The faunal phases and palaeoecology of ostracod-mussel bands in the coal measures of the north and England

Abstract

The stratigraphy of three ostracod-mussel bands occurring respectively in the Lower Modiolaris, Upper Communis and Upper Similis-Pulchra Zones of the Northumberland and Durham Coal Measures is described. The Hopkins' Band in the Dower Modiolaris Zone is compared with similar bands at an equivalent stratigraphical horizon in the Midgeholme and Cumberland coalfields. The dominant non-marine lamellibranchs in this band resemble, or are related to, Anthracosia regularis and Garbonicola oslancis. The Three Quarter ostracod-mussel band from the Communis Zone contains Ourvirimula and Garbonicola declevis, while the Claxheugh Shell Bed from the Upper Similis-Pulchra Zone contains Naiadites and forms resembling Anthraconauta phillipsi. The ostracod, annelid, plant and fish remains in these bands are also described. The validity and distinction of the ostracod genera Jonesina and Geisina are established and the species J. fastigiata, G. arcuata and G. subarcuata are described in detail. The ostracod species Carbonita cf. evelinae, C. of. rankiniana, C. humilis, G. pungens. G. inflata, C. secans and C. concava sp. nov. are described from the Hopkins' Band and the presence of moults and dimorphic forms of these species is shown in the ostracod populations of the bands. The synonomay, morphology and stratigraphical ranges of the above species of ostracods from the British Coal Measures are reviewed and discussed. The true generic assignment and growth forms of the worm Microconchus (Spirorbis) pusillus are discussed. Vertical sequences of six "faunal phases" for the Hopkins' Band and three for the Three Quarter Band are deduced from a study of the respective faunas at several localities. These faunal phases are subsequently related to changing ecological conditions. The conditions under which the sediments of the Hopkins' Band were laid down are deduced from a study of the petrology of the sediments and their chemical composition. The orientation and associations of fossils in the sediments are recorded and the published information on palaeoecology and comparative ecology of living mussels and ostracods is reviewed. It is concluded that the Hopkins' Band was deposited in a brackish lagoonal environment at the beginning of a typical Coal Measures cyclothem. At first the richly carbonaceous bottom sediment of this lagoon prohibited the establishment of a benthonic fauna, but a change in the source and type of sediment and increase in turbulence enabled a mussel fauna to thrive with a reduction of the previously abundant pseudo-planktonic ostracod fauna. Increased sedimentation eliminated this fauna and the conditions became more deltaic. Similar environmental conditions are deduced for the Three Quarter Band, but the Claxheugh Shell Bed is believed to have formed in a small isolated brackish or fresh water lake within the main delta or swamp.