Starfish diversity in the Wenlock of England

Although their record extends back to the Early Ordovician, the occurrence of fossil starfish (Echinodermata: Asteroidea) is dependent almost exclusively upon horizons of exceptional preservation. Thus, asteroids found in Silurian obrution deposits of the English Midlands and Welsh Borderlands are particularly significant to an understanding of the early diversity of the group. Six species are described here: Hudsonaster? carectum sp. nov. (Hudsonasteridae), from the lower part of the Lower Elton Formation; and, from the Much Wenlock Limestone Formation, the hudsonasterids Doliaster brachyactis gen. et sp. nov. and Siluraster? ketleyi (Spencer, 1916), the lepidasterids Lepidaster grayi Forbes, 1850 and Lepidactis wenlocki Spencer, 1918, and the palasterinid Palasterina orchilocalia sp. nov. Though few in number, they show a diverse range of body morphologies when compared with Ordovician taxa: L. wenlocki had long, slender rays when fully grown whereas D. brachyactis is the first asteroid with the short-rayed body form of extant cushion stars. Most distinctive of all is L. grayi, the earliest multiradiate taxon known, all complete specimens of which have 13 rays. This morphological variety is interpreted as indicating that by the Early Silurian starfish were exploiting a wide range of feeding habits and ecological niches.

Abundant in modern seas, starfish have an extremely uneven distribution in the geological record. From the first unequivocal examples in the Tremadoc (495 Ma) (Dean 1999b), well-preserved fossil asteroids are limited almost exclusively to event beds. This is true of many echinoderms, as their bodies are composed of individual calcite plates (ossicles) held together by soft tissue that rapidly decays after death, causing the ossicles to separate and disperse on the sea floor. In the case of vagile epifaunal echinoderms such as starfish, a sudden and quite substantial influx of sediment is thus required to bury the animal while it is still alive or very recently deceased, in order for complete preservation to occur (see Goldring and Stephenson 1972). Additionally, there is considerable variation in the preservation potential of different species of asteroid, demonstrated by LeClair (1993) as being primarily dependent on two factors: the arrangement of soft tissues holding the ossicles together, and the environment in which the starfish lived.

Obrution horizons in the Much Wenlock Limestone Formation of the English Midlands and Welsh Borderlands, particularly those of Wren's Nest and Castle Hill, Dudley, have yielded numerous species of echinoderms. Crinoids are the most varied and abundant, with at least 56 species known (see Widdison 2002). Thirteen fossil asteroids have been found, belonging to five species, listed alphabetically as follows: Doliaster brachyactis gen. et sp. nov., Lepidactis wenlockiSpencer, 1918, Lepidaster grayiForbes, 1850, Palasterina orchilocalia sp. nov. and Siluraster? ketleyi (Spencer, 1916). Additionally, one species, Hudsonaster? carectum sp. nov., is described that is of somewhat uncertain provenance, possibly the Much Wenlock Limestone Formation, but more probably the overlying Lower Elton Formation. These make up the total asteroid fauna of the English Wenlock, although a small number of specimens of the ophiuroid Lapworthura sp. (J. Dean, pers. comm. 1999) have been found in the Nantglyn Flags Group (Upper Sheinwoodian–Homerian) of Llanrwst, North Wales, and there are two further asterozoans, Eoactis simplexSpencer, 1916 and Urosoma hirudo (Forbes, 1848), with specimens of possible Wenlock age.

A scarcity of asteroid-bearing horizons is prevalent throughout the British Silurian: the Llandovery has only a single locality, Gutterford Burn in the Pentland Hills near Edinburgh, from which more than a single starfish has been found; and the Ludlow only two, the Upper Leintwardine Formation of Church Hill, Leintwardine, Herefordshire, and the Bannisdale Slates around Kendal, Cumbria. The specimens from Gutterford Burn were originally described as being of Wenlock age (Peach and Horne 1899; Spencer 1916, 1919, 1922, 1927), and new species such as Taeniactis wenlockiSpencer, 1922 and Protactis wenlockensisSpencer, 1922 were named on that basis, but subsequent work has shown the rocks to be of Late Llandovery (Telychian) age (Lamont 1947; Robertson 1989).