Transactions of the Royal Society of Edinburgh: Earth Sciences, 85, 123-130, 1994 The Upper Cambrian bradoriid ostracod Cyclotron lapworthi is a hesslandonid Mark Williams, David J. Siveter, Adrian W. A. Rushton and Vivianne Berg-Madsen ABSTRACT: Cyclotron lapworthi (Groom, 1902), one of the few British Cambrian bradoriid ostracod species known from a large number of specimens, is redescribed. It has an interdorsum, thus indicating a hesslandonid affinity. The ontogeny of C. lapworthi does not demonstrate a clear distinction of its individual moult stages but does show marked changes in the development of nodes and lobes. C. lapworthi shows wide variation in carapace shape possibly indicating that its valves were thin and relatively flexible. In England and Canada C. lapworthi is restricted to the Upper Cambrian Olenus Zone. C. lapworthi may also occur coevally in the Upper Cambrian of Sweden. KEY WORDS: Ostracoda, ontogeny, biostratigraphy, palaeoecology, taxonomy. All Cambrian ostracods and ostracod-like fossils have conven- tionally been referred to the Order Bradoriida. However, the taxonomic status of Bradoriida is contentious; reviews are given in Jones and McKenzie (1980) and Briggs (1983). Originally Muller (1964, 1979) considered the Phosphato- copina as a suborder of the Bradoriida (Ostracoda). Jones & McKenzie (1980) treated the Phosphatocopina as an early, short-lived ostracod group and the suborder Bradoriina as an heterogeneous grouping which included some possible ances- tral ostracods together with forms probably more closely related to other crustacean groups. Later Muller (1982) raised both the Phosphatocopina and Bradoriina to ordinal level, distinguishing them on the basis that the former had a primary phosphatic component and the latter may have a calcareous component to the carapace. More recently, on the basis of studies of their soft-part anatomy, the assignment of the Phosphatocopina to the Ostracoda has been questioned (Muller & Walossek 1991). However, as noted by Hinz- Schallreuter (1993a), a detailed analysis of these arguments is yet to be published. In preference to subdividing Cambrian ostracods into Phosphatocopina or Bradoriina, Hinz- Schallreuter (1993a) elected to use the taxon Archaeocopida Sylvester-Bradley, 1961 for all such fossils, characterising them as an ostracod group distinct from 'true' ostracods by having a different shell composition and differences in the development of the dorsal and admarginal surfaces. A thorough evaluation of the systematics of Cambrian ostracods is beyond the scope of our paper. Herein, we follow the majority of published opinions (Muller 1964, 1979; Jones & McKenzie 1980; Briggs 1983) in retaining the Phosphatocopina as a suborder within the Bradoriida and the Ostracoda. British Cambrian ostracods are largely unstudied. Rushton and Siveter (in press) give a summary of the present state of knowledge. Cyclotron lapworthi (Groom, 1902) represents one of the few species for which an adequate number of specimens is available. Having studied all known material of C. lapworthi we aim to: • demonstrate its ontogenetic and morphological variation • establish its geographical and stratigraphical distribution and potential for correlation • present new evidence regarding the taxonomic affinity of the genus Cyclotron. 1. Materials, methods and terminology The British material studied herein comprises mostly Groom's (1902) specimens of C. lapworthi, which are deposited in the University Museum Oxford (OUM) and the Lapworth Museum of Birmingham University (BU). This material, recovered from Groom's (1898) locality 257 in the southern Malvern Hills of the Welsh Borderland, comprises over 100 complete valves and carapaces. The Lapworth Museum also contains material of C. lapworthi collected from Warwickshire, central England, by Lapworth and subsequently donated to Groom. The British Geological Survey (BGS) houses material recovered from outcrop and the Merevale No. 3 Borehole in the vicinity of Nuneaton, Warwickshire, central England. The North American material, from southeastern Newfoundland, is housed at the Smithsonian Institution, Washington, D C (USNM) and the Scandinavian material at the Swedish Geological Survey (SGU), Uppsala, Sweden. All of the known valves of C. lapworthi occur on small slabs of mudrock. The shell is often incomplete and many specimens occur as internal and external moulds. Valves are generally preserved with low relief and often appea' to be flattened or distorted, thus possibly reflecting an originally soft and somewhat flexible valve. Specimens have been photographed mainly by light photography (methods of Siveter 1990). In order to enhance contrast, specimens were coated with a thin film of black opaque and then, immediately prior to photogra- phy, with ammonium chloride. Photographs were taken using an Aristophot mounted with a Leica camera. Smaller specimens were studied with the scanning electron microscope. In the description of C. lapworthi we use the morphological terms node and lobe. These lobal structures (external, broad- based domiciliar inflations of the valve) are defined as follows: • lobe: a distinctly elongate lobal structure • node: an ovoid to circular lobal structure. The numbering of the nodes of Cyclotron (Fig. 1) in part follows Grundel (in Grundel & Buchholz 1981) and Hinz- Schallreuter (1993a). We follow Cowie et al. (1972) in their use of the term Upper Cambrian with respect to the British sequence.