An impact of a large asteroid at the end of the Cretaceous, accompanied by massive volcanism, led to an important turning point in earth history in which about 75% of all animals and plants went extinct. Following this global catastrophe and extinction of non-avian dinosaurs, ecosystems rapidly recovered and mammals quickly evolved and diversified to become the dominant terrestrial animals. This episode of explosive mammal diversification and evolution occurred under a “hot house” climate that prevailed through most of the Paleogene (about 66 to 40 million years ago) in which global temperatures were generally warm and equable, but temperatures occasionally spiked during brief, episodic hyperthermal events. Hyperthermal events are poorly understood, but appear to be related to a sudden influx of methane, a powerful greenhouse gas, into the atmosphere. The effects of hyperthermal events on earth’s biota are poorly understood, but may provide insight into changes happening in modern times.  

In order to better understand how changing climates affected early Paleocene terrestrial ecosystems, we examined the geologic record preserved in the San Juan Basin of northwestern New Mexico. The San Juan Basin contains one of the best records of terrestrial environments and ecosystems spanning the Cretaceous - Paleogene boundary and the ensuing four million years of the early Paleocene. We determined the age of early Paleocene strata of the San Juan Basin with high precision by correlating the paleomagnetic signature preserved in San Juan Basin strata with the known ancient shifts of earth’s magnetic field. We also dated volcanic ashes and volcanic minerals using high-precision radioisotopic dating techniques. We reconstructed early Paleocene climate and environments using several different methods; we estimated mean annual temperature and mean annual precipitation using leaf fossils, estimated mean annual temperature from oxygen isotopes extracted from the fossilized teeth of mammals, and reconstructed depositional environments from careful examinations of strata included detailed analyses of fossil soils. Finally, we compared these records to those constructed from fossil mammals collected from the same strata to see if sudden changes in climate or environments coincided with changes in mammal communities through the early Paleocene.

Several Important results from this research are: 1) the San Juan Basin record across the Cretaceous – Paleogene boundary is much more complete than was previously thought – it includes a diverse fauna of latest Cretaceous vertebrate animals, including dinosaurs; 2) early Paleocene floras and faunas of New Mexico recovered from the end-Cretaceous mass extinction very rapidly and tended to be much more diverse than faunas and floras of the same age recovered from the Upper Great Plains of western North America; 3) significant differences between New Mexico latest Cretaceous faunas and early Paleocene floras and faunas and those from the Upper Great Plains suggest that there was a great deal of heterogeneity in plants and animals across western North America during this time; 4) we found evidence for at least two hyperthermal events in the early Paleocene – one of these coincides with the Late Danian Event found in deep sea cores, but one appears to be new – it corresponds to a major change in mammalian faunas; 5) preliminary evidence suggests that at least one mammal, the phenacodontid ‘condylarth’ Tetraclaenodon, underwent a significant reduction in size within one of the hyperthermals – this ‘dwarfing’ has been observed at the Eocene thermal maximum 1, a later and bigger hyperthermal event.

This project provided educational and research opportunities for a number of graduate and undergraduate students and volunteers including a number of Native American undergraduate students, a group that is underrepresented in the geosciences. All students and volunteers received extensive training in field and lab work and benefited from their participation in multi-disciplinary, collaborative work with a diverse group of researchers and students. Several students participated in research, were included as co-authors of research papers, attended scientific meetings, and presented research results at scientific meetings.

Results of this work were disseminated through incorporation into permanent exhibits at the New Mexico Museum of Natural History and Science, through public lectures at schools and other institutions, through publication of many papers in peer-reviewed journals including several in very high impact journals, through numerous presentations at scientific meetings, through several theses and dissertations, and through various media outlets including television, radio, newspaper, and online media.

Last Modified: 11/28/2017
Modified by: Thomas E Williamson