Developmental transformations in Jurassic driftwood crinoids

Abstract

The internal, multi-element skeleton of echinoderms can differentiate almost as much as that of the vertebrates. The implied probability of post-mortem disarticulation was bypassed in pelagic species, whose carcasses could sink directly onto toxic sea bottoms. Among them were isocrinids attached to driftwood. In the Toarcian Posidonia Shales they are represented by two easily distinguishable forms: (1) the giant and long-stemmed Seirocrinus was probably a passive tow-net filtrator; (2) the smaller and heavily cirrated Pentacrinites may have produced its own filter current, even though this assumption is in conflict with the distribution of muscles in present-day benthic Isocrinida. In a newly discovered colony of the smaller Early Liassic Pentacrinites fossilis, both strategies are found together. This leads to the hypothesis that the two Toarcian forms may also represent developmental stages of the same species. After the Toarcian, Seirocrinus (as either a taxon or a developmental stage) became extinct, probably due to shipworms reducing the drifting time of logs. In contrast, the proposed feeding style of Pentacrinites allowed it to return to the benthic realm, but in an inverted attitude. During this complex history, most adaptational changes required only heterochronic shifts in the development of tegmen, arms, pinnules, stem, and cirri. Unrelated pseudoplanktonic crinoids of earlier times show convergent adaptations with different pathways.