Rare and problematic taxa from the Much Wenlock Limestone Formation

Abstract

The Much Wenlock Limestone Formation (Silurian: Wenlock, Homerian) of England and Wales contains a diverse invertebrate fauna including many rare and problematical taxa. This study investigates the palaeobiology and palaeoecology of five such groups: asteroids (starfish), the crinoid Calyptocymba mariae gen. et sp. nov., rostroconch molluscs, machaeridians and cornulitids. Six species of asteroids are recognized, including three new forms (Palasterina orchilocalia sp. nov., Hudsonaster? carectum sp. nov., and Doliaster brachyactis gen. et sp. nov.), that show a diverse range of morphologies. Most distinctive is the 13-rayed Lepidaster grayi Forbes, 1850, the oldest known multiradiate starfish. The variety of asteroid body shapes indicates a diversification of behaviour, particularly feeding strategies. By functional convergence, L. grayi is interpreted as an active predator. C. mariae is an unusual species of camerate crinoid, with a small, thinly plated calyx. This, combined with its slender body morphology, is interpreted as an adaptation to life in a low energy environment. Four rostroconch taxa [Mulceodens aedicula sp. nov., M? aequicostatus (Phillips, 1848), M. latus sp. nov., and Redstonia sima sp. nov.] occur, and are interpreted as mobile, semi-infaunal, deposit feeders. Differences between Mulceodens and Redstonia probably reflect adaptations by closely related taxa to specific environments, with Mulceodens living in higher energy conditions, rather than indicating that they belong to separate families as has been suggested previously. Machaeridians are problematical taxa having elongate bodies covered with serially repeated calcitic sclerites, and three species – Turrilepas wrightiana (de Koninck, 1857), Lepidocoleus ketleyanus (Reed, 1901) and L? extraplax sp. nov. – are recognized in the Much Wenlock Limestone Formation. By analysis of their body morphology, skeletal structure and growth pattern, machaeridians are interpreted here as molluscs, probably the sister group of Polyplacophora + Conchifera. Using a similar approach, Cornulites is interpreted as a stem group anthozoan. Cornulites species show various life positions: C. scalariformis Vine, 1882, attached itself to live brachiopods, with its aperture positioned close to the host’s feeding currents, C. gremialis formed clusters growing upon one another, and C. cellulosus was solitary and unattached