THE ROLE OF GEOGRAPHIC VARIATION IN INTERPRETING STRATIGRAPHIC PATTERNS OF MORPHOLOGICAL CHANGE

Phenotypic variation is the raw material acted upon by natural selection. Nearly all traits will show some plasticity because phenotypes result from the interaction of genetics with environment. The response of phenotypic plasticity to variable environmental conditions requires further understanding, especially in deep-time studies. The power of the fossil record is that it allows us to measure morphological change within a species over longer time scales, but can paleontology contribute to evolutionary biology by improving our understanding of the relationship between phenotype and environment over time? This requires an understanding of how phenotypic plasticity within a species translates into the stratophenetic patterns documented in the fossil record. There are two steps to achieve this: (1) document the geographic distribution of phenotypic variability within a species and how it corresponds to spatial environmental gradients, and then (2) evaluate this relationship over time.

The relationship between morphological variability and environment is quantified to interpret stratophenetic patterns in the trilobite species Flexicalymene granulosa from the Upper Ordovician Kope Formation. Previous work has defined a geographic morphocline and established a statistical correlation between morphology of F. granulosa and paleoenvironment regionally across a single bed. Here, we expand on this to include a temporal view of this relationship by sampling several beds over a relatively small stratigraphic interval (the Alexandria Submember) over the region (approximately 90 km). In this study, morphological patterns are measured using geometric morphometrics, which quantifies shape change at high resolution. Environmental gradients are measured through faunal gradient analysis, using both detrended correspondence analysis and nonmetric multidimensional scaling. The results of these two approaches can be compared. Further corroboration of biological signals is achieved by comparing them to general sedimentological interpretations within the Kope Formation. We propose that morphological patterns through time are the result of lateral shifts in morphotypes as environment shifts, with little to no net change in morphology.