NEW EVIDENCE FROM THE AXIAL SKELETON IN EARLIEST CRINOIDS INFORMS CRINOID ANCESTRY

An extensive survey across well-preserved and extant crinoid taxa shows consistent yet previously unreported cover plate patterns in crinoid arms. Among all seven earliest crinoids (Tremadocian), cover plates are arranged in two tiers, with lateral and medial series. Lateral cover plates articulate and align with calcified floor plates one-to-one, completing the axial skeletal complex. Axial morphology similar to this occurs deeper in echinoderm phylogeny, among Cambrian edrioasteroid-like stem pentaradiates. A hypothesis of crinoid arm origination from these stem pentaradiates requires extension of the axial and nearby extraxial anatomy at ambulacral tips, along with plate reduction and differentiation, but no de novo plate acquisition in either region. Timing of appearance indicates changes leading to modern crinoid arms were asynchronous; the crinoid arm evolved in stages. The earliest, most diagnostic, change involved the extraxial region, with the acquisition of aboral ray plates (radials and brachials). The basic axial skeletal construct of earliest crinoid arms was conserved during this key transformation. Floor plates are absent in nearly all Late Floian and younger crinoids, but the two-tiered cover plate pattern occurs frequently in Late Ordovician forms with robust arms, while a single tier characterizes pinnulate and other forms with diminutive construction. Early blastozoans (eocrinoids, “cystoids”), often cited as exclusive sister group of the Crinoidea, and displaying plesiomorphies for that clade, differ from earliest crinoids in expressing a single cover plate tier. Post-emergent acquisition of this trait in crinoids supports homoplasy as the cause of their similarity to blastozoans, with the one-tiered pattern as likely synapomorphic for the Blastozoa. Middle to Late Paleozoic blastoids express two-tiered plating on the oral surface of brachioles, but this differs from that of crinoids; in particular, the outer series does not roof the ambulacral groove.