Many evolutionary studies focus on how populations of organisms adapt and change in the face of environmental changes. The raw material upon which natural selection acts is plasticity, or variability, in an organism?s phenotypes (the physical expression, such as body size, shape, and others, of the genetic make-up of an organism). Nearly all phenotypes will show some plasticity because traits that are expressed result from the interaction of genetics with environment. The relationship between the plasticity of phenotypes and variable environmental conditions, however, still requires further understanding. While numerous biological studies have addressed this relationship with modern organisms, even extending into the young fossil record, this area of research has been lacking in settings that are much older. The power of the fossil record is that it allows us to measure changes within a species over longer time scales; but can paleontology?s contribution to evolutionary biology improve our understanding of the relationship between phenotype and environment over time? There are two primary steps in achieving this goal: 1. the geographic distribution of phenotypic variability within a species and how it corresponds to spatial environmental gradients (known as a reaction norm) must be accurately documented. 2. The reaction norm for a species must then be evaluated over time. The goal of this research is to describe the relationship between paleoenvironmental conditions and variability in the shape of two trilobite species within rocks of the Kope Formation (Ordovician Period, around 450 million years old). In this study, morphology is measured using geometric morphometrics, which quantifies shape change at high resolution. Paleoenvironmental change is measured through gradient analysis, which uses the distribution and abundance of fossil remains to characterize environment. Preliminary research shows a statistically significant portion of shape variability in the cranidium (head) of the trilobite Flexicalymene granulosa is related to the environment in which this species is preserved. This relationship between shape and environment lead us to propose that patterns of change through time in this trilobite species is the result of populations following their preferred environments (also known as environmental tracking). To test our hypothesis, the next step is to systematically track this established relationship through time. To this end, we will compare cranidial shape to general geological interpretations, such as rock type, of environment, and to a quantified measure of environment acquired through gradient analysis in the Kope Formation. Intensive field work conducted over the next three years will provide a comprehensive dataset of trilobite cranidial shape and environment through time and space from which to test our hypothesis. This study has direct implications for understanding how variation within a trilobite species contributes to evolution within trilobites as a group. It compares morphological change to a high-resolution measure of environmental change using a novel approach, which can be used on many organisms in many geological settings. An important component of this study will be the integration of scientific research with science education through a workshop developed at Cincinnati Museum Center designed to provide training for regional science teachers in geology and paleontology.
The primary goal of this research was to quantify the relationship between phenotypic plasticity and environment to interpret stratophenetic patterns in the trilobite species Flexicalymene granulosa from a 2 million year interval within the Upper Ordovician (450 million year old) Cincinnatian Series in its type setting. Morphological change was measured using geometric morphometrics, or landmark-based analyses, which quantifies shape and shape change at high resolution within an integrated morphological complex. Environment was characterized qualitatively using depositional features such as lithology and sedimentary cycles, and quantitatively using high-resolution gradient analysis applied to strata of the type Cincinnatian. Previous work had defined a geographic morphocline and established a statistical correlation between morphology of F. granulosa and paleoenvironment in the Kope Formation. Sampling over the course of 4 field seasons has resulted in a high-resolution dataset from 63 beds from the Snag Creek, Alexandria, Grand View and Grand Avenue submembers from six outcrops spanning approximately 90 km of the outcrop area. Based on these analyses, we have found a consistent upramp-to-downramp trend in Flexicalymene morphology throughout the sampled interval. Up-ramp specimens have narrower cranidia and more inward- and forward-positioned eyes than down-ramp specimens. These geographic morphoclines are stable throughout the studied interval, although data now suggests that their intensity, as related to the strength of environmental gradients, varies along an environmental gradient. These morphological patterns extend from the Snag Creek through the Grand Avenue submembers of the Kope Formation. The Snag Creek and Alexandria submembers record a time of stable environmental conditions with a shift towards a more rapid shallowing upward trend (as recorded by stratigraphy, sedimentology, and DCA scores) into the Grand View and Grand Avenue submembers. Within this interval of more rapid environmental change, morphologies shift significantly from the quieter Snag Creek and Alexandria morphologies, but the norm of reaction remains stable. Hence, while morphologies shift with environmental shifts, these changes are well within the pattern of the norm of reaction established by geographic studies. The conclusions that can be drawn from this research are: 1) The stability of the reaction norm through time, along with changes in intensity as a function of morphological distance (geographic differentiation), indicate a dynamic, yet persistent, clinal regime in which morphotypes track shifts in the environmental gradients along the paleoramp through time. 2) A lack of novel morphologies in this interval suggests that this regime is not a setting in which adaptive change accounts for stratophenetic patterns. 3) Stratigraphic patterns mirror geographic patterns, therefore it is crucial to have an understanding of geographic variation prior to the interpretation of stratophenetic patterns to determine if they arise through evolutionary processes or reflect shifting morphotypes over time. 4) While shifts in morphology do occur within the study interval during times of environmental change (changes in water depth), these are an extension of the norm of reaction established by geographic studies, and therefore, represent clinal translocation.
