What's left of Cretaceous mammals is most commonly their teeth. Dense and covered with hard enamel, teeth are the most resistant bones in the body. Most of the fossils we are finding are from the beds of ancient rivers, which disarticulated the animals and destroyed many less resistant parts. Fortunately, mammal teeth are very informative for taxonomy and systematics, and can also tell us about diet. Unfortunately, since the mammals themselves were small, their teeth are usually only a millimeter or two long. This makes Cretaceous mammals a lot harder to find than some larger vertebrates that lived at the same time. The techniques for finding these tiny fossils are different than those used in recovering large ones.

Here I have included some pictures of field and lab techniques. I have omitted two crucial preliminary steps: researching maps and notes to get an idea of the best places to look for fossils before we went into the field; and getting a permit from the Bureau of Land Management to work on federal lands under its jurisdiction.

The first and third photos on this page were generously donated by professional photographer Ray Nelson. Please do not copy them without permission.

A tiny bone on the surface, or maybe a few fish scales on the side of a hill, can be the sign that there is a productive microvertebrate site. To find out, however, you have to "put your nose to the outcrop." Here we crawl over the surface of a site that we will later quarry.

Picking the visible fossils off the top like this has three advantages: we get an idea of what taxa are in the site, we avoid stepping on tiny fossils and inadvertantly destroying them, and the fossils we find are spared a soaking in water which can damage them if they are fragile.

Here is a fossil locality at the base of a white sandstone channel in the De-Na-Zin Member of the Kirtland Formation. To collect the fossils, we use chisels to break out small chunks of the thin fossil-rich layer, and sack up those pieces of rock. Usually these productive lenses are only a few feet wide. When we have chiseled out the fossils, we fill the quarry in again. After a good rain, it's very hard to tell we worked there.

There is no running water in our field area. Removing the fossils from the rock matrix requires a brief soak in water, or sometimes weak acetic acid. This means we have to get the matrix to a tank.

Because we are working in a National Wilderness Area, we cannot drive a truck to our sites -- not even when we have hundreds of pounds of rock. We usually have to backpack the sacks out one at a time. In this case we were lucky to be able to wheel them a couple of miles along the bed of a wash.

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 screens. Fossils and small pebbles are left in the screens. We are using window mesh screens nested in a second screen of much finer mesh.

Sometimes we can reduce the volume of rock even further by separating the remaining particles from the bones and teeth with a high-density ("heavy") liquid.

While recovering microvertebrate fossils isn't as photogenic as excavation of a large dinosaur, the results can be more interesting. Instead of getting one spectacular animal, we discover bits and pieces of whole ecosystems.

These fossils are not from the San Juan Basin, but they illustrate the kind of diversity represented in microvertebrate localities. At the top is a toe bone of a large theropod dinosaur, in the middle are a variety of alligator teeth, and at the bottom is a pile of garpike scales. We also commonly find pieces of turtles and lizards, as well as many types of dinosaur and mammal teeth.

Even after screenwashing, the best finds are difficult to see. Duke undergraduate students and volunteers at the New Mexico Museum of Natural History and Science use microscopes to sort through the material left in the screens. Each tooth is carefully put in its own glass vial, and all the fossils are catalogued at the NMMNH.