DELINEATING SPATIAL AND TEMPORAL MORPHOLOGICAL TRENDS IN FLEXICALYMENE (TRILOBITA): A QUANTITATIVE APPROACH TO IDENTIFYING ECOPHENOTYPY IN THE TYPE CINCINNATIAN (UPPER ORDOVICIAN)

Trilobites have played a key role in the documentation of evolutionary patterns in the fossil record. Differentiating true morphological evolution within trilobites from the stratigraphic expression of ecophenotypic variation presents a challenge to paleobiological investigations. As environment plays a key role in both spatial and temporal expression of morphology, it is essential to distinguish between ecophenotypic variation within a single species as a result of environmental gradients, and evolutionary change through time. At the extreme, what appears as evolutionary change may in fact reflect the lateral migration of ecophenotypes (phenotypic expression that is related to environment) as they track their preferred environments through time. As noted by Jablonski (2000), this problem was recognized over 40 years ago, but only a few studies, predominantly from strata within a temporal scope of the last 15 million years, have addressed it in detail.

There are two important steps to recognizing ecophenotypy. First, it must be established that the morphologic variability within a taxon is related to environmental variables. This is a determination of whether differences in some environmental variable (such as substrate conditions, salinity, or others) are at least associated with, if not causing, different morphotypes within a taxon. Previous analyses have shown that cranidial morphology in Flexicalymene corresponds with changing environmental factors. Secondly, if a relationship between morphology and environment can be established, then it must be assessed geographically (i.e., across contemporaneous populations) and stratigraphically (i.e., over time).

Here, a quantitative approach to this second step is applied. Utilizing geometric morphometrics to depict the cranidial morphology of Flexicalymene and gradient analysis to capture fine-scale environmental fluctuations, the spatio-temporal relationship between morphology and paleoenvironmental conditions in 3 localities of the Snag Creek Submember of the lower type Cincinnatian Series is explored.