The comet that killed the dinosaurs opened the evolutionary door for one of Earth's most diverse groups of creatures: mammals. David Archibald, Ph.D., a professor of evolutionary biology at San Diego State, has made this transition from dinosaurs to mammals his expertise.

Archibald studies early mammalian fossils and is trying to constrain the origins of the phylum to which humans belong. His research has taken him around the world in search of the remains of terrestrial creatures.

Archibald's interest is in the explosion of mammal diversity that occurred after the K-T boundary, a point which marks the extinction of the dinosaurs. Mammals diversified on an ordinal level at this time.

"(Order) is one of the categories or ranks used in classification," Archibald said. "It has no biological meaning other than indicating the level or rank in a classification."

There are about 28 classifications of mammals. The first appeared approximately 200 million years ago and showed very little early diversification, but there is debate about when the diversification into orders began. The argument is whether mammals were diversifying concurrently with dinosaurs or after they went extinct. Scientists like Archibald are studying this using two approaches: fossil evidence and molecular studies.

Molecular studies, which analyze the base pairs that make up DNA, suggest that mammal diversification was occurring well before the dinosaurs went extinct. The fossil evidence can be contradictory, with most mammalian fossils appearing after the K-T Boundary occurred.

Archibald has focused his research in deserts of Uzbekistan on fossils that occurred just before the K-T boundary. He and his graduate students have researched with URBAC (Uzbekistan Russian British American & Canadian Scientists), paleontological team, organized for this location.

Nine expeditions in the past 12 years occurred specifically to collect rare fossils. The expeditions to Uzbekistan usually lasted from late summer to mid-fall and would carry the team of scientists into remote regions of the country.

Alongside the team was a group of locally hired Uzbeks who helped maintain the camp and process more than two tons of material from the quarries. The Uzbeks were given a crash course in fossil hunting and provided extra eyes to help hand-pick the fossils.

The popular image of a paleontologist is that of a khaki-clad scientist removing large bones from the ground, but the reality of the URBAC team is quite different. They're actually searching for mammalian fossils that are very small, usually teeth. Occasionally there is even a partial jaw found.

The mammals that inhabited the area are the ancient ancestors of rodents, bats and even small-hoofed animals that rarely reached the size of a large dog. They are looking at the teeth specifically because they preserve better than any other body part and are distinct features.

"In the case of most mammalian teeth, there are at most two generations of teeth - our milk and permanent teeth - that are under quite tight genetic control and do not change morphologically, except by wear or disease, after they erupt," Archibald said.

The team has helped to develop the picture of what the Uzbekistan area looked like 90 million years ago. They have discovered more than 85 species of creatures that were coexisting in this area. Turtles, lizards, birds and mammals lived alongside dinosaurs in a low, swampy coastal plain, similar to barrier islands off the shore of Texas.

Archibald admits that this project is in its mature state and is winding down. The next major expedition to Asia will be to Kazakhstan to see if they can find similar fossils in slightly older sediments because the Uzbekistan sites have been exhausted.

The fossils collected have added more information about which mammals were present on Earth during the time of the dinosaurs. Their data has also provided information that would suggest that although most mammals diversified after the K-T Boundary, some diversification occurred before the dinosaurs' extinction.

Even as the Uzbekistan project closes, Archibald's five graduate students are working on new projects. One graduate student, Justin Strauss, who just arrived at SDSU from the South Dakota School of Mines and Technology, is doing preliminary work on another set of teeth.

Strauss will be looking at the origin of mammalian carnivores and how the shearing edges developed on teeth. This summer, Archibald will take Strauss and another graduate student into the field in Montana; thus, one project closes and another one begins, with new questions that will yield new answers.