MORPHOLOGICAL CHANGES IN FLEXICALYMENE (TRILOBITA) FROM THE CINCINNATIAN SERIES: GEOGRAPHIC VARIABILITY AND THE EFFECTS OF TEMPORAL AVERAGING ON MICROEVOLUTIONARY PATTERNS

The fossil record presents definitive evidence of evolutionary change through time, but identifying the causes responsible for such change presents a serious challenge. Fortunately, conceptual and technical advances made in the last five years offer improved opportunities for understanding both the detailed nature of morphological change (or lack thereof) within lineages of fossils. This study has focused on a uniquely well preserved lineage of trilobites recovered from a stratigraphic setting that is particularly favorable for the dissection of micro-evolutionary change.

Currently over 2000 specimens of Flexicalymene have been collected from 47 trilobite-bearing beds from throughout the Cincinnatian Series, of over which 1000 have been digitized for landmark analysis. Previous taxonomic assessments have suggested that the cranidial morphology of these trilobites is stable. Morphometric analysis has shown that specimens within the Richmondian show remarkable stability in cranidial landmarks. This is not always the case. Specimens within the Edenian (Kope Formation) show morphological stability in the glabella, however, analyses have documented adaxial and anterior migration of the eye which apparently occurred gradually and iteratively. The disparity of evolutionary patterns within a single lineage of Flexicalymene in the Cincinnatian Series is striking, but factors contributing to this disparity are unclear and currently being explored.

The assessment of geographic variability is key in deciphering whether the observed morphological changes are ecophenotypic or evolutionary in nature. Preliminary results suggest that geographic variability is limited, and that long term morphological trends exceed geographic variance among contemporary populations. In addition, initial results suggest that morphological variance in "time-rich" beds is low compared to overall variance throughout the section, regardless of the degree of time-averaging within any individual bed.