CONTRASTING TAXONOMIC AND ECOLOGICAL CHANGE ACROSS THE MID-LATE DEVONIAN INTERVAL

The Mid-Late Devonian was an interval of elevated faunal turnover culminating in the Frasnian/Famennian mass extinction. In addition to the severe taxonomic changes that occurred during this time, communities also experienced severe ecological changes.

We investigate patterns of taxonomic and ecological change through the Mid-Late Devonian interval using faunal counts from bulk samples in New York State. Taxonomic and ecological changes are measured by classifying organisms into genera and ecological lifestyles, a method of grouping organisms by their role in an ecosystem regardless of their evolutionary history. Two-way cluster analyses and ordinations were used to analyze samples across the Givetian/Frasnian and Frasnian/Famennian boundaries.

A two-way cluster analysis of samples and genera from each stage shows several distinct biofacies identified by previous studies. When samples are coded for these biofacies they describe a depth gradient along axis 1 of a DCA ordination for the Givetian and Famennian stages, and axis 2 of the Frasnian stage. When these depth zones are considered for ecological communities, Givetian samples show little differentiation according to depth; however, Frasnian and Famennian communities show distinct separation by biofacies. This suggests that while similar Givetian ecological communities could exist along a depth gradient, Frasnian and Famennian ecological communities changed as water depth increased.

When the data for each stage is combined, ordination among genera shows progressively smaller groups for the Givetian through Famennian, suggesting communities become increasingly similar. Givetian samples separate primarily along axis 2, possibly representing a depth gradient. Frasnian samples also separate along axis 2, albeit to a lesser degree, likely representing a depth gradient. Famennian samples cluster closely, with no clear distinction of depth. When considering the entire data set for ecological communities, no distinction is seen along axes 1 or 2 in terms of depth. Instead, samples separate based on the abundance of the two primary lifestyles, surficial-attached-suspension feeders and surficial-unattached-suspension feeders, with communities becoming predominantly surficial-attached-suspension feeders by the Famennian.