A Review of Silurian Discinoid Brachiopods from Historical British Localities
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A review of Silurian discinoid brachiopods from historical British localities MICHAL MERGL The present article revises descriptions of Silurian discinoid brachiopods by J. de C. Sowerby and T. Davidson in the mid-nineteenth century, and of old collections from classic palaeontological sites in Britain, currently housed in the Nat- ural History Museum, London. Orbicula forbesii Davidson, 1848, O. rugata J. de C. Sowerby in Murchison, 1839, O. striata J. de C. Sowerby in Murchison, 1839, Discina verneuilii Davidson, 1848 are re-described. The genus Rugadiscina and the species Acrosaccus woolhopensis, Acrosaccus cocksi and Schizotreta walkeri are erected. Discina? cf. rarissima Barrande, 1879 and the genus Lochkothele are documented from the Silurian of Britain for the first time. Although the generic composition of the British Wenlock discinoid fauna is similar to that of the Barrandian area in Bo- hemia, differences at the species level confirm the separation of these terrains during the Silurian. • Key words: Brachiopoda, Discinida, taxonomy, Silurian, Great Britain, Bohemia. MERGL, M. 2006. A review of Silurian discinoid brachiopods from historical British localities. Bulletin of Geosciences 81(4), 215–236. Czech Geological Survey, Prague. ISSN 1214-1119. Manuscript received September 21, 2006; accepted in revised form November 5, 2006; issued December 31, 2006. Michal Mergl, University of West Bohemia at Plzeň, Department of Biology, Klatovská 51, 306 19 Plzeň, Czech Repub- lic; [email protected] Lower Palaeozoic discinoid brachiopods are relatively type species of the genus Orbiculoidea d’Orbigny, 1847, poorly known, even though they have received some atten- and the species Discina rugata (Sowerby in Murchison, tion from a few workers in recent decades (Bassett 1986, 1839). Both species names have been widely used by Holmer 1987, Havlíček & Mergl 1988, Havlíček 1994, other authors and collectors to identify subcircular Sutton et al. 2000, Mergl 2001). This lack of knowledge is discinoids of early Palaeozoic age, regardless of the reflected in the revised edition of the Treatise on Inverteb- highly variable morphology and various stratigraphical rate Paleontology (Holmer & Popov 2000), in which all levels involved. the discinid genera are grouped into two families, the Tre- matidae and the Discinidae. Many Lower Palaeozoic discinoids were described Material during the first half of the nineteenth century, but some species have never been fully revised, in particular those The present study is a revision of the species erected by of Sowerby (1839) and Davidson (1848, 1866). However, Sowerby (1839) and Davidson (1848, 1866). The research similar species from the Barrandian area of the Czech Re- described here is based exclusively on material stored in public, mostly erected by Barrande (1879), have been re- the collections of the Natural History Museum in Lon- vised recently (Mergl 2001). The present revision of don, and the British Geological Survey in Keyworth. Ne- Davidson’s specimens, and other material collected wer collections have not been considered. The stratigra- mostly in the nineteenth century from the same localities, phic and locality data are often very limited in old indicates higher discinoid diversity than previously sug- material, and do not fit well within the present state of gested. While Sowerby (1839) and Davidson (1848, stratigraphic schemes. When possible, old data concer- 1866) noted only four species currently referred to ning locality and age have been re-interpreted in accor- discinoids, there are now at least nine recognised species. dance with names and stratigraphy presented by Al- Davidson (1848, 1866) confused some taxa, and although dridge et al. (2000). However, some of the original Cocks (1978) critically revised Davidson’s material stratigraphical and locality information has not yet pro- and identified the type specimens, some taxonomic ques- ved amenable to re-interpretation. The interpretation of tions remained unsolved. Of particular concern are the stratigraphic and locality data, and the corresponding species Discina forbesii Davidson, 1848, which is the species ranges, are given in Figs 1 and 2. 215 Bulletin of Geosciences Vol. 81, 4, 2006 ans, pelmatozoans, rhynchonelliform brachiopods, and other invertebrate groups in the shallow water environ- ments of an extensive reef that bordered the Welsh Basin. Seven generally medium- to large-sized species lived in that reef: Schizocrania verneuilii, Acrosaccus cocksi, A. woolhopensis, Orbiculoidea forbesii, Lochkothele sp., Schizotreta? cf. rarissima and S. walkeri. In the Barran- dian, however, shallow environments were restricted to the apices of submarine volcanoes or along the fringes of tem- porary volcanic islands. The extent of these shallow water environments was restricted, and they were immediately surrounded by deeper water. Extensive reef complexes were never present in Bohemia, and only small coral-crinoid bio- stromes were developed locally in the Wenlock and persist- ed into the Ludlow (Gorstian). Though the large- and medium-sized discinids Acrosaccus patelliformis and A. bu- bovicensis were uncommon, they were associated with smaller and more diversified discinids including Orbiculoi- dea karlstejnensis, Schizotreta? rarissima, Schizotreta sp., Acrosaccus vexatum, Ivanothele sp., and other lingulate bra- chiopods. In contrast to Britain, cemented craniids are rare and Schizocrania is unknown in Bohemia. Figure 1. Distribution of historical localities for the Wenlock and Lud- In contrast to the Wenlock Series, the more siliciclastic low Series, set against the paleogeographical elements of Silurian Britain. development of the Ludlow Series in Britain was colonised Modified from Aldridge et al. (2000, Figs 4.1, 5.1). by various discinoids. The discinoid brachiopod diversity subsequently decreased, with only Rugodiscina rugata, and Affinity of the British and Bohemian Schizocrania striata present, the latter commonly living in a Silurian discinoids specific habitat of empty, drifting cephalopod shells (Lockley & Antia 1980). Similar medium- to large-sized convexoplane Current palaeogeographical reconstruction (Cocks & Tor- discinids and trematids are unknown from the Ludlow of the svik 2002) locates the Silurian strata of England and Wales Barrandian. There, the limestone sedimentation continued in the Avalonian terrain, while the sequence preserved in into the Ludlow and Pridoli, and there was no substantial the Barrandian of Bohemia belongs to the separate micro- siliciclastic input until the Middle Devonian. Numerous small plate of Perunica (Havlíček et al. 1994). It is not surprising and poorly known discinoids (Acrosaccus, Schizotreta, that, although separated by an oceanic barrier, similar sub- Ivanothele, Kosoidea, Sterbinella, minute Orbiculoidea)oc- tropic palaeolatitudes and shallow marine paleoenviron- cur in the Ludlow and Pridoli of the Barrandian. The ments are reflected by closely related benthic communities convexo-plane discinoids and Schizocrania appeared in Bo- in the Wenlock of Britain and Bohemia (Calef & Hancock hemia during the Early Devonian (Orbiculoidea magnifica, 1974, Watkins 1979, Watkins & Aithie 1980, Havlíček & Schizocrania sp.), probably as a consequence of the spread of Štorch 1990). This similarity in brachiopod communities is a more uniform Early Devonian fauna. also evident among the Wenlock discinoid brachiopods of England and Bohemia. There are pairs of discinid species (Acrosaccus woolhopensis – A. patelliformis, A. cocksi – A. Systematic palaeontology bubovicensis) that represent vicarious species in related pa- laeocommunities. Schizotreta? rarissima and related spe- The materials described in the present paper are from the cies occur in both areas, but the species is not well known collections of the Natural History Museum in London (B in Britain or Bohemia. The encrusting Schizocrania verne- and BB), the British Geological Survey in Keyworth, Not- uilii is unknown in the Silurian of Bohemia, although this tingham (BGS and GSM), and the National Museum in genus does occur there in the Ordovician and rarely also in Prague (NM L). the Devonian of the region. Lochkothele sp., which is pre- sent in the Wenlock of Britain, is represented in the earliest Devonian of Bohemia by abundant L. intermedia (Bar- Order Lingulida Waagen, 1885 rande, 1879). British Wenlock discinoids lived in remar- Superfamily Discinoidea Gray, 1840 kably rich biostromes of corals, stromatoporoids, bryozo- Family Trematidae Schuchert, 1893 216 Michal Mergl British Silurian discinoid brachiopods Genus Schizocrania Hall & Whitfield, 1875 Type species.–?Orbicula filosa Hall, 1847; Trenton Group; Ordovician; New York State, USA. Schizocrania striata (J. de C. Sowerby, 1839) Figure 3A–I 1839 Orbicula striata J. de C. Sowerby; Sowerby in Mur- chison, p. 610, pl. 5, fig. 12. 1854 Orbicula striata J. de C. Sowerby. – Murchison, pl. 20, fig. 3. 1859 Discina striata (J. de C. Sowerby). – Murchison, pl. 20, fig. 3. 1866 Discina striata (J. de C. Sowerby). – Davidson, p. 6, pl. 6, figs 1–4. 1867 Discina striata (J. de C. Sowerby). – Murchison, pl. 20, fig. 3. 1872 Discina striata (J. de C. Sowerby). – Murchison, Figure 2. pl. 20, fig. 3. The global Wenlock and Ludlow stratigraphy and ranges of revised British discinoid brachiopods. 1902 Orbicula striata J. de C. Sowerby. – Blake, p. 6. 1963 Schizocrania striata (J. de C. Sowerby). – Holland et al., p. 154. men being 18 mm long. The dorsal valve is subcircular, 1978