Last dinosaur before mass extinction discovered

A team of scientists has discovered the newest dinosaur preserved in the fossil record before the catastrophic meteor impact 65 million years ago, which many scientists believe caused their extinction. The finding indicates that dinosaurs did not go extinct prior to the impact and provides further evidence of the impact as cause of extinction. Results of the study appear online July 13, 2011 in the journal Biology Letters.

Yale researchers found the horn of a ceratopsian, likely a Triceratops like the one pictured here, close to a boundary marking the time of mass extinction. Image Credit: EncycloPetey

Researchers from Yale University discovered the fossilized horn of a ceratopsian – likely a Triceratops – in the Hell Creek formation in Montana in 2010. They found the fossil buried just five inches below the K-T boundary, the geological layer that marks the transition from the Cretaceous period to the Tertiary period at the time of the mass extinction 65 million years ago.

K–T boundary exposure in the Raton Basin of Colorado, showing an abrupt change from dark to light colored rock. Via Wikipedia

Since scientists first proposed the impact hypothesis, more than 30 years ago, for the demise of the dinosaurs, many have come to believe the meteor caused the mass extinction, but a sticking point has been an apparent lack of fossils buried within the 10 feet of rock below the K-T boundary. The seeming anomaly has come to be known as the three-meter gap. Until now, this gap has caused some paleontologists to question whether the non-avian dinosaurs of the era – which included Tyrannosaurus rex, Triceratops, Torosaurus and the duckbilled dinosaurs – gradually went extinct sometime before the meteor struck. (Avian dinosaurs survived the impact and eventually gave rise to modern-day birds.)

Tyler Lyson, lead author of the study, said:

This discovery suggests the three-meter gap doesn’t exist. The fact that this specimen was so close to the boundary indicates that at least some dinosaurs were doing fine right up until the impact.

While the team can’t determine the exact age of the dinosaur, Lyson said it likely lived tens of thousands to just a few thousand years before the impact:

This discovery provides some evidence that dinosaurs didn’t slowly die out before the meteor struck.

A 1904 illustration of Triceratops. Image Credit: Charles R. Knight
A 2008 illustration of Triceratops on a German postage stamp. Image Credit: Deutsche Post AG

Researchers sent soil samples to a laboratory to determine the exact location of the boundary, which is marked by the relative abundance of certain types of fossilized pollen and other geological indicators (but difficult to determine visually).

Yale graduate student Stephen Chester, pictured here, and Eric Sargis discovered the dinosaur horn. Image Credit: Tim Webster

Because the dinosaur was buried in a mudstone floodplain, the team knew it hadn’t been re-deposited from older sediments, which can sometimes happen when fossils are found in riverbeds that may have eroded and re-distributed material over time.

Lyson suspects that other fossils discovered in the past may have been closer to the boundary than originally thought and that the so-called three-meter gap never existed. He said:

We should be able to verify that, using the more sophisticated soil analysis technique rather than estimating the boundary’s location based solely on a visual examination of the rock formations while in the field, which is what has typically been done in the past.

Bottom line: Yale scientists discovered a ceratopsian – likely a Triceratops – close to a geological layer called the K-T boundary, providing evidence that dinosaurs did not gradually die out before a meteor impact 65 million years ago. Results of their study appear online July 13, 2011 in the journal Biology Letters.

Read more at Yale University Office of Public Affairs and Communications

David Helfand on risk from killer asteroids

Gerta Keller: “Volcanoes killed the dinosaurs”

July 14, 2011

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