Terrestrial ecosystem recovery following the end-Cretaceous mass extinction 65 million years ago is best known from studies of the North American Western Interior. This recovery includes the appearance of "new" mammals whose lineages are unknown from North America in the preceding Late Cretaceous period. That may be because knowledge of latest Cretaceous mammalian faunas has been geographically restricted to the northern region of the Western Interior. (more on why this project is interesting)

In 2000, Tom Williamson discovered site L4005, which is yielding a diverse bunch of mammals, many of them new species! The discovery proved that the Naashoibito Member of the Kirtland Formation was rich in fossil mammals. We have since discovered several other sites. In the San Juan Basin of northwestern New Mexico, this is the first diverse assemblage of latest Cretaceous age from south of Wyoming. We hope that our discoveries will shed light on the origins of "new" mammals that appear later in the record. (more on how we find microvertebrate fossils)

In 2003, we also worked on the regional stratigraphy and took palynological samples, hoping to constrain the age of the Naashoibito a little better. (pictures and text about our 2003 field season)

The second, third and fifth photos on this page were generously donated by professional wildlife photographer Ray Nelson. Please do not copy without permission.

This is the uppermost part of the Kirtland Formation. The purple stripes are ancient soils, which are laterally interrupted by white sand channels. The area in this photo, now within the Bisti/De-Na-Zin Wilderness Area, is famous for the dinosaurs collected by Barnum Brown in the early 1900's. Because Brown did not find fossil mammals here, for a long time it was assumed that there weren't any.

Just below the Naashoibito Member is the De-Na-Zin Member of the Kirtland Formation. Note that this mudstone is a very different color! There is probably an unconformity between the two, meaning that the De-Na-Zin is significantly older. Here Dr. Tom Williamson of the New Mexico Museum of Natural History signals in what might be a good direction to look for more fossils.

This tiny tooth is one of the first mammal fossils we found from site L4005. It is the lower fourth premolar of a multituberculate mammal. Most of the mammals that we have found at this site are multis. The most common is called Essonodon. Essonodon is present, but rare, in fossil faunas of the same age in Montana. One of the major differences between New Mexico and Montana at this time - maybe 67 million years ago - is the relative abundance of Essonodon in New Mexico.

At least, we made the first page of the "New Mexico and the West" section of New Mexico's "Sunday Journal" on August 6, 2000. The top picture is a molar of Essonodon - and that white thing it's sitting on is the head of a pin!

To get those tiny specimens out of the rock, we don't use a hammer and chisel. Instead, we dissolve the rock away from the bones by soaking it in water and letting the smaller particles drift out through a screen. In the lower picture, field assistant Jodge Meserve is holding up a screen full of fossils. Using this method, we are able to recover the remains of entire faunas, including fish, turtles, amphibians, mammals, and sometimes even birds.

This method is very different from the excavation of a huge, single dinosaur skeleton, which is what many people first think of in relation to vertebrate paleontology. Instead of getting one spectacular animal, we get a sample of many different species.

A well-earned rest

After a long day of hiking and prospecting for microvertebrate sites in temperatures over 100 degrees (F), followed by cooking dinner and cleaning up, the summer of 2000 field crew takes a break. We work some long days. But everyone looks forward to relaxing and enjoying the phenomenal desert sky.