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It is a long-standing debate in paleontology: the dinosaurs prospered when an asteroid hit the earth a fateful spring day 66 million years ago, or were they already going out, and the space rock gave an final and devastating blow?
To find answers, a team of researchers studied the fossil file of North America, focusing on 18 million years before mass extinction at the end of the Cretaceous period. The new analysis, published Tuesday in The Revue Biologie CurrentAdd to an increasing assembly of evidence that the dinosaurs worked very well before the deadly impact of the asteroid.
However, at their nominal value, the fossils available for the study from that moment – more than 8,000 – suggest that the number of dinosaurs culminated about 75 million years ago, then decreased in the 9 million years preceding the asteroid strike.
“This comes back to the fossil file and its fidelity, or its quality. And there is therefore an awareness since the 1970s that the fossil file is not correct, but it is a biased reflection in the past,” said Chris Dean, member of paleontology research at the University College.
“It was only in recent years that we have started to see the full extent of (the bias problem), when you use these large databases of fossil occurrences,” he said.
To better understand what was going on at the time of the disappearance of dinosaurs, Dean and his colleagues turned to a statistical approach called occupation modeling to estimate the probability that a dinosaur is present on a site. Used in current ecology and conservation, the occupation modeling aims to explain the fact that a species can be neglected or not detected even when it is present in a particular area. This study marks the first time that the approach was used to watch the dinosaurs and on a large scale, said Dean.
“The application of a new technique is really difficult,” noted Dean. “I don’t think it will be the last word. I’m sure there is much more to say.”
For the new study, the researchers examined four main families of dinosaurs: the ankylosauridae (shielded planting dinosaurs such as club -tailed ankylosaurus), ceratopsidae (large herbivores with three horns, including triceratops), Hadrosauridae (Dinosaurs à Vachette à Three Corns) and Tyrannosaurae (Carnivores like tyrannosaurs).
“We have examined these larger groups so that we can have more data, effectively,” said Dean. “We have climbed into North America in a large space grid (and determined) the places where we can find fossils (the places where we) have physically found fossils and how many times people have gone fossil (in these places).”
The information was introduced into a computer model, and Dean and his colleagues compared the physical fossil file with that proposed by the model and found a gap.
The model suggested that, during the period of 18 million years in question, the proportion of land of the four dinosaurs clades Probably occupied has remained global constant, which suggests that their potential housing area has remained stable and the risk of extinction remained low.
One of the factors that could have darken the real models of dinosaurs was the lack of rock exposed to the surface of the earth during this time window – and therefore available for fossil hunters today to examine.
“In this study, we show that this apparent decline is more likely the result of a reduced sampling window, caused by geological changes in these terminal mesozoic fossil layers – driven by processes such as tectonics, mountain uprising and maritime level retirement – rather than authentic fluctuations in biodiversity,” said the co -author of the study. Alessandro Department of Earth Sciences of the London College, in a press release.
“Dinosaurs were probably not inevitably dedicated to extinction at the end of the mesozoic,” said Chiarenza. “If it was not for this asteroid, they could always share this planet with mammals, lizards and their surviving descendants: birds.”
The study has helped to underline which bias can affect the understanding of scientists of the true model of dinosaurs leading to the extinction event, said Darla Zelenitsky, paleontologist at the University of Calgary in Alberta which was not involved in research.
“Due to the nature of the rock record, (paleontologists) found it more difficult to detect dinosaurs and thus understand their diversity models in this time window just before the extinction of the mass,” she said.
“This certainly makes sense because we know that there are biases linked to rock recording which can obscure the real biological models. The more rocks exposed to the surface (today), the best are our chances of finding dinosaurs in this rock, which in turn leads to a better understanding of their diversity models.”
Mike Benton, professor of vertebrate paleontology at the British University of Bristol, described the newspaper “detail and detailed”, but said that he did not prove that there was no reduction in the diversity of dinosaurs before the extinction event. Benton’s work suggested that dinosaurs were declining before asteroids tried them. He was not involved in the new study.
“The current document suggests that” reduction “can be explained as a statistical artifact,” said Benton by e-mail. “What he shows is … simply that the reduction could be real or could be explained by a reduced sampling, in my opinion.”