PLESIOMORPHIC CHARACTER STATES AMONG THE EARLIEST CRINOIDS

Upon first glance, it is often challenging to assign newly discovered Early Ordovician crinoids to well established crown groups. This is partly a consequence of the many morphological characters shared by most Tremadocian and Floian crinoids from all subclasses. Although it is likely crinoids originated in the Cambrian, thus far the oldest undisputed crinoids are from the earliest Ordovician.

Arguably, the earliest known crinoid is Apektocrinus from the lowermost (Tremadocian) Garden City Formation of Idaho. It shares many morphological characters with most early crinoids, traits that are largely absent in some clades by the Late Ordovician. These include: 1) a stem composed of many thin meres rather than holomeric columnals; 2) a stem with a very large lumen; 3) fixed brachials and interradials in the calyx; and 4) irregular plating of the mid-cup, possibly due to the asymmetry of the posterior interray.

These and a host of additional character states are shared by Apektocrinus, Titanocrinus, and newly prepared camerate and disparid crinoids from the Garden City Formation. These traits, which are largely associated with the arms, include: 5) uniserial arms that lack pinnules; 6) floor plates with podial pores; 7) “free interradials” that extend from the dorsal cup into the arms, often forming an imbricated mosaic of plates between the brachials and cover plates; and 8) two sets of biserial cover plates. Unlike some taxa from the later Ordovician, all known Floian and Tremadocian crinoids have 9) polyplated discoidal holdfasts.

The morphological similarity of Early Ordovician camerates, cladids, and disparids may make them difficult to differentiate, but it also suggests we are getting to closer to understanding the shared common ancestor of all known crinoids, and by extension, the phylogenetic relationship of crinoids to other classes of echinoderms.