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Why Did Mammals Survive the 'K/T Extinction'?
Picture a dinosaur. Huge, menacing creatures, they ruled the Earth for nearly 200 million years, striking fear with every ground-shaking stride. Yet these great beasts were no match for a 6-mile wide meteor that struck near modern-day Mexico 65 million years ago, incinerating everything in its path. This catastrophic impact -- called the Cretaceous-Tertiary or K/T extinction event -- spelled doom for the dinosaurs and many other species. Some animals, however, including many small mammals, managed to survive.
How did they do it?
"They were better at escaping the heat," said Russ Graham, senior research associate in geosciences at Penn State. "It was the huge amount of thermal heat released by the meteor strike that was the main cause of the K/T extinction."
He said underground burrows and aquatic environments protected small mammals from the brief but drastic rise in temperature. In contrast, the larger dinosaurs would have been completely exposed, and vast numbers would have been instantly burned to death.
After several days of searing heat, the earth's surface temperature returned to bearable levels, and the mammals emerged from their burrows, but it was a barren wasteland they encountered, one that presented yet another set of daunting conditions to be overcome, Graham said. It was their diet which enabled these mammals to survive in habitats nearly devoid of plant life.
"Even if large herbivorous dinosaurs had managed to survive the initial meteor strike, they would have had nothing to eat," he said, "because most of the earth's above-ground plant material had been destroyed."
Mammals, in contrast, could eat insects and aquatic plants, which were relatively abundant after the meteor strike. As the remaining dinosaurs died off, mammals began to flourish. Although representatives from other classes of animals also survived the K/T extinction -- crocodiles, for instance, had the saving ability to take to water -- mammals were clearly the main beneficiaries and they have since spread to nearly every corner of the planet.
TRICERATOPS WAS LAST DINOSAUR STANDING
A Triceratops may have been the last dinosaur standing, according to a new study that determined a fossil from Montana's Hell Creek Formation is "the youngest dinosaur known to science."
The Triceratops, described in the latest Royal Society Biology Letters, dates to 65 million years ago, the critical period of time associated with the Cretaceous-Tertiary (K-T) extinction event that wiped out all non-avian dinosaurs and many other animals and plants.
Since this rhinoceros-looking, three-horned dinosaur lived so close to the mass extinction moment, it could negate an earlier theory that dinosaurs gradually died out before 65 million years ago.
"Our paper suggests that dinosaurs did not go extinct prior to the impact," lead author Tyler Lyson told Discovery News. "The fact that this dinosaur is so close to the K-T boundary lends support to the idea that they went extinct as a result of a meteorite impact."
Lyson, a researcher in Yale University's Department of Geology and Geophysics, and his team discovered the remains of the Triceratops, including its over 1.5-foot-long horn, just 5 inches below the pollen-calibrated K-T boundary at Camel Butte, a hill at the Hell Creek Formation in southeastern Montana.
By studying the region's geological layers, the scientists can see how dinosaurs suddenly disappeared after the catastrophic event, which Lyson and many other experts believe was a meteorite strike that directly hit Earth at Mexico's Yucatan Peninsula.
Lyson said that "we don't fully understand the kill mechanism," but other researchers "have a proposed a nuclear winter, while others have proposed a thermal pulse."
The prior theory that dinosaurs gradually died out before 65 million years ago was often based on what is known as the "3-meter gap," which referred to an apparent geological zone devoid of dinosaur fossils before the K-T event.
The Hell Creek Triceratops, however, was not only found within that 3-meter region, but it also exists at the upper reaches of it, proving that at least one dinosaur and presumably more were still alive when the meteorite blasted into Chicxulub, Mexico.
Co-author Stephen Chester of Yale's Department of Anthropology told Discovery News that the Camel Butte site is important both because it has "the most recent dinosaur specimen" and "because we are finding a great diversity of small mammals that are first documented directly after the extinction event."
Chester continued, "Although the K-T mass extinction event is mainly known for the disappearance of the non-avian dinosaurs, it is also an extremely important event in mammalian evolution because once the dinosaurs vanished, mammals underwent a large adaptive radiation and began occupying diverse ecological niches in the Paleocene."
These mammals included condylarths, which were hoofed animals proposed to be ancestral to some modern orders of hoofed mammals. They also included multituberculates, which Chester described as being "extinct rodent-like animals with a very specialized dentition."
It remains unclear why certain mammals, turtles and other animals survived the K-T extinction event, but Lyson explained that species with generalist, rather than specialized, diets tended to fare better, as did smaller animals and water dwellers.
Kirk Johnson is vice president of Research & Collections and chief curator at the Denver Museum of Nature & Science.
Johnson told Discovery News that he agrees the Triceratops is indeed "the last known non-avian dinosaur of the Cretaceous." He said, "The 3M Gap is a weak concept to begin with," and that his own work on plants and insects supports the idea that the meteor impact was the "direct and immediate cause of habitat destruction and extinction of more than 50 percent of North American plant and insect species."
It Took Earth Ten Million Years to Recover from Greatest Mass Extinction
It took some 10 million years for Earth to recover from the greatest mass extinction of all time, latest research has revealed.
Life was nearly wiped out 250 million years ago, with only 10 per cent of plants and animals surviving. It is currently much debated how life recovered from this cataclysm, whether quickly or slowly.
Recent evidence for a rapid bounce-back is evaluated in a new review article by Dr Zhong-Qiang Chen, from the China University of Geosciences in Wuhan, and Professor Michael Benton from the University of Bristol. They find that recovery from the crisis lasted some 10 million years, as explained May 27 in Nature Geoscience.
There were apparently two reasons for the delay, the sheer intensity of the crisis, and continuing grim conditions on Earth after the first wave of extinction.
The end-Permian crisis, by far the most dramatic biological crisis to affect life on Earth, was triggered by a number of physical environmental shocks -- global warming, acid rain, ocean acidification and ocean anoxia. These were enough to kill off 90 per cent of living things on land and in the sea.
Dr Chen said: "It is hard to imagine how so much of life could have been killed, but there is no doubt from some of the fantastic rock sections in China and elsewhere round the world that this was the biggest crisis ever faced by life."
Current research shows that the grim conditions continued in bursts for some five to six million years after the initial crisis, with repeated carbon and oxygen crises, warming and other ill effects.
Some groups of animals on the sea and land did recover quickly and began to rebuild their ecosystems, but they suffered further setbacks. Life had not really recovered in these early phases because permanent ecosystems were not established.
Professor Benton, Professor of Vertebrate Palaeontology at the University of Bristol, said: "Life seemed to be getting back to normal when another crisis hit and set it back again. The carbon crises were repeated many times, and then finally conditions became normal again after five million years or so."
Finally, after the environmental crises ceased to be so severe, more complex ecosystems emerged. In the sea, new groups, such as ancestral crabs and lobsters, as well as the first marine reptiles, came on the scene, and they formed the basis of future modern-style ecosystems.
Professor Benton added: "We often see mass extinctions as entirely negative but in this most devastating case, life did recover, after many millions of years, and new groups emerged. The event had re-set evolution. However, the causes of the killing -- global warming, acid rain, ocean acidification -- sound eerily familiar to us today. Perhaps we can learn something from these ancient events."
