New mammalian local faunas from the first ca. 80 ka of the Paleocene in northeastern Montana and a revised model of biotic recovery from the Cretaceous–Paleogene mass extinction

ABSTRACT

The earliest phases of mammalian recovery following the Cretaceous–Paleogene (K–Pg) mass extinction are incompletely known but crucial to understanding the development of modern terrestrial ecosystems. Here we report new mammalian faunal data from three vertebrate microfossil assemblages in the Hell Creek region of northeastern Montana, the deposition of which we constrain to within the first 28–80 ka of the Paleocene using new stratigraphic observations within a high-resolution chronostratigraphic framework. We quantified the taxonomic diversity among these three assemblages and five other assemblages from both the Hell Creek region and Denver Basin, together spanning the first ca. 300 ka post-K–Pg mass extinction. Our results allowed us to sub-divide the established ‘disaster’ and ‘recovery’ phases of recovery into the following sub-phases: (i) early disaster, characterized by the presence of ‘dead clades walking,’ high relative abundance of bloom taxa, and the appearance of post-mass-extinction immigrants, (ii) late disaster, characterized by a reduction in the number of ‘dead clades walking,’ continued high relative abundance of bloom taxa, and a more diverse assemblage of immigrants, (iii) early recovery, characterized by decreased relative abundance of bloom taxa, and continued immigration, and (iv) late recovery, characterized by the onset of in situ diversification. We note important differences in the pattern and timing of mammalian faunal succession between the Hell Creek and Denver Basin, suggesting that post-K–Pg mammalian recovery was spatially heterogeneous. Our results provided a new model for post-K–Pg mammalian biotic recovery that can now be tested with other earliest Paleocene assemblages across western North America.

ACKNOWLEDGMENTS

We acknowledge that the fossils in this paper were collected on lands that are the traditional territory of the Fort Belknap Assiniboine & Gros Ventre Tribes and Fort Peck Assiniboine & Sioux Tribes. Future field trips will be respectful to the original peoples and sovereignty. We would like to express our deep gratitude to our late mentor and friend W. A. Clemens; without his contributions to the study of the Hell Creek region’s fossil and rock record none of this work would have been possible. We also thank the people of Jordan, Montana for welcoming us into their community each summer, Jane and Dale Tharp for granting access to these field areas, I. Fendley and C. Sprain for discussions on Hell Creek chronostratigraphy, S. Smith for help with taxon identifications and discussions about recovery, current and former members of the Wilson Mantilla Lab (A. Brannick, H. Fulghum, D. Grossnickle, B. Hovatter, A. Peng, and P. Wilson Deibel) for fieldwork assistance and feedback throughout the writing process, current and former undergraduate and volunteer assitants E. Armstrong, B. Knobbe, L. Knowles, B. LeFevre, and S. Majidi for help in both the field and the lab, P. Holroyd for providing access to UCMP collections, former UCMP field crews for vertebrate microfossil collecting, K. Anderson for managing the UWBM vertebrate paleontology collections, the UW PaleoPod for fruitful discussions and support, the U.S. Bureau of Land Management and the Montana State Department of Natural Resources and Conservation for providing paleontological permits, and G. Liggett, D. Melton, and P. Rennie for facilitating those permits. Financial support of the Hell Creek Project was provided by the Myhrvold and Havranek Charitable Family Fund, the David B. Jones Foundation, the University of Washington Department of Biology, and the Burke Museum of Natural History and Culture. Financial support was also provided to JRC and LNW through National Science Foundation Graduate Research Fellowships and LNW through NSF EAR-PF 2052992.

AUTHOR CONTRIBUTIONS

JRC and GPWM conceived of the project; JRC, LNW, TST, and GPWM conducted fieldwork related to the project, both specimen collections and stratigraphic observations, and contributed to the manuscript; JRC identified specimens, took photographs, conducted analyses, made figures, and wrote the main body of text.

SUPPLEMENTARY FILES

Supplementary Data 1.docx: Supplementary descriptions of specimens, specimen measurements, and additional statistical tests.