An artist's rendering of the moment of impact when an enormous space rock struck the Yucatán peninsula at the end of the Cretaceous.

Credit: Don Davis, NASA

Kirk Johnson from the Denver Museum of Nature & Science discusses a recent review of evidence supporting the impact hypothesis as the cause of the dinosaur-killing extinction at the end of the Cretaceous.

Credit: National Science Foundation/Denver Museum of Nature and Science

The K-Pg boundary as exposed along the side of Interstate 25 near Raton Pass in southern Colorado. The obvious white layer is the K-Pg ejecta layer. It contains elevated levels of iridium and shocked mineral grains. Pollen and spores from Cretaceous plants are found immediately below this layer but not above it, a pattern that is seen from the southern United States all the way north to the Arctic Ocean. This direct link between impact ejecta and plant extinction suggests a very strong cause and effect relationship between impact and extinction.

Credit: Kirk Johnson, Denver Museum of Nature & Science

A three-dimensional Bouguer gravity map of the Chicxulub Crater. Petroleum exploration boreholes (C1, S1, and Y6) penetrated the crater structure, which is buried several hundred meters below the surface of the Yucatán Peninsula of Mexico. The Yucatán-6 (Y6) borehole is the discovery hole where Kring and others found shocked quartz and other evidence of impact origin. That discovery prompted the drilling of the Yaxcopoil-1 (Yax-1) borehole, which was the first scientific borehole drilled into the structure by the International Continental Drilling Program (ICDP) in 2001-2002. As described in the current Science paper, data from that drilling project, when integrated with other K-Pg boundary analyses from around the world, provide a strong link between the Chicxulub impact crater and a mass extinction event 65 million years ago. Proposals to drill into other portions of the immense structure are pending with the ICDP and the Integrated Ocean Drilling Program.

Credit: David A. Kring

Scanning electron micrographs of late Paleocene nannoplankton that evolved several million years after the K-Pg boundary extinction. These specimens are 8-10 microns in size.

Credit: Timothy Bralower and Jon Schueth, Penn State University

The Cretaceous Creekbed diorama in the Prehistoric Journey exhibit at the Denver Museum of Nature & Science shows two Stygimoloch spinifer dinosaurs fighting in the woodlands of North Dakota. The scene, based on an actual fossil site, represented the final ecosystem of the Cretaceous. Paleontology from this site documents the extinction of the dinosaurs as well as more than 50 percent of plant and insect species.

Credit: Denver Museum of Nature & Science

A skull of Triceratops collected in a Denver suburb in 2004. Triceratops was one of the most common dinosaurs in the central part of North America at the end of the Cretaceous and as such, one of the more famous victims of the Chicxulub impact.

Credit: Denver Museum of Nature & Science

Edmontosaurus, a duck-billed dinosaur, from Dawson County Montana. Duck-billed dinosaurs and Triceratops were the most common herbivorous dinosaurs in the American West at the end of the Cretaceous. The tail of this specimen bears a healed bite mark from a Tyrannosaurus rex. This animal survived that attack but neither species survived the asteroid impact.

Credit: Denver Museum of Nature & Science

The researchers' finding appears in the March 5, 2010 issue of the journal Science.

Credit: Copyright AAAS 2010