couple of aircraft. This association grew to become more and more widespread amongst crinoids from the Carboniferous onwards, permitting them to perform in additional high-current habitats. The place of the arms and pinnules might be adjusted to optimise filtration from the water column, whereas the present offered raise to the crown so it didn’t should be mechanically supported by the stem alone. The effectiveness of this association is attested to by the lengthy historical past of the pelecocrinids. However, the end-Permian extinction was to finish their lineage together with that of so lots of their contemporaries.
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 College of Kansas: Lawrence (Kansas).