Skeletal homologies, phylogeny and classification of the earliest asterozoan echinoderms

Abstract

Synopsis A thorough reappraisal of extant and fossil asterozoan plate systems is used to construct working hypotheses of primary plate homologies for Ordovician Asterozoa. Results indicate that inferomarginal ossicles of asteroids, somasteroids and the primitive ophiuroid Phragmactis are likely to be homologous with edrioasteroid marginals. The intermediate virgals and coverplates of somasteroids are identified as modified actinals and the sub‐laterals of certain stenurids are axial, here termed podial plates, with laterals in these taxa homologous to true lateral plates and adambulacrals of ophiuroids and asteroids, respectively. The nature of the asterozoan mouth frame is reinvestigated as a result of new study of the mouth frames of key taxa, with a reappraisal of existing developmental and symmetry data. It is argued that primitive asterozoan mouth angle plates were entirely ambulacral and probably not spine‐bearing, but that mouth spines evolved in asteroids and ophiuroids either on the existing first ambulacrals or by the superposition of first adambulacrals adoral to first ambulacrals. These new data are used to undertake a cladistic analysis of all known genera of Ordovician asterozoan echinoderms (38 taxa), based on a dataset of 150 morphological characters, rooted on the Middle Cambrian edrioasteroid taxa Stromatocystites and Cambraster. The resulting tree is fully resolved although some deeper branches show poor support. It reveals that Ordovician asteroids form a monophyletic group (defined by the presence of an unpaired axillary inferomarginal, opposing ambulacrals and blocky abutting adambulacrals) and are a sister group to the remainingasterozoans, including the somasteroids. Somasteroids are monophyletic and, with the primitive taxon Phragmactis, are a sister group to the remaining ophiuroids. The class Ophiuroidea is diagnosed by the near‐simultaneous appearance of apomorphies including an interradial disc with highly separated arms and aboral and oral interambulacral longitudinal musculature. A phylogenetic tree constructed for asterozoans based on current stratigraphic occurrence data indicates that at the time of their first appearance, asteroids and ophiuroid body plans were already established. This suggests a significant but unobserved pre‐Ordovician history. Comparison of observed and phylogenetic diversity together with the appearance of relatively young taxa close to the base of the tree point to a poor asterozoan record, particularly in the Lower Ordovician. A revised classification for stem‐group asterozoans is proposed based on well‐supported clades with the new taxon Eopentaroida erected. Mapping of character states on the most parsimonious solution suggests that whilst asteroid morphological evolution was conservative in the Ordovician, ophiuroids underwent a profound morphological diversification. Functionally, these changes are linked to the adoption of carnivory by derived Ordovician ophiuroids. In contrast, asteroids probably remained dominantly deposit‐feeders throughout the period. Character distribution on the cladogram refutes none of the hypotheses of primary plate homology, although it suggests that podial ossicles were developed in two clades independently and that the marginals of eopentaroids may not be homologous to those of other asterozoans and edrioasteroids.