39915-AC8
Bridging the Biotic-Geologic Divide: Morphological Evolution in the Ordovician Trilobite Flexicalymene in its Temporal, Geographic, and Phylogenetic Context
This project has demonstrated a clear parallel between the pattern of geographical variation among populations of a Paleozoic marine invertebrate, and a long-term temporal trend in morphology expressed at the scale of 104 to 105 years. The position of the eye in the late Ordovician trilobite Flexicalymene granulosa is located in a more axial and a more anterior position among individuals living in shallower water. In parallel to this, as the oceans shallowed during the early part of the Cincinnatian time interval, so the mean position of the eye shifted accordingly. Thus, in this species, variation this character shows an extremely subtle but statistically significant trend related to water depth. The adaptive value of this variation is unclear, but it is persistent. The project is the first study in the Paleozoic to demonstrate a scalar relationship between morphological variation on a short geographic/temporal timescale to that operative at million-year time intervals. In our time of rapid and unprecedented global change it is essential to view the geological record as an empirical testament of the experience of past environmental change. Geological sequences that are preserved with outstanding fidelity offer special opportunities for high-resolution analyses of the interactions between organisms and their environments. The project has been a detailed analysis of variation in morphology of the outstandingly well-preserved calymenid trilobites from these rocks. The special preservation in the Cincinnatian series of both the original mineralogy of the exoskeleton and of the original 3-D shape, allows for particularly detailed analysis of biologically-related changes in morphology. The high fidelity of the record of temporal change offers the chance to compare such variations at a variety of spatial and temporal scales. This project began with the bed-by-bed analysis of the preservation of intact articulated exoskeletons in a 0.46 m interval of shale at Mt Orab, Ohio. We showed that the 7+ beds represented in this interval have similar signatures of preservation via swamping, and that the entire interval likely accumulated in an interval of between 10 and 10,000 years. Thus, variation witnessed accrued at a timescale accessible from evolutionary studies of living organisms (Hunda et al. 2006). Using this as a basis, we were able to expand the study to look at a series of significant morphological transitions. Firstly, we explored an important evolutionary episode high in the Cincinnatian Series, in which Flexicalymene retrorsa retrosa is replaced by its miniaturized sister taxon which Flexicalymene retrorsa minuens. We showed that, despite previous interpretations, the transition did not involve a simple case of dwarfism, in which the descendent species simply slowed its rate of development. Rather, the change involved a subtle repatterning of growth dynamics (Hunda and Hughes, 2007). Another analysis of morphological variation, lower in the Cincinnatian series, demonstrated the temporal change in eye position discussed above, and related this to depth-related changes in the ecological assemblages of marine shelly organisms (Webber and Hunda, 2007). The publications listed resulted from the work of graduate student Brenda Hunda, whose PhD project was on this subject and who obtained a position as a professional paleontologist at the Cincinnati Museum Center during the course of this award. Dr. Hunda and PI Hughes then recruited another graduate student, Autumn Thompson, to undertake a Master's degree continuing the research that Dr. Hunda had initiated. Ms. Thompson graduated in March 2008 and is about to take up position as Geology Instructor at Charleston Southern University in Charleston, South Carolina. Hence this project as shown marked success in training and graduating successful students, both of whom have won competitive positions in the field. Ms Thompson's Master's degree research concentrated on the connection between geographic variation and that seen temporally. By selecting a particularly distinctive marker bed that could be traced over 10's of kilometers in the Cincinnatian Series it was possible to detect geographical variation on the finest time-scale presently assessable in these rocks: likely a maximum of 103 years. This was then compared statistically with the previously reported temporal trend in the Cincinnatian Series (Webber and Hunda 2007) that we know to reflect long-term changes in water depth. The clear parallel between the two provides the connection that shows that the longer term trend is a scaling of that seen at the shorter term (Hunda et al. 2008). This is a result of great importance because it makes the connection that was outlined in the original title of the proposal, and bridges the gap between micro- and macroevolution. The project has yielded 4 important publications to date, in journals including Evolution, Evolution and Development, Palaios and as a book chapter. More will be forthcoming. Female graduate students have been trained and successfully placed in related positions, and important inactions with the public have been gained through the amateur Cincinnati fossil group the Dry Dredgers.
