Unattached paleozoic stemless crinoids as environmental indices

Stemlessness in Paleozoic crinoids was apparently an adaptation that allowed some crinoids to exploit environments and feeding methods not commonly used by their stemmed counterparts. Most stemless crinoids inhabited shallow, high-energy, sometimes turbid environments at the edges of transgressive and regressive seas. Common environments include shoals, reefs, shoreface, and delta destruction. Development of stemlessness in a number of families at the same time in local areas may reflect episodes of synchronous convergent evolution. Such episodes occurred during the Early-Middle Ordovician in Europe, the Early Devonian in Europe and North America, the Carboniferous in North America, and the Early Permian in northeastern Europe and the East Indies. Accompanying adaptations in the arms and calyx enabled life directly on the bottom and possible part-time deposit feeding.