A field guide to the Silurian Echinodermata of the British Isles: Part 2 – Crinoidea, minor groups and discussion

Abstract

Crinoids are the most common and diverse group of echinoderms in the Silurian of the British Isles. This guide describes examples of all nominal crinoid groups recognized from this interval and region (48 genera, about 100 species). In contrast, coronate, edrioasteroid, mitrate and cyclocystoid echinoderms are rare, the first three higher taxonomic groups being represented by a single species each, and the last by two species. Of 145 species of echinoderms in the Silurian of the British Isles, 74 are known from the Much Wenlock Limestone Formation of Wenlock age at Dudley in the West Midlands. For comparison, the most diverse faunas from the Llandovery Series (North Esk Inlier, Pentland Hills, Scotland) and Ludlow Series (Lower Leintwardine Formation, Leintwardine, Herefordshire) consist of only 16 and 17 species, respectively. If the echinoderm-rich succession at Dudley was unknown, the three older series of the Silurian, the Llandovery, Wenlock and Ludlow, would each include about 30 known species of echinoderm. It is the extreme diversity of the Much Wenlock Limestone Formation at Dudley that makes the echinoderm diversities of the Llandovery and Ludlow appear depauperate. (BMNH), but also in those of the British Geological Survey, Keyworth (BGS GSM), the James Mitchell Museum, University College Galway (JMM), the Geological Museum, University of Copenhagen (MGUH), the National Museum of Wales, Cardiff (NMW), and the Sedgwick Museum, Cambridge (SM). Individual taxa are illustrated in Figures 1-15. In order to keep the survey of crinoids to a manageable length, systematic descriptions are mainly at the hierarchical level of genus. This entails examination of 48 genera, including about 100 species (Donovan et al. 2008, tables 16.1-16.3). Where a taxon is monospecific in the study area, it is referred to under the specific epithet. Taxa are listed within the major groups of Palaeozoic crinoids (Simms & Sevastopulo 1993), that is, diplobathrid and monobathrid camerates, cladids, sagenocrinid and taxocrinid flexibles, and disparids. The descriptions are based on the personal observations of the authors, supported by Ramsbottom (1954), Moore & Teichert (1978), Widdison (2001a) and other references as indicated. Unless stated otherwise, columns are assumed to be holomeric.

Perforocycloides nathalieae new genus and species, an unusual Silurian cyclocystoid (Echinodermata) from Anticosti Island, Québec, Canada

“…g Ventral view of marginal ossicles (top of the crest). c contiguous area of lateral suture of marginal, g groove on distal surface suture between marginal ossicles, p intermarginal sutural pore, r radial ducts, s peripheral skirt, v ventral disc plating ◂ Table 1 Stratigraphic and geographic distribution of Silurian cyclocystoid taxa (genera and species) compiled after Smith and Paul 1982;Frest et al 1999Frest et al , 2011Kutscher 2006, 2010;Donovan et al 2008 Description. Circular outline, 13 mm total diameter, composed of a few distal parts of the ventral disc plating, 17 prominent marginals, trapezoidal in dorsal view, organised into alternating groups of large and small plates, and a multiplated, marginal skirt.…”

The poorly illustrated crinoid

“… Reconstruction of the Silurian (Wenlock, Dudley) cladid crinoid, Dictenocrinus decadactylus (Salter), in a rheophobic attitude (after Bather 1900; fig. 3; compare with Donovan et al. 2008, fig.…”

Asterozoans from the Ludlow Series (upper Silurian) of Leintwardine, Herefordshire, UK

“…Apparent fluctuations in diversity may be masked by preservational bias, as noted by Donovan et al . (), in discussion of echinoderm faunas of the British Llandoverian and Wenlockian. However, taken at face value, the diverse Ludfordian Leintwardine fauna provides an invaluable resource to further our understanding of diversification during the Silurian.…”