ONTOGENETIC AND ALLOMETRIC SCALING WITHIN THE LATE CRETACEOUS PLACENTICERAS OF ALABAMA: A LINEAR MIXED MODEL APPROACH

High morphological variability in the Late Cretaceous Placenticeras genus of ammonites has historically resulted in a proliferation of species designations within this group, but recent studies have recombined many taxa, acknowledging dimorphism, polymorphism, and ontogenetic change in this genus. Quantifying morphological changes through ontogeny within Placenticeras populations is a prerequisite to understanding where boundaries between Placenticeras species are supported. While the ontogenetic stages for some placenticeratids have been previously described qualitatively, this study is the first to use linear mixed models (LMMs) to quantitatively assess ontogenetic and allometric scaling within Placenticeras.

We sampled 113 Placenticeras specimens from the Late Cretaceous (Santonian to Maastrichtian) of Alabama and collected morphometric measurements at 1-7 different ontogenetic positions per specimen, depending on available preservation, for a combined total of 301 measured positions. Measured variables include standard ammonite conch parameters (e.g. whorl height, whorl width, etc.) and variables describing ornament placement. We used LMMs to control for the non-independence of measurements from the same specimen, allowing morphological variation within and among individuals to contribute to our evaluation of allometric versus isometric scaling within the Placenticeras of Alabama. Using log-transformed data, we first evaluated whether or not the linear relationship between each measured variable and specimen size (whorl height) indicated isometric/allometric scaling across the sample; we then evaluated a (more complex) threshold linear relationship (allowing a change in slope after a given threshold size).

Several variables (whorl width, venter width, radius, radial umbilicus, and the distance between umbilical tubercles) demonstrated biphasic allometric scaling within the sample, with a significant change in slope (p < 0.05) occurring at whorl heights between 41 and 62 mm, suggesting a transition between ontogenetic stages within that range of whorl height sizes for the sample. By contrast, some other variables were best fit by a linear relationship with whorl height and scaled isometrically within the sample (e.g. septal spacing).