THE PHENOMIC TOOLKIT AND PALEONTOLOGY: A CASE STUDY USING PALEOGENE MARSUPIALS (Invited Presentation)

Automated three dimensional geometric morphometric analysis (auto3dgm) is a promising high-throughput tool for studying shape variation across space and time. It is important to recognize that such high-dimensional methods complement, rather than displace, other morphometric approaches. Here, we use the fossil record of peradectid marsupial molars from before, during, and after the Paleocene-Eocene Thermal Maximum (PETM) in the Bighorn Basin, Wyoming, to exemplify uses and challenges of auto3dgm. The question of whether PETM peradectids belong to the same taxon as non-PETM peradectids remains open, complicated by long-standing difficulties discriminating isolated second and third lower molars (M₂, M₃) in order to compare homologous tooth positions.

Using 3D digital models, we assembled a dataset of lower molars (N = 65) following previously published protocols for surface creation and alignment. Most (N = 52) were peradectid M₂s or M₃s. A minority (N = 13) were either of another family or tooth position included to demonstrate the outcome of analyzing misidentified specimens. In a principal components analysis (PCA) those “mistakes” were not clearly differentiated from correctly identified specimens in PCs 1-2 but occupied the edges of specimen clusters. These results illustrate that while auto3dgm is not immune to the “garbage in, garbage out” problem, a careful check of which surfaces occupy the margins of distributions can help avoid spurious results in downstream analyses.

After misidentified specimens were removed, PCA decomposed the dataset into two non-overlapping clusters of PETM vs. non-PETM specimens along PC1. These results support a two-taxon interpretation across this interval. Within each cluster, PC2 partially separated known M₂s from known M₃s. Heat maps of mean differences between known M₂s and M₃s highlighted trigonid proportions as important for discrimination. A resulting trigonid length-to-width ratio became a simple tool for identifying isolated molars, including broken and worn specimens, to tooth position. Overall, auto3dgm forms one part of a broader morphometric toolkit necessary to document faunal change.