A new highly variable non-marine bivalve fauna and a new species from basal Westphalian D near Osnabriick, Germany with an Appendix: New internal and external features in the genusAnthraconaia

Abstract

AbstractThe study is based on over 170 bivalve specimens collected in the last seven years by three independent collectors in basal Westphalian D strata at Piesberg Quarry, near Osnabrück, northern Germany. All the material has come from the roof shales of either the Bänkchen or the Dreibänke coal, 25 m above it, most of it from above the Dreibanke coal. Both faunal bands have closely similar sediments, shells and floral associations, mainly of hydrophyllic plants, including those that characterise waterlogged swamps. The palaeoenvironments were, on average, of low energy deltaic lakes.Both faunal bands constitute life assemblages of small, sparseAnthraconaiamingled with abundant plants and plant debris in richly carbonaceous shale with a variable carbonate content. Siphonal and pedal gapes of the valves indicate that the bivalves lived in a steeply burrowing position probably below the surface of the sediments. After death most valve pairs lay with their median planes parallel to the bedding planes of the sediment. Later overlying pressure, normal to bedding planes and varying according to the time of carbonate formation, led to a pattern of breakdown of the convexity of the shells essentially the same as that found in largerAnthraconaiain Westphalian B. In both cases it appears that lateral profiles of shells have been unaffected by vertical crushing. It is therefore reasonable to treat the profiles of uncrushed and vertically crushed shells together.On the basis of measurements and morphology two groups of shells have been distinguished at Piesberg, that ofAnthraconaia pruvosti(Tchernychev) Weir, comprising about 96 per cent of the fauna, and the remainderAnthraconaia piesbergensissp. nov. Each is illustrated by a variation diagram or pictograph and is in part defined by growth equations in terms of shell length, height and anterior end. The smallA. piesbergensis, which also yields evidence of posterior gape, is formally described.Anthraconaia pruvosti, which has not been previously recorded in Germany, shows extremely wide variation with a number of new varieties, but all intergrade with previously known varieties of this species and includeAnthraconaia weissiana(Geinitz). The mean size of the shells is half that of the holotype, from northern France, but size ranges are comparable with those from the U.K. The mode of the fauna lies around small, elongate, subtriangular shells with nearly straight to slightly reflected ventral margins.The elements of the Piesberg fauna fall into place stratigraphically in the sequence ofAnthraconaiafaunas in Westphalian C to late Westphalian D, mainly in the southern part of the U.K. Moreover, very small, scarce shells recently described above No. 10 coal, late Westphalian D at Writhlington, Somerset, U.K., appear related toA. piesbergensis. At this horizon they had different associates and a different palaeohabitat from those of the largerAnthraconaia pringlei(Dix & Trueman) which succeededA. pruvosti. It is therefore likely thatA. piesbergensismay have occupied a different palaeoecological niche from that ofA. pruvostiand thatA. piesbergensiswas collected from a horizon slightly different from those yieldingA. pruvostiat Piesberg Quarry, possibly from a fauna which lived in shallower water.Re-figuring of uncrushed, excellently preserved material ofAnthraconaiaaff.pruvostifrom late Westphalian C of Pembrokeshire, SW Wales, reveals new evidence of the musculature of the genusAnthraconaia. There is also clear evidence of siphonal and pedal gapes in closed valves, both being features previously unrecognised inAnthraconaia, and characteristic, to date, only of Westphalian late C and D Stages in Germany and the U.K.