North American material, from southeastern Newfoundland, is housed at ... Figure 1 Schematic representation of the ontogeny and terminology for nodes and ...
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 conventionally 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 Phosphatocopina 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 ancestral 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 HinzSchallreuter (1993a), a detailed analysis of these arguments is yet to be published. In preference to subdividing Cambrian ostracods into Phosphatocopina or Bradoriina, HinzSchallreuter (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 photography, 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, broadbased 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 HinzSchallreuter (1993a). We follow Cowie et al. (1972) in their use of the term Upper Cambrian with respect to the British sequence.
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particular the relatively rapid expansion of lobes LI and L2 are shown diagramatically in Figure 1. In contrast to bradoriid ostracods with univalved juvenile carapaces (Zhang & Pratt 1993) C. lapworthi shows a differentiation of left and right valves of the carapace, together with a narrow but clearly developed interdorsum, even in the smallest specimens (Fig. 6n).
3. Stratigraphic significance
Figure 1 Schematic representation of the ontogeny and terminology for nodes and lobes of C. lapworthi (left valves).
2. Ontogeny of C. lapworthi The ontogenetic development of the carapace of Cambrian bradoriids is poorly known. Miiller and Walossek (1991) have indicated the presence of large numbers of phosphatocopines (over 50000 specimens) in their material from the Upper Cambrian 'orsten' of Sweden, but the ontogeny of the several species is yet to be documented in detail. Topotype specimens of C. lapworthi range from 0.56 mm to 5.00 mm long. Most specimens are 1.00 mm to 3.50 mm long and rarely exceed 4.00 mm. Plots of all known topotype and Canadian material of C. lapworthi did not reveal clearly defined ontogenetic stages for the species (Fig. 2). Similar size distributions are indicated for the bradoriids Monasterium oepiki from the Middle Cambrian of Australia (Hinz 1992) and Bidimorpha bidimorpha from the Middle Cambrian of Scandinavia (Hinz-Schallreuter 1993a). The lack of clearly differentiated moult stages in the latter material could possibly be due to mixed populations, the result of ecological factors, or be a primary feature of growth of early ostracods (Hinz 1992; Hinz-Schallreuter 1993a). Muller (1979) considered that phosphatocopine ostracods moulted at short intervals continually throughout their life. The development of nodes and lobes during growth, and in
Bradoriids have considerable but neglected potential for use in biostratigraphy, especially in the North Atlantic region (e.g. Siveter et al. 1993; Rushton & Siveter in press). C. lapworthi. a short-ranging form, is restricted to the lower Upper Cambrian (Fig. 3). In Warwickshire bradoriids occur throughout the Outwoods Shales Formation but C. lapworthi appears to be confined to the Olenus Zone (Taylor & Rushton 1971). The occurrence of C. lapworthi in borehole and surface exposures in the Outwoods Shales of the Nuneaton area (Figs 4, 5), provides some indication of its stratigraphic distribution. In Merevale No. 3 Borehole [SP 3071 9574] definitive C. lapworthi ranges from 15.06 m to 53.96 m, though other, undescribed material of Cyclotron is known to occur down to 77 m. Based on the occurrence of the trilobite Homagnostus obesus, the base of the Olenus Zone in the Merevale No. 3 Borehole is placed at 61.16 m (Rushton 1983). The first appearance of definitive C. lapworthi closely corresponds to the base of the 0. gibbosus Subzone (54.96 m) and the species also occurs in the O. truncatus and O. wahlenbergi subzones (Figs 4. 5) of the borehole. At Purley Quarry [SP 305 962] C. lapworthi is present near the base of, or possibly just below, the Olenus Zone (see Taylor & Rushton 1971, figs 3, 4) and at Old Wharfe Inn [SP 3624 9046] it occurs possibly in the O. cataractes Subzone associated with O. teles Rushton (1983). C. lapworthi is also present at: Bar Pool Trench, North of Stockingford Railway Cutting [SP 3425 9227], where it occurs low in the Olenus Zone (probably in the O. gibbosus Subzone): Oldbury Reservoir [SP 307 953] in the O. cataractes Subzone: and Stockingford Railway Cutting [SP 343 921], possibly in the O. cataractes Subzone. At Perch Hill [SP 370 887] its presence is taken to indicate the Olenus Zone there. In the Malvern Hills, the type locality for C. lapworthi, its occurrence in a thin shale unit at the base of the WhiteLeaved-Oak Shales has been used to infer the presence of the Olenus Zone (Taylor & Rushton 1971). Rushton (1983) has also indicated the presence of the Olenus Zone in the 'Lingula' Flags of the St Davids district, South Wales. A single bradoriid (BMNH 12745) from the 'Lingula R-igs appears to be a Cyclotron species but is too poorly preserved for a specific assignment to be made. C. lapworthi has not yet been identified from the Olenus Zone of North Wales. In Newfoundland Martin and Dean (1988) have recovered specimens of Cyclotron sp. from the Olenus Zone in the Elliott Cove Formation of Manuels River, Conception Bay. At least one of their figured specimens (1988, pi. 6, fig. 11) is referable to C. lapworthi. Similarly, Polyphyma marginata Ulrich & Bassler, 1931, collected from black shales of the Olenus Zone at Manuels River (=the Elliott Cove Formation of current usage), is conspecific with C. lapworthi. Thus, C. lapworthi can be used to recognise the Olenus Zone in Cambrian sequences bordering the North Atlantic. A single specimen from the Upper Cambrian of Oland. Sweden, closely resembles C. lapworthi (Fig. 6p) but is incomplete posteriorly. This specimen (SGU 8667), recovered
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CYCLOTRON LAPWORTHI FROM BRITAIN AND CANADA
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