TRACE FOSSILS AS EVIDENCE IN THE EVOLUTION OF CARBONICOLA

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Abstract
SUMMARY: In Standedge cutting near Marsden, Yorkshire, the Butterly Marine Band, of late Kinderscoutian (Namurian R 1C ) age, is gradationally overlain by a small-scale (7 m) coarsening-upward sequence. This is characterised by parallel-laminated mudstone-dominated units at the base and by parallel-laminated fine sandstone/siltstone-dominated lithofacies at the top. The sequence, which probably reflects the infilling of a shallow interdistributary bay by sediment-laden incursions from distributaries, is truncated by erosion at the base of the Upper Kinderscout Grit. Planolites occurs sporadically in the 2 m of laminated silty mudstones lying immediately above the Marine Band. In the succeeding thin sandstones, siltstones and silty mudstones trace fossils are abundant and include ‘ Scolicia ’, Cochlichnus and hitherto undescribed forms. Pelecypodichnus and Pelecypodichnus escape shafts are extremely numerous in these beds and both are commonly connected to trails which are parallel to bedding planes. The association of trails with burrows indicates that bivalve movement was initially across the substratum, then steeply downward and finally more or less vertically upward. The structures fit the recently described marginally marine bivalves provisionally designated of. Sanguinolites Hind non M’Coy from the top of the Upper Kinderscout Grit in Sabden Brook, near Clitheroe. On the basis of comparative internal and external shell morphology this form appears to be ancestral to Carbonicola (Eagar 1977). Burrows and escape shafts at Standedge fit well into the sequence of burrow morphologies in which progressively longer escape shafts develop from the middle of the Shale Grit to the middle of the Marsdenian. Thence they merge upward into forms ascribed to Carbonicola, which are associated with trace fossils suggestive of fresher water. Escape shafts of Carbonicola, as far as they have been investigated, tend to show more pronounced current orientation of their long axes than shafts attributed to cf. Sanguinolites.