QUANTITATIVE ASSESMENT OF FAUNAL STABILITY AT DIFFERING SPATIAL SCALES WITHIN AN ECOLOGICAL-EVOLUTIONARY SUBUNIT: A TEST CASE FROM THE HAMILTON GROUP AND TULLY FORMATION OF NEW YORK AND PENNSYLVANIA

Exceptional faunal stability, both in biofacies composition and rank abundance, has previously been suggested to characterize the diverse Hamilton fauna within the well studied late-Middle Devonian (Givetian) Hamilton and Tully formations of the Appalachian Basin. At present, little has been done to quantitatively characterize a final reappearance of the diverse, coral-rich fauna within the upper portion of the Tully. This recurrence appears after a major faunal change in the lower Tully, wherein an entirely new association of taxa become established, and just prior to a global faunal disruption known as the Taghanic bioevent. This latter event marks the end of the Hamilton/Tully ecological-evolutionary subunit, after which the Hamilton fauna is mostly absent from the basin. A detailed study of the composition and structure of this ultimate faunal occurrence can provide insight into the nature of biofacies change through time within an ecological-evolutionary subunit.

A rigorous quantitative examination of the faunal elements contained within the highest coral bed in the Hamilton Group (the Lansing bed) and the aforementioned bed in the overlying Tully Formation (the West Brook shale) was conducted to assess differences in biofacies composition and structure within and between beds. Replicate bulk samples were collected to assess faunal change at three differing spatial scales: within outcrop, between outcrops locally, and regionally across New York and Pennsylvania. Preliminary analyses indicate that although many taxa present in the Lansing bed persist through the lower Tully event into the West Brook, overall compositional diversity is greater within the older Lansing coral bed. Initial results also reveal that relative abundances and rank orderings of taxa vary at differing spatial scales within and between beds. One might anticipate that greater faunal diversity should occur at the end of the Hamilton/Tully ecological-evolutionary subunit due to the mixing of biotas during community reorganization; however, results do not support this conclusion.