Changes in morphological disparity in eutherian
mammals across the K-Pg boundary and Palaeocene-Eocene Thermal Maximum using discrete
morphofunctional characters

Emmerson, Rebecca

Abstract

The mammalian clade Eutheria comprises placental mammals (the dominant group of mammals today, with >6000 living species, ~93% of living diversity) plus their stem relatives. A major event in Earth history that had a profound influence on Eutheria was the Cretaceous–Paleogene (K–Pg) extinction event, ~66 million years ago. Prior to this extinction event, during the Mesozoic, known eutherians were small and apparently predominantly insectivorous, showing relatively limited morphological disparity. After it, during Palaeogene, there was a major increase in eutherian disparity, with a much wider range of body sizes and ecomorphologies (including specialized herbivores and carnivores) appearing in the fossil record, reflecting the availability of new ecological niches. A second key event known to have influenced eutherian evolution is the Palaeocene-Eocene Thermal Maximum (PETM), a period of particularly high global temperatures ~55 million years ago, which coincided with major faunal turnover. However, the extent to which the PETM affected eutherian disparity, and how this compares to the changes seen across the K-Pg boundary, has been less well studied. Previous studies of mammalian disparity using discrete characters have typically used phylogenetic character matrices, which are intended to resolve evolutionary relationships, and so score non-homologous but functionally equivalent structures differently; this may be problematic for estimating changes in disparity. Here, a different approach is used: 61 discrete character traits of the dentition and lower jaw are identified that reflect morphofunctional attributes but that do not take into account specific homologies (e.g. the particular dental locus), and which should more accurately reflect ecomorphological disparity. These characters are scored for 245 eutherian taxa from the Late Cretaceous, Palaeocene and Eocene. The resultant matrix was then used to quantify morphological disparity and changes in morphospace occupancy in eutherians over time using Principal Co-Ordinate Analysis (PCoA) and non-metric multidimensional scaling (NMDS), using both epoch and stage/age time bins, and comparing the global pattern with results for individual landmasses. Disparity changes were then quantified for the PCoA analysis using Sum of Variances (SOV), Sum of Ranges (SOR), Distance from Centroid (DFC) and Weighted Mean Pairwise Disparity analyses. Results confirm a major increase in morphospace occupancy across the K-Pg boundary, with the Late Cretaceous morphospace entirely within the much larger Palaeocene morphospace. From the Palaeocene to the Eocene, morphospace occupancy again increases to a larger extent, and part of the Palaeocene morphospace is not represented in the Eocene, suggesting that some ecomorphologies were lost across the PETM. Morphological variance disparity increases marginally over the K-Pg, before seeing a much larger increase over the PETM. This study provides a new perspective on trends in eutherian morphological disparity during the late Mesozoic and early Palaeogene, and shows that discrete morphofunctional characters can be used to effectively quantify changes in morphospace occupancy.

Citation

morphofunctional characters. (Dissertation). University of Salford

• Thesis Type: Dissertation
• Deposit Date: Apr 12, 2022
• Publicly Available Date: Mar 2, 2024
• Award Date: Dec 1, 2022