Early Paleozoic starfish

Abstract

The fossil starfish described here are mainly from the Lower Ordovician (including the Tremadocian). Only brief descriptions of these starfish have been given previously. They are the earliest starfish known, and arrived in a series of transgressions which began in the Tremadocian and persisted throughout the immediately succeeding beds. Each of the transgressions brought in new faunas. Cephalopods arrived in the Tremadocian and locally graptolites in the basal Arenig. The grouping of the starfish adopted here is based upon the activities of the arms, especially during feeding. Three groups of starfish are recognized. Two of these groups are recognizable as Asteroidea and Ophiuroidea. In both these groups the arms are functional, but the arm activities have different mechanisms. The asteroid arm from the beginning is adapted for a carnivorous diet of large food, whilst the arm of the primitive ophiuroid is adapted for feeding on the small food present in or near the sea bottom. It is suggested that the primitive Ophiuroidea, like the recentAmphiura, had a ‘burrowing habit’. The third group had no functional arms. The forms belonging to this group appear slightly earlier (Tremadocian) than do either the Asteroidea (Lower Arenig) or the Ophiuroidea (basal Arenig). The tube feet are placed in position by changes in body shape. They seem to have lived on small food either from the planktonic shower (Villebrunastern.g.) or from that in the upper layers of the bottom (Archegonaster). Their skeleton shows many primitive characters. The name Somasteroidea n.subclass is suggested for this third group. Its members show the first stages in the differentiation of a starfish. It is noticeable that these first stages show no sign of an ambulacral groove, a character which previously has been regarded as primitive and used to link starfish with certain non-crinoid Pelmatozoa (Edrioasteroidea Bather; Thecoidea Jaekel). The view held (Spencer 1905) that the asteroid groove was present in early Echinoidea must also be abandoned. Early representatives of all these groups have characters which link them with a common ancestral structure. Each tube foot is housed on two ambulacralia in all Asteroidea (living and fossil), in the Somasteroidea and in the extinct primitive Ophiuroidea. Some groups of Ophiuroidea retain this character throughout the early Palaeozoic, whilst other groups acquired ‘vertebrae’. Each vertebra, a modified ambulacral, acquires its own tube foot. All early representatives have a mouth frame with radial V’s. It is suggested that the presence of the V’s is associated with a primitive oral opening, the stomodaeum, which had radial slits, the buccal slits.* The Somasteroidea have an aboral skeleton composed of ossicles which are little more than spicules radiating from a number of centres. The arrangement can be compared to that in the early stages of the skeletal components of recent forms. No other fossil echinoderm is known with ossicle components in a condition as primitive as this. It would seem that starfish arose at a very early stage in the development of echinoderm stocks.