QUANTIFYING ORNAMENTAL VARIABILITY IN AN AMMONOID CLADE USING GIS SPATIAL ANALYSES

Ammonoids are known for their intraspecific and interspecific morphological variation through ontogeny, specifically in ornamentation. Ornamental features are difficult to homologize, which makes qualitative descriptions and widely-used morphometric tools inappropriate for quantifying the complex morphologies of ammonoids. However, spatial analyses such as those applied in geographic information systems (GIS) allow for global quantification and visualization of shell form. Here, I present a GIS-based method in which ornamental variability is assessed in order to evaluate evolutionary patterns in two heteromorph ammonoids.

I applied GIS-based techniques to sister species from the Late Cretaceous Western Interior Seaway: the ancestral and more variable Hoploscaphites spedeni, and descendant and less variable H. nebrascensis. I created digital models exhibiting the shells’ lateral surfaces using photogrammetric software and imported the reconstructions into a GIS environment. Orientation Patch Count (OPC), traditionally used to quantify dental complexity, was used as a terrain roughness index to quantify the degree of tuberculation. This 3D analysis exposed the overlapping morphologies of H. spedeni and H. nebrascensis, with H. nebrascensis specimens exhibiting similar ornamentation to the most tuberculate H. spedeni. In order to evaluate the 2D distributions of ornament on each shell, I digitized the tubercles as points and the ribs as polylines. Average Nearest Neighbor analysis revealed that the distribution of ribs is constrained in at least H. spedeni, and the distribution of tubercles is more variable through ontogeny between both Hoploscaphites species.

The results of the GIS-based spatial analyses demonstrate that the target for evolutionary change in this clade resides in the macroconch body chamber. Specifically, H. nebrascensis is a paedomorphic descendant of H. spedeni, derived by means of prolonged tubercle expression through later ontogenetic stages. However, H. nebrascensis microconchs retain developmental flexibility from H. spedeni. These results are consistent with previous research on these species and demonstrate the versatility of GIS-based spatial analyses to successfully and objectively quantify and visualize ammonoid characters and character states.