July 13, 2011
Scientists Find Fossil Below K-T Boundary
Lawrence LeBlond for redOrbit.com - Your Universe Online
The debate whether dinosaurs went extinct due to a large space rock that struck the Earth 65.5 million years ago (MYA) may have been answered with the discovery of a distinctive brow horn from a Ceratopsian dinosaur just 13 centimeters (5.1 inches) below the K-T boundary -- the distinct layer of geological sediments separating the Cretaceous and Tertiary periods.Rocks laid down 65.5 MYA show a thin layer abundant in rare elements like Iridium, spherules and shocked Quartz that could only have come from a meteorite impact. Since no fossils have ever been found in sediments above the K-T boundary, conventional wisdom has it that the end of dinosaurs came with an asteroid impact that caused firestorms, acid rain and a nuclear winter that blotted out the Sun.
But that theory had a hole in it. The fossil record showed an apparent lack of dinosaur fossils in the last few million years leading up to the impact, suggesting that the "three meter gap" proves that dinosaurs went extinct long before the catastrophic impact.
Scientists working in the Hell's Creek formation in the Montana badlands say they have resolved that dispute.
Writing in the Royal Society journal Biology Letters, the paleontologists report on the new discovery of the closest dinosaur fossil ever found to the K-T boundary.
The ancient remains uncovered in Montana belong to the last known dinosaur to ever walk the planet and gives weight to the theory that dinosaurs were in fact wiped out by an asteroid impact. All other dinosaur fossils found are either much older, or were unearthed after being washed from their original graves into much younger sediments, long after they died.
The fossil is most likely of an adult triceratops, a dinosaur growing up to 30 feet long and weighing up to 13 tons. The nearly 18-inch fossilized brow horn was found just 5 inches below the K-T boundary.
Researchers spotted the horn in a 100m-thick slab of mudstone in southeastern Montana's badlands. The region is one of the few in the world that preserves fossils before and after the period of the mass extinction.
"This is the youngest dinosaur that has been discovered in situ. Others can be found in younger deposits, but those have been put there by geological processes and are actually much older," said Tyler Lyson, a paleontologist at Yale University.
The discovery undermines the theory that gained ground in the 1980s, that dinosaurs died out due to climate change or rising sea levels long before the planet was struck by a space rock. The theory carried some weight due to a lack of fossils found within the "three meter gap" of the K-T boundary.
The finding "demonstrates that dinosaurs did not go extinct prior to the impact and that at least some dinosaurs were doing very well right up until we had the impact," Lyson told the Guardian.
"The in situ specimen demonstrates that a gap devoid of non-avian (bird) dinosaur fossils does not exist and is inconsistent with the hypothesis that non-avian dinosaurs were extinct prior to the K-T boundary impact event," the team of scientists wrote.
Gaps in the fossil record - which is patchy at best - are nothing new. Another, covering tens of millions of years, is clearly evident in the Hell's Creek formations some 60 meters (200 feet) below the K-T boundary.
But because similar dinosaur fossils are found both above and below this gap it is assumed the absence of fossils has more to do with geological processes, or simply blind prospecting luck, than any extinction event and subsequent miraculous reintroduction.
The "three meter gap" prior to the K-T boundary is unique because dinosaur fossils never reappear in the geological record.
Dr. Paul Barrett of the Natural History Museum said the discovery was strong evidence that dinosaurs were killed off in North America by a catastrophic event, but the evidence is not conclusive globally.
"It shows that in this part of the world dinosaurs were still viable and still roaming around at the time the meteorite hit. But what it doesn't tell us is what was going on in the rest of the world, and it could be that in other parts of the world dinosaurs were dying out at different rates and for different reasons because of other things going on at the time," he told BBC News.
He argues that just one brow horn discovery doesn't resolve the dispute over dinosaur extinction.
Lyson admits, though, that it is intriguing that no fossils at all were found in a 50-inch-deep layer of sediment after the catastrophic impact and calls for more work to explain the mystery.
Upon spotting the fossil, the researchers dug a trench next door to the fossil and removed rock samples from various depths. These samples were sent to Antoine Bercovici at the China University of Geosciences, who analyzed pollen grains in the rocks to identify the K-T boundary. When the asteroid hit, the existing plant life died out, and was later replaced with growth of ferns.
While the Chicxulub asteroid impact is largely uncontested, the manner in which it killed the dinosaurs is still open to debate.
"The impact may have kicked up dust and blocked out the sun and caused a nuclear winter that killed off the plant life," said Lyson.
"Another idea is that the collision produced a thermal pulse, a microwaving effect of the entire Earth, so anything that was out on the surface, that couldn't burrow in the ground, or go underwater, was fried," he added.
Image Below: Yale graduate student Stephen Chester discovered the last known dinosaur before the catastrophic meteor impact 65 million years ago. Credit: Tim Webster
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