MORPHOLOGICAL PATTERNS WITHIN NARROW RANGES OF ENVIRONMENTAL CONDITIONS

Morphological analyses on the trilobite Flexicalymene granulosa (Cincinnatian Series, Upper Ordovician) has shown a significant correspondence between patterns of cranidial landmark positions and environment quantified through gradient analysis. Interpretation of these trends is complicated because morphological variability can arise through a variety of processes. Of particular importance is differentiating potential evolutionary change from geographic shifts in phenotypes through time. Distinguishing genetic from non-genetic morphological variation in this fossil species is not possible, but, in either case, environment will play a key role through processes such as selective pressure or habitat-tracking. For this reason, many studies on morphological change through time rely on depositional settings with limited environmental variability (usually from single lithofacies) with the goal of capturing evolutionary changes. The quantified model used here depicts a co-varying relationship between morphology and environment that provides a means to control each of these factors through space and time in an effort to decipher process from pattern.

With this in mind, the goal of this research is to isolate morphological changes from environmental change. Because environmental variability in this analysis is quantified, it is possible to examine morphological variation while minimizing environmental variation. In effect, this will enable a determination of whether morphology is conserved within similar environments. This has an advantage over using lithofacies alone, because gradient analysis has been shown to detect subtle environmental signals within the single lithofacies of the Kope Formation. If morphology holds constant at a particular environment, we can say that habitat-tracking (either through lateral movement of ecophenotypes or geographic subpopulations) is occurring. However, if the expressed phenotype is outside of the norm of reaction for a particular environment, then there are two possible explanations: the null model may not be capturing all the environmental parameters; or if a new morphology is expressed within a particular environment, then F. granulosa is not habitat-tracking and evolutionary change may have occurred.