Evolution & Systematics

Fossil Record

There is a rich fossil record of Crinoids due to the large number of ossicles that are embedded throughout the body’s connective tissues (Ruppert et al 2004). The information provided by these fossil records help follow the evolutionary history of crinoids. Over 6000 species have been described through the fossil record, with the earliest crinoid found to resemble the sea lily’s stalked form (Alender et al 1966, Meyer and Macurda 1977).

Evolution

A number of evolutionary aspects of crinoids can be traced through the fossil record and through its phylogenetic relationship with other echinoderm classes.

The fossil record provides a view of the ancestral body in the form of a stalked crinoid and the evolution of the free living crinoids, such as C.glebosus, which have evolved several times in the past (Alender et al 1966).

The wide distribution of the vitelleria (doiliodoria) larva among echinoderms suggests that this larval form was the ancestral echinoderm larva (Alender et al 1966).

The regeneration abilities of crinoids are comparable to holothurians but not to asterozoans due to its inability to asexually reproduce by spontaneously subdividing, making asexual reproduction relatively advanced derived echinoderms feature (Alender et al 1966).

Comatulid crinoids have been found to be in the highest grade of evolutionary development within Crinoidea. Their evolved traits include the role of spination and stiffening of the oral pinnules for defense, crawling and swimming capabilities that allow for predator avoidance, movement to greater food availability and stress avoidance, the loss of the attachment stalk for greater dispersal and movement, and the development and adaptive features of the cirri to allow for rocky substrate attachment and movement (Meyer and Macruda 1977).

Crinoids have adapted many new body forms and structures that allow it to survive through various geological periods to become better suited to a changing world but increasing its mobility and feeding structures.

For further reading on Comatulid evolution please see Meyer and Macruda (1977).

Phylogeny

Recent molecular analysis of extant echinoderms have confirmed the paleontological evidence that crinoids are the sister group to the remaining extant echinoderms (Cohen et al 2004), with the current classifications of crinoids dividing it into Bourgueticrinida, Millericrinida, Cyrtocrinidia and Isocrinidia as those comprising 95 species of‘Sea Lily’ (Roux et al 2002) and the Comatulidia which contains 500 species of ‘Feather Stars’ (Messing 1997).