Reports: UR852083-UR8: Biostratigraphic and Paleoecologic Analysis of the First Micro-Mammal Community from the Type Location of the Middle Eocene, Late Uintan North American Land Mammal Age

James Westgate, PhD, Lamar University

Field & Lab Work

Field work at the WU-26 fossil locality in the Uinta Basin in northeastern Utah was conducted in July & August of the 2012 and 2013 field seasons. Three undergraduate students assisted in the fieldwork in 2012, while four under-graduate and one graduate student assisted P. I. Westgate in the field in 2013.

A total of seven tons of bulk sample was removed from the target stratigraphic horizon in the quarry during the two field seasons. The bulk sample was screen-washed in the Green River and reduced to a concentrate of about 500 kg which was driven to the Paleontology Lab at Lamar U. for further processing. This involved oven drying of specimens with subsequent overnight soaking in naptha, draining off the naptha, then overnight soaking in water, followed by screen-washing using screens with a 500 micron mesh size. This process reduced sand-sized lithic claystone fragments to clay-sized particles which passed through the 500 micron mesh screens and significantly reduced field concentrate volumes. After drying, the reduced samples were sorted by size using a stack of Standard screens with mesh sizes of 700 microns, 1.0 mm, 1.6 mm, 2.0 mm, 2.8 mm & 4.0 mm. Fractions 2 mm and larger were sorted for fossils using the naked eye, while fractions smaller than 2 mm were processed using binocular microscopes.

Samples collected in 2012 were sorted for fossils during the fall of 2012-spring of 2013 by four part-time Geology major lab assistants. Samples collected and concentrated in the field in Utah in 2013 are currently being reduced in volume by the three undergraduate geology majors who helped collect them.

The 2012 field crew also measured a detailed stratigraphic section from the WU-26 quarry upward to the base of the Brennan Basin Member of the Duchesne River Formation. This measured section indicates that WU-26 lies in the upper part of the Uinta C Member of the Uinta Formation, approximately 40 m. below the basal sand channels of the Duchesne River Formation. Field crews in both 2012 and 2013 were shown the regional middle Eocene stratigraphic setting in the Uinta Basin via driving a north-south transit (opposite dip) through the basin and stopping at key outcrops to see the lithologic differences between the Green River Fm.; Uinta Fm. Members A, B & C; and the Duchesne River Fm.

The 2013 field crew also participated in a survey for additional potential micro-mammal sites in Uinta C and recovered the partial remains of a 3 meter-long crocodile during this survey. We also collected a test bulk sample of 70 kg from a stratum exposed at WU-18 in Uinta B having a large concentration of gar scales. This evidence of standing water has similarity to conditions which existed at WU-26 and suggests the potential for bearing micro-mammal remains. This new WU-18 bulk sample will be processed after the WU-26 sample.


The processed sample collected during the 2012 field season at micro-mammal locality WU-26 increased the sample size collected from the site by about 45% and now totals more than 300 mammal specimens. Approximately 60% are rodents, 10% primates, 10% insectivores, 10% other placental mammals, and 10% are marsupials. WU-26 is the first known Uinta C micro-mammal site and provides a unique glimpse into the mammal community which inhabited the Uinta Basin near the end of deposition of the Uinta Formation.  Individual species’ sample sizes are approaching the level where biostatistical analyses may be run with confidence in their results.

The WU-26 sample size is also now approaching a level large enough to make meaningful comparisons with Uinta C micro-mammal communities outside the Uinta Basin. More than 1000 micro-mammal specimens have been collected at Texas Memorial Museum locality 42486 from the tropical and parallic, late middle Eocene, Laredo Formation, at Laredo, Texas, which is a Uinta C correlate. The two faunas display many similarities at the ordinal level, including each having four species of primates. Several genera are present in both faunas including Mytonius, Epihippus, Amynodon, Protoreodon, Mytonomys, Microparamys, and Pauromys, indicating that both environments shared habitats compatible for these taxa. However, it appears that the two communities shared few species as Protoreodon parvus is the only species common to both faunas. That number may grow as the WU-26 sample size increases and genera that have not been identified to species level are better known. It is significant that the earliest known North American lagomorph, Mytonolagus petersoni, is present at WU-26, but absent from TMM 42486.

Students have benefitted from this research in multiple ways. They have learned many new paleontologic and stratigraphic field and lab techniques which are not taught in typical university courses. They have also learned to think as researchers and not just classroom students. And some have added publications to their resumes, which will be a plus in applying to graduate school. Project progress has brought the P. I. closer to the point where a detailed manuscript on the WU-26 micro-fauna will be produced.