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Were dinosaurs headed for extinction even before massive asteroid strike? Scientists offer new clues 
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It’s a long-standing debate in paleontology: Were dinosaurs thriving when an asteroid hit Earth one fateful spring day 66 million years ago, or were they already on their way out, and the space rock delivered a final, devastating blow?

To find answers, a team of researchers studied North America’s fossil record, focusing on the 18 million years before the mass extinction at the end of the Cretaceous period. The new analysis, published Tuesday in the journal Current Biology, adds to a growing body of evidence that the dinosaurs were doing just fine before the asteroid’s deadly impact.

However, at face value, the fossils available for study from this time — more than 8,000 — suggest the number of dinosaur species peaked about 75 million years ago and then declined in the 9 million years leading up to the asteroid strike.

“It comes down to the fossil record and its fidelity, or its quality. And so there’s been an awareness since the 1970s that the fossil record is not accurate, but it is a biased reflection in the past,” said lead study author Chris Dean, a research fellow in paleontology at University College London.

“It’s only in very recent years that we’ve started to see the full extent of (the bias issue), when using these large databases of fossil occurrences,” he said.

To understand better what was going on at the time of the dinosaurs’ demise, Dean and his colleagues turned to a statistical approach called occupancy modeling to estimate the probability of a dinosaur being present at a site. Used in present-day ecology and conservation, occupancy modeling aims to account for the fact that a species may be overlooked or not detected even when present in a particular area. This study marks the first time the approach has been used to look at dinosaurs and over a large scale, Dean said.

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