QUANTIFYING MORPHOLOGICAL VARIABILITY IN AMMONOIDS USING GIS SPATIAL ANALYSES

One difficulty in determining the evolutionary relationships among ammonoid taxa stems from the high morphological variability present in many species and genera. It is common to use characters related to shape and ornamentation in evolutionary studies; however, with no effective way to quantify the variance of these characters and the covariance among them, phylogenetic analyses often are inconclusive. The intent of this study has been to test a new methodology for quantifying morphological variability in ammonoid shell shape and ornamentation using GIS.

First, I acquired photographs of the left lateral surface of 30 specimens described as either Hoploscaphites (Jeletzkites) spedeni or H. (J.) nebrascensis from the Late Cretaceous Western Interior. This heteromorphic ammonoid genus is known for its intraspecific variability. After scaling the photographs, I then used ArcGIS 10 to trace the shape of the body chamber and phragmocone, the ribs, and the tubercles as polygons, polylines, and points, respectively. Next, I constructed spiral transect lines through the entire exposed shell of each ammonoid representing 25%, 50%, and 75% of the shell’s whorl height; intersections of the transect lines and polylines and various ratios serve as indices to quantify variations in rib density within and among specimens. I also assessed variability in the spatial distribution of tubercles on the lateral surface of each shell and evaluated correlations between locations of tubercles and ribs using spatial statistics tools in ArcGIS 10. Finally, I compared body chamber shapes by overlaying polygons of multiple specimens.

Although this methodology simplifies the lateral surface of an ammonoid shell and assumes no injuries occurred in the specimen’s ontogeny, it provides a new way to quantify the spectrum of variability present in many morphological characters, specifically shape and ornamentation, used in phylogenetic studies. In combination with new 3-D geospatial approaches to the analysis of fossil forms, these GIS techniques will help ammonoid paleontologists to better define character states and produce more resolved phylogenies for ammonoid clades.