MOUTH FRAME CONSTRUCTION IN PALEOZOIC EDRIOASTEROIDS, BLASTOZOANS, AND CRINOIDS

The oral region in primitive echinoderms has numerous characters of potential phylogenetic significance. The mouth, or peristome, is formed by the plates of the peristomial border. The peristomial border may be composed of either oral plates (interradial elements) or oral frame plates (radial elements presumably related to floor plates). Two fundamentally distinct Peristomial Border Systems (PBS) exist where the peristomial border is formed from oral plates (type A) or from oral frame plates (type B). In PBS A1, observed only in the middle Cambrian edrioasteroid Kailidiscus, the peristomial border is formed by both oral plates (integrated interradial plates) and biserial inner ambulacral floor plates. In PBS A2, found in the middle Cambrian edrioasteroid Cambraster and certain Ordovician blastozoans, the peristome is bordered by oral plates with the smaller orals 2 and 5 outside of the peristomial border. In PBS A3, observed in different Ordovician blastozoans plus plesiomorphic crinoids, the peristome is bordered by all the oral plates and covered by small movable cover plates, including both shared ambulacral cover plates and the five primary peristomial cover plates (PCPP). In PBS A4, observed in coronates and more-derived crinoids, the peristome is bordered by all the oral plates and covered by the five PPCP. In PBS B1, observed in Ordovician edrioasterid edrioasteroids, the peristomial border is formed from oral frame plates that are bordered by small oral plates. In PBS B2, observed in isorophid edrioasteroids, and middle Cambrian gogiid and imbricate eocrinoids, the peristomial border is formed exclusively from oral frame plates. This classification scheme for mouth frames does not directly imply an evolutionary ordering, but it does provide key homologies for developing phylogenetic hypotheses to explore the evolutionary relationships among these groups and ultimately resolving the origin of pentaradiality.