What makes an ophiuroid? A morphological study of the problematic Ordovician stelleroid Stenaster and the palaeobiology of the earliest asteroids and ophiuroids

Extant asteroids and ophiuroids [EchinodermataJ are distinguished by differences in arm support, water vascular system structures and in details of arm and jaw structure. However, some lower Palaeozoic taxa show combinations of both asteroid-like and ophiuroid-like characters and their morphology and functional biology is poorly understood. This paper redescribes one such taxon, the middle-upper Ordovician stellate echinoderm Stenaster and clarifies its phylogenetic status. Characters in common with extant and Ordovician ophiuroids, include arm support due primarily to ambulacral ossicles, presence of extensive longitudinal arm musculature, a mobile jaw and an internalised radial water vessel with internalised podial pores. In addition, Stenaster lacks several characters which are conventionally considered to be asteroid-like, for example an axillary, madreporitc, marginal ossicles and a true ambulacral groove. However, in overall shape Stenaster is remarkably asteroid-like, showing short, broad-based arms shared podial basins and a small disc. A cladistic analysis of early asteroids, ophiuroids and somasteroid taxa consistently places Stenaster within the ophiuroids and suggests secondary convergence to asteroids. In functional terms, Stenaster is interpreted as an ophiuroid which has secondarily adopted a semi-infaunal, deposit-feeding mode of life, analogous to that of some extant paxillosid asteroids.