North Dakota Fossil Site Evidence Suggests the Dinosaurs May Have Died in the Spring

New research points to when massive asteroid impact happened, and why only some animals survived.
Picture taken from behind a woman with blond and pink hair, who is working to excavate a skeleton of a paddlefish, with tools and supplies lying on the rock and sand nearby.

Melanie During working to excavate a paddlefish in the Tanis deposit

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Charles Q. Choi, Contributor

(Inside Science) -- The age of dinosaurs may have ended in springtime in Mexico, which may help explain the pattern of extinctions that resulted, a controversial new study now claims.

After dominating the planet for roughly 135 million years, the dinosaurs' reign ended about 66 million years ago, with the killing blow most likely dealt by an asteroid roughly 6 miles (10 kilometers) wide. The result was a crater more than 110 miles (180 km) across, near what is now the town of Chicxulub in Mexico's Yucatán Peninsula.

The meteor strike would have released as much energy as 100 trillion tons of TNT, more than a billion times more than the atomic bombs that destroyed Hiroshima and Nagasaki combined. The cosmic impact had global effects, killing off about 75% of all species on the planet, including all dinosaurs except birds, as well as the winged reptiles known as pterosaurs and the mollusks known as ammonites.

Now scientists, analyzing fossils of fish, think they have pinpointed the season in which the collision occurred to springtime in the Northern Hemisphere. They suggest their findings could help explain why birds and mammals survived while most dinosaurs perished.

The researchers investigated a fossil trove unearthed in North Dakota in the rock bed known as the Hell Creek Formation. In the past, when the animals were alive, the site, dubbed Tanis, experienced a temperate and seasonally wet climate, with summer highs of roughly 20 degrees Celsius (68 degrees Fahrenheit) and winter lows of 5 C (40 F). Nowadays the area can reach as hot as up to 45 C (113 F), "but the real struggle were the biting flies, who really tried to eat me alive," recalled study lead author Melanie During, a vertebrate paleontologist at Uppsala University in Sweden.

Tanis preserved remains of crocodiles, sharks, turtles and other animals that apparently died en masse from a kind of wave known as a seiche, which can occur in confined bodies of water shaken by continental disturbances, such as in swimming pools during earthquakes. Previous research suggested this seiche may have been triggered by, and occurred minutes to hours after, the Chicxulub impact, although a number of researchers remained unconvinced these deaths were linked directly with the meteor strike.

The scientists focused on the remains of filter-feeding sturgeons and paddlefish. They discovered tiny spherical glass beads that rained down after the cosmic impact lodged in bony projections known as gill rakers, which these fish used to sieve out food particles from the water. However, this debris was not seen further down in their guts, suggesting they probably died after getting buried alive when the seiche triggered a giant surge of water and soil.

The researchers also examined 3D growth patterns of jaw bones and fin spines from the fish. Much as tree rings can through their number and composition reveal the age of trees and the growing conditions of each season, so too these bones preserve seasonal changes from embryonic development all the way to the death of these fish. 

The scientists found that bone cell density and volumes were on the rise but had not yet peaked during the year of death, which suggested the fish stopped growing in the spring. "The bone micro-structures were preserved in tremendous detail," During said.

In addition, the scientists examined two carbon isotopes within one of the paddlefish -- carbon-12, which has six neutrons within its nucleus, and carbon-13, which is heavier with seven. The zooplankton that were the paddlefish's prey of choice are enriched in carbon-13, and their availability peaked in summer. The carbon isotopes in the fossil suggested its feeding season had not yet climaxed, supporting a spring death.

The possibility that the Chicxulub impact hit in the spring in the Northern Hemisphere may help explain why some groups of animals went extinct while others survived, the scientists noted. For example, in many animals, reproduction and growth take place during springtime, and most dinosaurs likely required long incubation times for their eggs compared with birds, potentially making them more vulnerable to a catastrophe in the spring. In addition, prior work suggested ecosystems in the Southern Hemisphere, which correspondingly would have experienced the impact during its autumn, recovered up to twice as fast as ones in the Northern Hemisphere, consistent with a seasonal difference.

These findings may suggest a strategy other scientists could use to examine fossils in their own collections for connections to this ancient catastrophe that "may have been entirely missed," said vertebrate paleobiologist Jeff Liston at the Royal Tyrrell Museum of Palaeontology in Canada, who did not take part in this research. "If there is one site like this created by the impact, then there are highly likely to be others where this methodology could be replicated and help to build a much greater global view of the impact aftermath."

Future research may look at not just what fish experienced during the moments of the disaster. "Are there invertebrates that could also provide more information, or even larger vertebrates, such as dinosaurs?" said vertebrate paleontologist Femke Holwerda at the Royal Tyrrell Museum of Palaeontology, who also did not take part in this study. "I'd like to see this methodology applied to as many organisms as possible."

However, not everyone is convinced by the new study's conclusions. Although paleohistologist Holly Ballard at Oklahoma State University, who did not take part in this research, said it does present strong evidence that the fish all died in the spring, she added that many scientists remain skeptical the mass death at Tanis directly resulted from the meteor strike in the Yucatan. "The jury is still out on the Chicxulub connection," she noted. "Many researchers feel that the Tanis site being connected to the Cretaceous end of days is far from a foregone conclusion."

During remained more hopeful. "More Tanis-like deposits are waiting to be found, and I am very much looking forward to what has been buried alive in those," she said.

The scientists detailed their findings this week in the journal Nature.

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Charles Q. Choi is a science reporter who has written for Scientific American, The New York Times, Wired, Science, Nature, and National Geographic News, among others.