BIVALVES FROM THE PENNSYLVANIAN FINIS SHALE (VIRGILIAN) OF NORTH CENTRAL TEXAS – NOTES ON TAXONOMY, LIFE HABITS AND PALAEOECOLOGY

The uppermost Pennsylvanian succession of North Central Texas is united as Cisco Group, each of the six formations included represents an almost complete megacycle of transgression and regression. The lowermost of these six cycles is known as the Graham Formation, with the Finis Shale, a complete second order cycle, near the base. The Finis Shale Cycle mostly consists of dark coloured mudstones and shales yielding an exceptionally well-preserved and moderately high diverse invertebrate fauna consisting of almost all invertebrate groups but dominated by brachiopods.

The bivalves, which rarely exceed 10 % of the complete fauna, are dominated by palaeotaxodonts and carditids. They make up more than 80 % of the bivalve fauna, pteriomorphs and heteroconchs are minor components. For our studies, the different modes of life have been inferred from data on Recent taxa.

It is assumed that all occurring genera of the Palaeotaxodonta are deposit feeders and belong to the mobile endofauna, whereas all other taxa are suspension feeders. The Carditida (Astartella), the Trigoniida (Schizodus) and the Grammysioidea are the only infaunal suspension feeders. Thus, c. 80 % of the bivalve fauna consists of infaunal taxa. The mode of life of Leptodesma (Pterioidea) is controversial (epifaunal vs. semi-infaunal). Except for Leptodesma, the rest of the bivalve fauna, making up only about 6 %, consists of epifaunally attached taxa of different groups.

The high percentage of infaunal bivalves indicates aerobic conditions in the muddy substrate, and the high number of deposit feeders shows that it was a vital biofacies with organic-rich, fine-grained substrate. The byssally attached epifauna was probably related to rare objects for attachment, e.g., other organisms, such as bryozoan colonies, corals, crinoids or algae may have been suitable attachment objects. The particularly high number of infaunal taxa is presumably a result of mutual food and space competition. A typical tiering was developed and the bivalves mostly settled the infaunal niches, where the competitive pressure was lower compared with the substrate surface dominated by brachiopods. An attempt to correlating the present results with the studies on faunal communities of previous authors is in progress. Their results, however, lack a taxonomic revision of the bivalve fauna.