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Tracing the Dinosaurs' End at Big Bend National Park

National Park Service photograph, by A. Atwater. Rock strata known as the Javelina Formation, near the west entrance of Big Bend National Park, contain a feature called the K-Pg boundary, which records the sweeping extinction that ended the Age of Dinosaurs.

If, as Emerson said, “the health of the eye demands a horizon,” Big Bend National Park is the ultimate tonic for the eyes. Visitors are awed by expansive desert-mountain vistas.

The story that landscape tells about the history of life is equally vast. Big Bend's fossil record is unparalleled among national parks. With more than 1,200 species, it's a nearly uninterrupted record of 130 million years of evolutionary history. 

It includes charismatic “stars” of the dinosaur age – tyrannosaurus, alamosaurus, quetzalcoatlus. But at multiple sites, Big Bend's rugged topography preserves a line, a layer of rock, where such fossils cease. In this feature of the West Texas landscape, we glimpse what was truly the end of an era.

One doesn't have to be geologically literate to suspect that Big Bend's soaring escarpments and striated badlands, its welter of mountains and mesas, tell a complex story. The park's importance to paleontologists is tied to that story.

Tectonic forces squeezed and stretched this land – raising massive limestone blocks to form the Carmen Mountains on the park's east side, the Mesa de Anguila on the west. But in between, the same rock was dropped almost 2,000 feet. While fossils in the uplands were eroded into sand and gravel, fossil-bearing strata in this down-dropped basin, the “Sunken Block,” were preserved.

The story they tell spans much of the Cretaceous Period, which began 145 million years ago. There are fossils of 40-foot-long marine lizards called mosasaurs, testifying to the Cretaceous seaway that covered this land. Later, this land was coastline, prowled by tyrannosaurs and giant crocodilians. Later still, it was an inland floodplain.

Don Corrick is park geologist.

“But then there's a really big event, about halfway through that 130 million years,” Corrick said, “that is the extinction of the dinosaurs, and the ushering in of the Cenozoic. We transition from the Age of Reptiles to the Age of Mammals. That's one of the really significant things that our park preserves: We have the beds exposed here that were laid down during that extinction episode.”

As soon as scientists discovered dinosaurs, they wondered at their disappearance. And here, in the Big Bend landscape, is a line, a boundary, that marks that extinction, 66 million years ago. It's known as the K-Pg boundary – “K,” from the German word for Cretaceous, and “Pg” for Paleogene, the geologic period that followed.  

It's exposed along the park's Glenn Springs Road, at formations known as the Black Peaks, and elsewhere.

“People visiting the park drive right across that boundary, if they come in the west entrance,” Corrick said. “You come in from Terlingua, as you drive into the park you're on older rocks, and you're driving up through time, basically. Between the sign and the booth, right there in the curve, you're crossing that boundary, where there's dinosaurs behind you, towards Terlingua, and the dinosaurs are gone in those rock layers right in front of you.”

Seventy-five percent of species vanished in the K-Pg extinction, including the dinosaurs. Or most of them. Scientists now agree that birds are surviving dinosaurs.

What caused this cataclysmic extinction?

Massive volcanic eruptions in present-day India were pumping sulphur into the atmosphere at the time, likely impacting climate. And sea levels were dropping, which may have disrupted the food chain. 

But there's consensus that the impact of a meteor – at least 6-miles-wide – at the Yucatan Peninsula was a major factor. Among other effects, the material it ejected likely blocked out the sun for years – bringing photosynthesis to a halt.

Around the world, scientists find high levels of iridium in the K-Pg boundary. Iridium is rare on Earth, but abundant in asteroids. Researchers are studying the K-Pg boundary in Big Bend. The meteor impact created a tsunami, likely 300 feet high or higher. There could be evidence here.

“This is a big deal,” Corrick said. “I don't know of any other national park units that have this preserved. Plus, we're so close to where the meteor struck, that we represent the closest to the meteor impact that were on land at that time.” 

Reptiles dominated the Earth for 180 million years. The K-Pg boundary marks the end of that world. But in the epochs that followed, new species emerged to fill the open niches. Mammals were at the forefront. The K-Pg boundary is the beginning of a new world – one that includes us.

Drew Stuart is the producer for the Marfa Public Radio series Nature Notes.