MORPHOLOGY AND EVOLUTIONARY IMPLICATIONS OF A PRIMITIVE EARLY ORDOVICIAN CRINOID

A single crinoid specimen from near the base of the Lower Ordovician Garden City Formation in southeastern Idaho may represent the earliest known crinoid. Conodont evidence indicates an age bracketed by trilobite zones B-E or lower to middle Ibexian (= lower to middle Tremadocian). The crinoid, representing a new family, genus, and species, was heavily weathered when originally collected, but because of its early age and potential phylogenetic importance has been extensively excavated from its hard limestone slab revealing nearly 80% of the cup, arm, and proximal stem morphology.

The new crinoid displays a mixture of early and later derived traits, unlike any other taxon. Lower cup plating is, in general, cladid-like dicyclic (3-circlet), although with significant irregularities and anomalies. Infrabasal, basal, and radial circlets are all six-fold, interrupted by apparent gap, inferradianal, and superradianal/anal X plates, respectively. A small raised radial is tentatively interpreted to mark the C-ray. New crinoid apomorphies include fixed arm brachials and intervening interbrachial fields and meric stem to the cup. The new crinoid has long atomous arms with floor plates demonstrably homologous with those found in the ambulacra of derived Cambrian edrioasteroids but not eocrinoids. Along with cover plates, these diverge from brachials and interbrachials in the lower arms and extend onto the tegmen. This shows that crinoid arms are unique composite coelom-bearing structures that evolved during the divergence of crinoids near the Cambro-Ordovician boundary.

The new crinoid's morphology is remarkably divergent compared to other Ordovician crinoids, although it shares some similarities to the slightly younger Aethocrinus. Both have cladid-like dicyclic cups and early crinoid traits such as a pentameric stem and circlet irregularities. However, unlike Aethocrinus, the new taxon lacks a fourth cup circlet (lintels), has plesiomorphic floor plates and accessory plating in the arms, and has cup plate irregularities in different places.