USING GEOMETRIC MORPHOMETRICS AND GRADIENT ANALYSIS TO QUANTITATIVELY COMPARE SPATIO-TEMPORAL MORPHOLOGICAL TRENDS TO ENVIRONMENTAL CHANGE

A challenge to investigations of temporal patterns of morphological change in fossil species is differentiating true morphological evolution from the stratigraphic expression of ecophenotypic variation. Patterns interpreted as evolutionary change in fact may reflect the migration of ecophenotypes into and out of the study region in response to environmental shifts through time. Many studies, especially those from relatively older strata, rely on stratigraphic architecture, sedimentological characteristics, or biofacies associations to gain insight into environmental conditions that may be coincidental with larger-scale morphological trends. These approaches can potentially ignore fine-scale trends, which may correspond to environmental gradients that are subtle and difficult to recognize, such as from depositional settings within single lithofacies. Furthermore, a clear understanding of the geographic variability of intraspecific morphology is hindered in depositional settings where geographic coverage is limited or where fine-scale stratigraphic correlations are lacking.

Here, the spatio-temporal relationship between the morphology of the trilobite Flexicalymene granulosa and paleoenvironmental conditions in lower type Cincinnatian Series is explored quantitatively and at high resolution. Morphological change in F. granulosa is measured using geometric morphometrics, a technique that quantifies shape change at a high resolution within an integrated morphological complex. Environmental change is captured through a bed-by-bed application of gradient analysis, which numerically compares the distribution and abundance of fossil remains and has been used to ordinate taxa according to underlying ecological gradients. The outcomes of these two quantitative approaches are compared directly to test whether F. granulosa morphology varies systematically with both stratigraphic and geographic environmental gradients. Previous analysis using these techniques has shown a general correlation between eye position and environmental factors related to water depth. These findings are expanded upon by testing whether geographic variation in morphology compares to spatial environmental gradients in nearly contemporaneous assemblages across the Cincinnatian paleo-ramp.