Were dinosaurs already on the way out when the asteroid hit?

A new study shows there were fewer dinosaur species evolving prior to their extinction 66 million years ago. But does that mean they were doomed even if the meteorite didn't hit? Belinda Smith reports.

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Dinosaurs were well past their prime when a meteorite hit Earth 66 million years ago and finished them off, new research suggests.

A trio of biologists in the UK, led by palaeontologist Manabu Sakamoto from the University of Reading, modelled the number of dinosaur species that evolved during their 160-million-year reign. They found, with only a couple of exceptions, fewer species emerged in their final 50 million years or so.

It's this declined "speciation", they report in the Proceedings of the National Academy of Sciences, that suggests dinosaurs overall were going extinct long before the meteorite impact. Some groups seemed worse off than others: the giant long-necked sauropods, for instance, declined faster than the therapod meat-eaters, a group that included Tyrannosaurus rex.

Bucking the trend, ceratopsids – horned dinosaurs – seemed to pick up their speciation rate in those final few millions of years.

"We were not expecting this result," Sakamoto says. "While the asteroid impact is still the prime candidate for the dinosaurs’ final disappearance, it is clear that they were already past their prime in an evolutionary sense."

While the researchers couldn't pinpoint specific pressures that might result in this declined speciation, they suggest continental land mass break-up and volcanic activity may have played a part.

John Long, a palaeontologist at Flinders University in Adelaide, Australia, calls the work "innovative and clever". But, he adds, "one thing is not counted in the argument, and that is the actual biomass of the species on the planet".

In other words, just because there are fewer species of an animal, it doesn't necessarily mean the animal is dying out.

Take koalas. Some 20 million years ago or so, Australia was home to around 16 species of koalas, Long says: "Then you look at today, and we have only one species of koala. But they're highly abundant. It doesn't mean they're on the way out."

Long says this isn't to say the dinosaurs weren't slowly moving towards extinction prior to the asteroid apocalypse. But without biomass or population numbers, it's impossible to say for sure. And the fossil record is not a particularly good indicator of population size, he says: "We can only, at best, guess it."

Regardless, he adds, some groups of dinosaurs would have been more susceptible to environmental changes.

Massive sauropods, which evolved long necks and teeth to reach and strip leaves from specific tree species, might have been among the first to go once the meteorite kicked up vast amounts of dust and blacked out the Sun, decimating plant life. "But many small dinosaurs, such as birds, came through unscathed."

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Belinda Smith is a science and technology journalist in Melbourne, Australia.