Writhlington Geological Nature Reserve Research Articles

The phylogenetic position of the extinct arachnid order Phalangiotarbida Haase, 1890, with reference to the fauna from the Writhlington Geological Nature Reserve (Somerset, UK)

The phylogenetic position of the extinct arachnid order Phalangiotarbida Haase, 1890, with reference to the fauna from the Writhlington Geological Nature Reserve (Somerset, UK)

The phylogenetic position of the extinct arachnid order Phalangiotarbida Haase, 1890, with reference to the fauna from the Writhlington Geological Nature Reserve (Somerset, UK)

ABSTRACTStudy of abundant phalangiotarbid (Arachnida: Phalangiotarbida) material – provisionally assigned here to Bornatarbus mayasii (Haupt in Nindel 1955) – from the Upper Carboniferous of Writhlington, UK, has revealed new information about some previously equivocal characters. The present authors report a trifurcate apotele, possible spiracles on sternite 5, and confirm the presence of 10 opisthosomal tergites plus a dorsal anal operculum. The affinities of phalangiotarbids are obscure, with most authors favouring affinities with Opiliones (harvestmen) and/or Acari (mites and ticks). Phalangiotarbida is scored for characters used in previous studies of arachnid relationships. A cladistic analysis based on 63 characters using 13 terminal arachnid taxa (plus a hypothetical outgroup), resolves Phalangiotarbida as sister group to (Palpigradi + Tetrapulmonata): the taxon Megoperculata sensu Shultz (1990). Even under cladistic analysis, the position of the Phalangiotarbida remains hard to resolve, but a prosomal sternite with distinct sclerites potentially groups them with the Megoperculata.

An Upper Carboniferous eurypterid from the Writhlington Geological Nature Reserve

The first recorded eurypterid from the Writhlington Geological Nature Reserve is described and is referred to the Westphalian species Adelophthalmus imhofi (Reuss). The palaeoecology of the animal is discussed and it is concluded that it was a freshwater predator living in a river channel system.

Arthropleurids from the Westphalian D of Writhlington Geological Nature Reserve, Somerset

Recent finds of the giant myriapod Arthropleura from the Writhlington Geological Nature Reserve are described and their palaeoecology discussed. The fossils are fragmentary and represent various parts of the animal including tergites, limbs and rosette plates. One specimen is identifiable as Arthropleura armata Jordan, 1854 and it is likely that all the material belongs to this species. A length of about 46 to 66 cm is inferred for the complete animals. Analysis of plant debris associated with the fossils suggests that the Writhlington Arthropleura lived in high diversity fern/pteridosperm forests growing on the river levees; the low diversity Lepidodendron forest which grew in the adjacent back-basin was probably flooded for much of the year and provided an unsuitable habitat. Comparison with other sites suggests that, in general, the less waterlogged levee forests may have been Arthropleura's preferred habitat.

Carboniferous fossil plant assemblages and palaeoecology at the Writhlington Geological Nature Reserve

The Writhlington Geological Nature Reserve has yielded a rich Upper Carboniferous fauna and flora from rocks probably derived from just above the no. 10 seam of the Farrington Formation, of upper Westphalian D age. The fossiliferous sediments were deposited in an upper delta plain environment and comprise a sequence of overbank/delta/crevasse splay and flood-plain lake sediments. They prograde rapidly into a back-basin mire, probably as the result of a crevasse splay. The fossil flora is of high diversity and contains in situ and transported elements. Several different fossil assemblages can be recognized. Three foliage assemblages contain the bulk of collected material and are considered to be derived from different plant communities. The community structure for the Writhlington forest is tentatively inferred to have comprised a low diversity lycopod forest in the back-basin, and higher diversity fern/pteridosperm forests in river levee and back-levee habitats. Sphenopsid stands may have grown in wet back-basin and back-levee forests, possibly also as pure stands beside water.

An Upper Carboniferous amblypygid from the Writhlington Geological Nature Reserve

The first amblypygid (Arachnida: Amblypygi) from the Writhlington Geological Nature Reserve (Upper Carboniferous, Westphalian D) is described. The ventral opisthosoma and the posterior walking legs are relatively complete. This small, possibly juvenile specimen is tentatively identified as Proto-phrynus carbonarius? Petrunkevitch, 1913 . The taxonomy of Carboniferous amblypygids is reviewed and the genera Thelyphrynus and Protophrynus are discussed as potential synonyms of the genus Graeophonus. Carboniferous amblypygids resemble Recent forms and appear to represent ‘living fossils’. However, there are problems reconciling characters used in fossil and Recent amblypygid taxonomy.

The minerals of Writhlington Geological Nature Reserve

The clay ironstone nodules found at Writhlington Geological Nature Reserve have yielded a number of minerals that are associated with coal-bearing rocks. In addition to these minerals, cavities within the nodules have been found to contain organic compounds that may have derived from the coal during the mineralization processes.

The palaeobiology of the Writhlington trigonotarbid arachnid

The morphology of the trigonotarbid arachnid from the Upper Carboniferous (Westphalian D) Writhlington Geological Nature Reserve is reconstructed, its opisthosomal segmentation determined, and its mode of life and palaeoecology discussed. Comparison of this material with other trigonotarbids in the family Eophrynidae from the Westphalian D of Mazon Creek and the Graigola Coal Seam, South Wales, suggests that all these forms can be referred to Pleophrynus verrucosa ( Pocock, 1911 ), which appears to have had two morphotypes, perhaps representing sexual dimorphism. P. verrucosa was widely distributed within the Westphalian D Coal Measures and supports the hypothesis of a single terrestrial faunal province in the European and North American Westphalian D.

Plant fossils from the Writhlington Geological Nature Reserve

This is currently the best locality for upper Westphalian D plant fossils in the Radstock Basin, one of the classic areas for studying the palaeobotany of this age. The assemblage includes a diverse range of gymnosperms, ferns, ‘horsetails; and ‘club-mosses’, and reflects the vegetation of the levee-banks within the Late Carboniferous coal-forming swamps. The assemblage belongs to the Dicksonites plueckenetii Subzone (upper Lobatopteris vestita Zone) and can be compared with coeval assemblages from South Wales, the Forest of Dean and the Maritime Provinces of Canada.

Xiphosurans from the Westphalian D of the Radstock Basin, Somerset Coalfield, the South Wales Coalfield and Mazon Creek, Illinois

Euproops kilmersdonensis Ambrose & Romano, 1972 is proposed as a synonym of Euproops danae ( Meek & Worthen, 1865 ) from Mazon Creek, Illinois. Five other species attributed to Euproops Meek, 1867 and one species attributed to Prestwichianella nitida Dix & Pringle, 1929 , from the Westphalian D of the South Wales Coalfield, described by Dix & Pringle (1929, 1930) are also synonymized with E. danae . In addition, six species described by Raymond (1944) from Mazon Creek are synonymized with E. danae . The taphonomic processes acting upon xiphosuran body fossils produce spurious morphological differences between specimens, which have been used in the past to define species. It is concluded that species diversity within the Carboniferous Xiphosura was low, contrary to previous reports ( Fisher, 1984 ). The mode of life of E. danae is re-evaluated in the light of trace fossils recently described by Pollard & Hardy (1991) from Writhlington Geological Nature Reserve, and from palaeophysiological considerations.

Non-marine bivalves from Writhlington Geological Nature Reserve, Avon

Fossiliferous material from the rock store of Writhlington Geological Nature Reserve has come from two horizons, the lower one above No. 10 Coal in the lower part of the Farrington Formation and the second one at its top, 200 m higher, above the Rock or Badger Coal. Both horizons lie in the Chronozone of Anthraconauta tenuis , in the Zone of Lobatopteris vestita , and the Subzone of Dicksonites plueckeneti , Upper Westphalian D. All the material from the Nature Reserve store which has received published descriptions has come from above No. 10 Coal. This horizon, which is rich in plants and land arthropods, has yielded uncommon, poorly preserved small short shells of Anthraconaia which are hitherto unknown in the Westphalian D. They are biometrically defined, illustrated and referred collectively to Anthraconaia cf. saravana (Schmidt). In the fauna above the Rock Coal the elongate biospecies Anthraconaia pringlei (Dix & Trueman) is also biometrically defined and is shown to have a posterior gape, a feature characteristic of a deep burrower and not previously recorded in the genus. Anthraconauta is represented by an apparent single community of complete shells and by many originally broken fragments. A study of the growth and variation of over 70 shells of Anthraconauta from Writhlington constitutes the first quantitative biometrical approach to this genus made in the UK.

The Writhlington phalangiotarbids: their palaeobiological significance

An unusually large number of specimens of the extinct arachnid order Phalangiotarbida have been recovered from Writhlington Geological Nature Reserve, a Westphalian D (Upper Carboniferous) locality in the West Country of Britain, near Radstock. The abundance of these specimens, and their co-occurrence with a diverse non-marine biota, permit preliminary tests and tentative corroboration of two hypotheses of Carboniferous palaeoecological dynamics. The excellent preservation of the Writhlington phalangiotarbids also corroborates previous interpretations of phalangiotarbid morphology and provides unique evidence regarding the homology of the opisothosomal segmentation, which bears directly on the phylogenetic relationships of the Phalangiotarbida.

A forest-litter animal community from the Upper Carboniferous? :notes on the association of animal body fossils with plants and lithology in the Westphalian D Coal Measures at Writhlington, Avon

Over 600 blocks containing animal body fossils have been examined from the Westphalian D of Writhlington Geological Nature Reserve. The rocks, silty mudstones, comprise three partially overlapping sublithologies. Plant material may be abundant and well preserved and 18 organ genera have been recorded. Based on the taxonomic composition of the contained flora, the blocks can be divided into three groups; a fourth lacks plant debris. These groups are, in decreasing abundance, ‘dominated’ by the Lycopsida; Pteridospermae and Pteridophyta; and Sphenopsida. Amongst the animals, the insect order Blattodea, the xiphosuran Euproops , and the arachnid orders Trigonotarbida and Phalangiotarbida are found to be strongly associated with shallow lake muds containing abundant lycopod debris. The latter may represent minimally transported and sorted forest litter. The insect order Protorthoptera occurs in the above rocks as well as those deposited possibly farther from the swamp, perhaps reflecting wider ecological tolerances. In comparison, Bivalvia and Conchostraca often occur in association with little flora.