ORIGIN OF HEMICOSMITID RHOMBIFERANS AND IMPLICATIONS FOR RHOMBIFERAN PHYLOGENY AND CLASSIFICATION

Hemicosmitids are stemmed blastozoan echinoderms that possess a theca with few circlets of large, cryptorhomb-bearing plates. They also have biserial, brachiole-bearing, exothecal ambulacra that resemble pinnulate crinoid arms. Hemicosmitid-like feeding appendages are present in Late Cambrian trachelocrinid eocrinoids, such as Tatonkacystis. However, trachelocrinids have numerous, irregularly arranged, thecal plates without respiratory structures.

A recently discovered eocrinoid from the Lower Ordovician Garden City Formation of southeastern Idaho and northeastern Utah suggests that hemicosmitids may have evolved from trachelocrinids. The new taxon is represented by five isolated exothecal appendages and a single theca with a long stem. The feeding structures are biserial and bear densely spaced biserial brachioles, and the theca is composed of many, irregularly arranged, slit-bearing, thecal plates. These respiratory structures are generally rhomb-shaped and are distributed across nearly all plate margins.

The feeding appendages and well-organized holomeric stem of the new taxon are similar to those of trachelocrinids and hemicosmitids. The plating arrangement and ridged ornament of the new taxon is similar to that of trachelocrinids, and the presence of respiratory structures in the Garden City specimen is a hemicosmitid trait. Although the slit-like respiratory structures of the new taxon differ from hemicosmitid cryptorhombs, the former could have produced the latter by covering the medial portion of the open respiratory slits, leaving only pore-like openings at their distal ends.

Based on their respiratory structures, hemicosmitids are currently classified as a small clade of Middle Ordovician-Devonian rhombiferans that include the much more diverse pectinirhomb-bearing glyptocystitoids and the enigmatic caryocystitids. If hemicosmitids evolved from trachelocrinids, as we interpret here, then the monophyletic origin of rhombiferans is rejected.