EVOLUTIONARY HISTORY OF CRINOID REGENERATION AND AUTOTOMY ABILITIES

Regeneration and autotomy abilities have been well documented among extant crinoids, but little heed has been given to the evolutionary history of these traits. Fossils indicate that crinoids have had the ability to regenerate since the Ordovician, an ability that was likely inherited from stem-group echinoderms.

Regeneration occurs in all parts of the crinoid skeleton and may be recognized by the presence of size discontinuities among adjacent plates, aberrant branching, and carbon isotopes. However, crinoids are not able to recover from all damage: reparative plates and overgrowths are evidence of non-regenerative healing. Paleozoic crinoids exhibit the same range of repair abilities as extant crinoids. With few exceptions, and despite evidence for increasing predation pressure through the Phanerozoic, crinoids show sparse evidence of changing regeneration potential through time.

However, crinoids’ abilities to autotomize, or voluntarily shed body parts, have changed through the Phanerozoic. For example, autotomy of the viscera is known to occur only in post-Paleozoic comatulids. Moreover, only comatulids bear true syzygies in their arms, which are more fully specialized for autotomy than other ligamentary articulations.

Arm autotomy is widely considered to be a post-Paleozoic phenomenon. However, fossil evidence suggests that crinoid arm autotomy abilities extend back at least as far as the Mississippian, and even among Ordovician crinoids, patterns of arm regeneration in some specimens are difficult to explain without invoking autotomy.

Definitive evidence for stalk autotomy is known only from the post-Paleozoic. Stalk autotomy would be expected to be advantageous only in clades with well-developed muscular arms and the ability to reattach if dislodged. It is therefore plausible that this ability began in the Paleozoic among some groups of advanced cladids.

Evidence from temporal changes in regeneration frequency and the evolutionary ecology of crinoids and their potential consumers indicate that biotic interactions are the most likely causes for changes in crinoid autotomy abilities through time.