The extinction at the end of the Cretaceous Period, 65 million years ago, is the most recent mass extinction event in Earth's history. The fossil record of this event and of the subsequent biotic recovery is particularly good in marine sediments, where it is documented in geologic sections around the world.

Our record of this event in terrestrial sediments is not as complete. The best terrestrial record of this interval is from the Western Interior of North America, where the mammalian fossil record is exceptional, and there are age constraints on many local and regional faunas.

I am particularly interested in how biogeographic variation in the Late Cretaceous might have contributed to recovery of terrestrial ecosystems in the earliest Cenozoic.

The San Juan Basin sediments, which include Late Cretaceous and earliest Paleocene mammals, turn out to be key in answering this question.

Although mammals did not go completely extinct in the end-Cretaceous event, it is a major extinction event in mammalian history, with significant faunal turnover at the Cretaceous-Tertiary boundary in North America.

The Cretaceous-Tertiary boundary is quantitatively different from previous episodes of mammalian faunal turnover. Such episodes in the Late Cretaceous are typified by about 30% of the new species appearing in the younger interval being immigrants from lineages not recognized in earlier Western Interior sediments. At the Cretaceous-Tertiary boundary, about 70% of the new mammalian species belong to immigrant lineages. So one question related to recovery is: Where did the new mammals come from?

Documentation of the Cretaceous-Tertiary interval is particularly good in the area between northern Alberta and north-central Mexico, in what were subsiding foreland basins east of the rising Rocky Mountains. In the Late Campanian (78-74 million years ago) the low-latitude mammalian faunas differ taxonomically from those in the northern Western Interior. This mammalian faunal transition seems to correspond with that of "lower" vertebrates, at paleolatitudes between 45 degrees and 49 degrees North. Even stronger evidence supports the existence of separate northern and southern faunas in the earliest Paleocene (65-64 million years ago).

But the intervening time -- the latest Cretaceous -- is the most important for understanding the end-Cretaceous extinction. And the problem has been that there have been no diverse mammalian faunas of this age recovered from the southern Western Interior. The mammalian fauna we have discovered in the San Juan Basin (white star) is the first.

The decision to search for fossil mammals in the Naashoibito Member of the Kirtland Formation was not a random one. The area is known to be fossil-producing, and Barnum Brown of the American Museum of Natural History collected dinosaurs in the Naashoibito around the turn of the century. Tyrannosaurus, Alamosaurus, and Torosaurus, all of which have been found in the Naashoibito, indicate that it is of latest Cretaceous age.

Also, one mammal tooth, a molar of the multituberculate Essonodon, was described by Tom Lehman. Essonodon is only known from the latest Cretaceous. And its presence suggested to us that if we searched specifically for microvertebrates (instead of large dinosaurs) we would find more mammals. We were right!

Although this project is focussed on the Late Cretaceous mammals of the Naashoibito Member of the Kirtland Formation, we also sometimes find other things. Left, a predentary of the ceratopsian Torosaurus found (and photographed) by Krister Smith; upper right, a condylarth jaw in matrix from the Nacimiento Formation (photo by Steven Semken); lower right, one of many Cretaceous mammal teeth from the Naashoibito. This single molar is about 2 millimeters wide.

We're working on the question of whether our newly discovered Naashoibito mammalian fauna could be a source of early Paleocene mammalian diversity. In any case, because many of the taxa appear to be new, it will expand known Late Cretaceous mammalian diversity.