Dinosaurs and the K–Pg Extinction

About 66 million years ago, Earth experienced a transformative event called the Cretaceous–Paleogene (called K–Pg or K-T) extinction event. This mass extinction wiped out around 75% of all species on the planet, including non-avian dinosaurs.

What Caused the K–Pg Extinction?

The Chicxulub Crater in Mexico

It is widely believed that the cause of this catastrophic extinction event was a massive asteroid impact. This asteroid, estimated to be about 10–15 kilometers in diameter, struck near what is now the Chicxulub crater on Mexico’s Yucatán Peninsula. The collision unleashed an energy equivalent to billions of nuclear bombs, triggering catastrophic environmental changes including:

• The release of enormous amounts of dust and aerosols into the atmosphere. This blocked sunlight and plant photosynthesis for months or years.

• The impact released sulfur gases, which likely contributed to acid rain and global cooling.

• The release of carbon dioxide which caused the ocean to become acidic which devastated marine ecosystems.

To K-T or to K-Pg? That is the question.

Historically, the boundary marking the end of the Cretaceous period and the beginning of the Tertiary period was referred to as the K-T boundary, with "K" representing "Cretaceous" and "T" representing "Tertiary". 

However, in 2005, the International Commission on Stratigraphy replaced the term “Tertiary” with “Paleogene,” (Pg) which is the first period of the Cenozoic Era. Thus, the new, correct terminology is K-Pg.

Where to see evidence of the K-PG Extinction

Of course, the first place to see evidence of the K-Pg Boundary is at the site of impact—the massive, 180-kilometer-wide Chicxulub Crater in the Yucatan Peninsula in Mexico.

One of the most well- documented sites with evidence of the K-Pg boundary is within the Hell Creek Formation in the states of Montana, North Dakota, South Dakota and Wyoming.

K-Pg Boundary from the Hell Creek Formation

If you know what you’re looking for, it’s possible to see a thin layer of sediment marking the K–Pg boundary. It's usually a dark-colored clay or shale layer, sometimes with a white or peach-colored appearance. The most obvious evidence is a layer of iridium, an element rare on Earth but abundant in asteroids, providing strong evidence of extraterrestrial impact.

At this stratigraphic layer, scientists have also found signs of a high-energy impact including shocked quartz and melted rock.

Finally, at the K-Pg boundary, there is a stratigraphic 1.76 meter “barren zone” that separates the last dinosaur fossils from the boundary, supporting an abrupt extinction.

The End of the Dinosaurs and the Survival of the Fittest

Scientists have found no definitive evidence of non-avian dinosaurs completely surviving the K-Pg extinction event. No articulated skeletons or in-situ specimens of dinosaurs exist above the K-Pg boundary.

The effects of the K–Pg extinction were both severe and selective. Many life forms were completely wiped out including:

• Terrestrial Losses: All non-avian dinosaurs, pterosaurs, and many mammals perished. Smaller species with adaptable diets—such as insectivores—survived

• Marine Devastation: Marine ecosystems were hit hard, with ammonites, mosasaurs, plesiosaurs, and many plankton species going extinct. Deep-sea organisms fared better due to their reliance on detritus rather than sunlight-dependent food chains

• Plant Extinctions: Fossil evidence shows widespread destruction of plant communities. Ferns and fungi briefly dominated before flowering plants re-emerged to reshape ecosystems

What life forms survived the K-Pg Extinction event?

• Avian dinosaurs are the only confirmed dinosaur lineage to survive the extinction. They diversified rapidly into a wide variety of birds that persist today.

• Small mammals adapted and in general, mammals diversified rapidly during the Paleogene period including the evolution of primates, whales, bats, and more.

• On a broader scale, ecosystems underwent profound changes. Tropical rainforests emerged as biodiversity hotspots

• Crocodilians survived and marine life rebounded with new species of fish and mollusks taking center stage

Join a dig at the Pioneer Trails Regional Museum and work with paleontologists to find evidence the life forms that changed and recovered after the K-Pg mass extinction.