ANTHRACONAIA PULCHELLA SP. NOV. AND A STUDY OF PALAEOECOLOGY IN THE COAL MEASURES OF THE OLDHAM AREA OF LANCASHIRE

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Abstract
A new species of non-marine lamellibranch, Anthraconaia pulchella, is proposed in place of A. pulchra (Wright non Hind) from the Lower similis-pulchra Zone of the Coal Measures. A detailed study is made of the relationship between the shells of A. pulchella sp. nov. and the enclosing sediments at Oak Victoria Colliery, Oldham. Shell size is shown to be directly related to the organic carbon and detrital quartz contents of the sediment and inversely related to the macroscopic pyrite content. The conditions under which the sediments were laid down are deduced, partly from study of the sediments themselves and partly from study of the disarticulation/ articulation ratios of the shells and the orientation of the shells relative to the sediments. It is concluded that A. pulchella sp. nov. thrived under conditions of relatively turbulent water but was dwarfed under conditions of relatively stagnant water. The cause of dwarfing is ascribed either to failure in food supply (possibly allochthonous organic debris in the main), or to the establishment of foul bottom conditions, or to a combination of both factors. In addition to Anthraconaia, the pulchella-bearing bed also carries occasional specimens of Anthracosia, which become more common towards the base, although they never dominate the faunal assemblage. Immediately beneath this bed, however, there is a band (10 inches thick) in which Anthracosia becomes dominant. Possible causes of the change from an Anthracosia-rich to an Anthraconaia-rich fauna are considered.