ECOLOGIC CHANGES ASSOCIATED WITH THE LATE DEVONIAN MASS EXTINCTION: EVIDENCE FROM FIELD AND LABORATORY STUDIES OF LIMESTONES FROM THE GREAT BASIN REGION OF THE WESTERN UNITED STATES

The Great Basin and Rocky Mountain regions of the Western United States offers an excellent opportunity to examine the ecologic changes associated with the Late Devonian mass extinction. This region contains a nearly continuous record of deposition on a carbonate shelf throughout this period, and outcrops are readily accessible.

In order to examine the ecologic changes during this interval, we examined limestone strata of middle Devonian to early Mississippian age. In total, nearly 4 km of section were logged in detail from 25 field localities stretching from southern Nevada to northern Montana. From this field data, a large database was constructed that includes over 3500 records with 58 fields.

The analysis of this data set demonstrates that a large shift in faunal abundance occurred in soft substrate communities from the pre-extinction Devonian, to the post-extinction Early Mississippian. The primary signal is a change from brachiopod rich communities in the Devonian to echinoderm-dominated communities in the Early Mississippian. For example, on average the fossil content of Devonian wackestones is approximately 50% brachiopod remains and 9% echinoderm fragments. In contrast, the fossil content of Early Mississippian wackestones is approximately 70% echinoderm material and only 20% brachiopod material.

In addition to field logging, several hundred samples were collected for laboratory analysis. The examination of thin sections and peels from these samples confirms the trends seen in the field data and demonstrates how thorough the pelmatozoan domination of the substrate was during the Early Mississippian. Thus the shift in faunal composition remains consistent at all scales of examination and within all categories of classification (e.g. packstones, grainstones, etc.).

Other associated changes highlighted by this analysis include an apparent decrease in the polytaxic nature of the communities from the Devonian to the Early Mississippian, and consistent though less dramatic changes in abundance within other faunal groups.