Non-marine bivalves from Writhlington Geological Nature Reserve, Avon

Abstract

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.