Abstract

The crinoids were the most diverse and numerically most common echinoderms in the Paleozoic, as well as being one of the most important groups of sessile invertebrates. Autotomy (self-mutilation) in the stem, that is, the casting off of the more distal part, is only known to occur in one extant group of crinoids, the post-Paleozoic isocrinine sea lilies. These crinoids have a long distal stalk with regularly spaced articulations (i.e., cryptosymplexies) adapted for autotomy. They are connected together by short, mutable collagenous tissues that undergo irreversible destabilization during autotomy; the distal opening of the axial canal is then sealed by new growth of calcite. Paleozoic crinoids lacked cryptosymplectial articulations, but have other adaptations that may have singly or collectively facilitated autotomy. These include the linking of adjacent short lengths of stalk (i.e., noditaxes) by short ligaments; the evolutionary trend to narrow the axial canal; occlusion of the axial canal in some species; and the ability to seal the axial canal. Other morphological features of relevance to autotomy include the common division of the stem into morphologically distinct regions and attachment by a distal, recumbent radicular runner. The ability to autotomize the distal stem in many Paleozoic crinoid clades was, at the very least, latent; if overt, it would have facilitated escape from environmental disturbance. It may, in part, explain the great abundance of disarticulated crinoid stem fragments in many Ordovician to Permian sedimentary deposits.

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