Earth and Planetary Sciences ETDs

Publication Date

Summer 7-30-2019


The Puercan North American Land Mammal Age (NALMA) is the earliest major North American terrestrial biochron of the Cenozoic era, spanning roughly the first one million years of the Paleogene period (Paleocene epoch, Danian stage; ~66.04-65.12 Ma). It is typified by the explosive ecomorphological diversification of the mammalian clade Eutheria (particularly our subclade, Placentalia), following the annihilation of non-avian dinosaurs and “archaic” mammal groups during the Cretaceous/Paleogene (K-Pg) mass extinction event. The spatiotemporal mode and tempo of Puercan eutherian diversification has long been the subject of debate, with disagreements over biogeographic zonation. The traditional model – based largely on well-sampled, well-constrained eutherian assemblages from Montana and Saskatchewan (Williston Basin), Wyoming (Bighorn Basin), Utah (Paradox Basin), and New Mexico (San Juan Basin) – postulates an increased norths/south dichotomy between faunal provinces and higher basin-level endemism in the later Pu2/Pu3 intervals relative to the earlier Pu1 interval, comparable to patterns observed in dinosaurian and mammalian faunas below the K-Pg boundary (Sloan, 1987; Buckley, 1994; Williamson, 1996). However, since the late 1970s-early 1980s, investigation of Pu1 faunas from the Great Divide and Hanna Basins in Wyoming, as well as the Denver Basin in Colorado, led to the proposal of a heterogeneous “transition- zone”, harboring unusual endemic arctocyonids and periptychids alongside taxa more typical of Pu2/Pu3 “southern” faunas in the Denver, Paradox, and San Juan Basins (Middleton, 1983; Eberle & Lillegraven, 1998; McComas & Eberle, 2016). However, the role of lithological and collecting biases in the formulation of these hypotheses has yet to be thoroughly tested.

This study analyzed the ecological biogeography of Puercan eutherian biogeography using species- level presence data for screen-washing and surface collection sites from the Pu1 and Pu2/Pu3 intervals, taking into account virtually the entire fossil record. Values of richness, evenness, average body size, and average trophic niche for each site were compared using Kruskal-Wallis tests of differentiation, based on geographic region, basin, and lithology. NMDS ordination scatterplots and AGNES dendrograms were constructed to observe gradients and clustering of Puercan localities, with ANOSIM tests analyzing the significance of region, basing, lithology, species body mass, and species dietary mode in ordination. Results provide conflicting perspectives on the role of biases in skewing potential biogeographic signals, though collecting methods have a first-order influence on occurrence data. There are no universal trends in differentiation due to richness, evenness, average body mass, or average diet; only Pu2/Pu3 screen-washing sites exhibited marked differentiation in all of these values except evenness. In the NMDS, ANOSIM, and AGNES analyses, similarity scores for screen-washing and surface localities in both the Pu1 and Pu2/Pu3 intervals were pulled by the large number of Williston Basin and San Juan Basin localities. Although geographic region, basin, and taxon dietary mode appear to be the dominant causes of ordinations after taking into account collecting methods, the signals are very weak.

The lack of an obvious geographic barrier to faunal dispersal in the Western Interior across the K/Pg boundary makes the notion of discrete biogeographic provinces in the Puercan rather suspect, in accordance with the results of this study. Larger samples from the Bighorn, Great Divide, Hanna, and Denver basins, as well as a more comprehensive understanding of the Paleocene paleogeography of North America and the phylogeography of early Paleocene mammals, are needed to resolve long- standing mysteries regarding the spatial scale of Puercan eutherian diversification.

Degree Name

Earth and Planetary Sciences

Level of Degree


Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Corinne Myers

Second Committee Member

Thomas Williamson

Third Committee Member

Jason Moore

Fourth Committee Member

Gary Weissmann

Project Sponsors

Geological Society of America, Paleontological Society, University of New Mexico




biogeography, Eutheria, Mammalia, K/Pg mass extinction, Puercan, Paleocene, taphonomy

Document Type


Available for download on Tuesday, July 27, 2021