Fossils found under tons of Kitty Litter. (Paleontology).

Excavations in southeastern Missouri have yielded fossils of ancient aquatic reptiles, as well as evidence of the impact that scientists suspect killed off the dinosaurs.

When miners were digging clay from the bottom of a large pit near Artiola in 1999, they exposed three layers of sediment, each about a half-meter thick, says Carl E. Campbell, a freelance paleontologist in St. Louis. The deepest layer contains traces of seafloor burrows and large numbers of phosphate nodules that include the fossils of ammonites, the extinct relatives of today's chambered nautilus.

The next layer up is a jumbled mix of broken limestone and fossils that range in size from microscopic plankton to handsize shells. This layer also yielded a 6-centimeter tooth and a vertebra that probably came from a large marine reptile, Mosasaur hoffmani, which could grow more than 17 meters long, says Campbell. Paleontologists had previously found this species only in the Netherlands and New Jersey, he notes.

Above this jumbled layer is a dead zone that contains virtually no fossils. Lying atop all of these sediments is a thick bed of absorbent montmorillonite clay, which the miners scoop up and sell as Kitty Litter. All the strata were laid down when the area rested at the bottom of a shallow ocean bay.

However, only the deepest of these layers was there about 65 million years ago, when scientists believe a comet or asteroid struck the shallow ocean just north of Mexico's Yucatan Peninsula. The tsunami kicked up by that impact swept northward into the bay covering what's now Missouri and deposited the jumbled layer of rocks and fossils, says Campbell. The melange left behind also contains an abundance of microtektites--crumb-size spherules that formed when drops of molten rock thrown into the air by the impact passed through the atmosphere and cooled.

The dead zone above that betrays the worldwide mass extinction that followed the impact from space, says Campbell. The thick, overlying bed of Kitty Litter clay is made of material that subsequently eroded from the ancient Appalachians. --S.P.