A new highly variable non-marine bivalve fauna and a new species from basal Westphalian D near Osnabruck, Germany

The study is based on over 170 bivalve specimens collected in the last seve n years by three independent collectors in basal Westphalian D strata at Pi esberg Quarry, near Osnabruck, northern Germany. All the material has come from the roof shales of either the Bankchen or the Dreibanke coal, 25 m abo ve it, most of it from above the Dreibanke coal. Both faunal bands have closely similar sediments, shells and floral associa tions, mainly of hydrophyllic plants, including those that characterise wat erlogged swamps. The palaeoenvironments were, on average, of low energy del taic lakes. Both faunal bands constitute life assemblages of small, sparse Anthraconaia mingled with abundant plants and plant debris in richly carbon aceous shale with a variable carbonate content. Siphonal and pedal gapes of the valves indicate that the bivalves lived in a steeply burrowing positio n probably below the surface of the sediments. After death most valve pairs lay with their median planes parallel to the bedding planes of the sedimen t. Later overlying pressure, normal to bedding planes and varying according to the time of carbonate formation, led to a pattern of breakdown of the c onvexity of the shells essentially the same as that found in larger Anthrac onaia in Westphalian B. In both cases it appears that lateral profiles of s hells 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 of Anthraconaia pruvosti (Tchernychev) Weir , comprising about 96 per cent of the fauna, and the remainder Anthraconaia piesbergensis sp. nov. Each is illustrated by a variation diagram or picto graph and is in part defined by growth equations in terms of shell length, height and anterior end. The small A. piesbergensis, which also yields evid ence of posterior gape, is formally described. Anthraconaia pruvosti, which has not been previously recorded in Germany, shows extremely wide variatio n with a number of new varieties, but all intergrade with previously known varieties of this species and include Anthraconaia 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 f auna 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 of Anthraconaia faunas in Westphalian C to late Westphalian D, ma inly in the southern part of the U.K. Moreover, very small, scarce shells r ecently described above No. 10 coal, late Westphalian D at Writhlington, So merset, U.K., appear related to A. piesbergensis. At this horizon they had different associates and a different palaeohabitat from those of the larger Anthraconaia pringlei (Dix & Trueman) which succeeded A. pruvosti. It is t herefore likely that A. piesbergensis may have occupied a different palaeoe cological niche from that of A. pruvosti and that A. piesbergensis was coll ected from a horizon slightly different from those yielding A. pruvosti at Piesberg Quarry, possibly from a fauna which lived in shallower water. Re-figuring of uncrushed, excellently preserved material of Anthraconaia af f. pruvosti from late Westphalian C of Pembrokeshire, SW Wales, reveals new evidence of the musculature of the genus Anthraconaia. There is also clear evidence of siphonal and pedal gapes in closed valves, both being features previously unrecognised in Anthraconaia, and characteristic, to date, only of Westphalian late C and D Stages in Germany and the U.K.