Dinosaurs gradually began to die out at least 40 million years before the devastating asteroid impact killed the rest in a dramatic way about 66 million years ago, concludes an extensive new study.
The research, outlined in the journal Proceedings of the National Academy of Sciences, is the first to incorporate phylogenetic information - meaning how species are related to one another - when studying speciation and extinction in dinosaurs.
The data provides evidence that things went from really bad to worse for most dinosaurs that did not evolve into birds.
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"Some dinosaurs definitely would have been instantly killed in the impact, those in the vicinity of the impact site for instance; others may have perished in the tsunami caused by the blast," lead author Manabu Sakamoto of the University of Reading's School of Biological Sciences told Discovery News.
He continued, "The majority of the remaining dinosaurs all over the rest of the world would have likely starved to death as vegetation died out owing to the layer of ash that blacked out the sky (nuclear winter)."
A model created by Sakamoto and colleagues Michael Benton and Chris Venditti found that most dinosaur populations were already declining as early as 48–53 million years before the asteroid impact, which left the massive Chicxulub crater in Mexico. They say that the Cretaceous Period was a time of extreme geological and environmental changes.
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"For instance," Sakamoto said, "sea level was fluctuating, climate was shifting from a hot house to a global cooling, there were prolonged volcanic activities, and continents were breaking apart, establishing the main continents we have today."
He explained that the supercontinent breakup would have resulted in less available land for dinosaurs to use for migration, which previously helped to fuel the evolution of new species. Climate change, both then and now, could induce major environmental and ecological shifts.
Sea level rise turned out to be more complex. Initially, it favored the evolution of more dinosaur species, but smaller land areas eventually restricted population growth.
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Our ancestors, the early mammals, could have posed a big threat to dinos as well.
"Another possibility is that mammals, which were small rodent-like creatures at that time, were in some way contributing to the dinosaur's downfall," Benton, a professor of vertebrate paleontology at the University of Bristol, told Discovery News. "Recent studies show evidence that mammals were on the rise prior to the K-Pg (extinction) event, so this scenario would be consistent with our finding."
Venditti, from the University of Reading, added that mammals might have outcompeted dinosaurs for resources, eaten their eggs, spread diseases or caused other problems for the once mighty dinos.
Aside from the dinosaur lineage that evolved to become birds, two other major groups of dinosaurs - the plant-eating hadrosaurs and ceratopsians - remained strong until the asteroid struck. Sakamoto explained that these dinos acquired specialized jaw structures that allowed them to process plant foods efficiently.
Paleontologist Steve Brusatte from the University of Edinburgh's School of GeoSciences said the new study is an impressive take on "a subject that has been studied to death."
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"Cretaceous dinosaurs were evolving into fewer new species, and going extinct at a faster rate, than earlier dinosaurs," said Brusatte. But he does not think that dinosaurs were doomed to extinction before 66 million years ago.
"The asteroid hit at a time when dinosaurs had already been around for a long time, and had already endured their really prolific periods of evolution," Brusatte said. "But the way I see it, the dinosaur extinction still came down to the asteroid. No asteroid, no extinction."
There is an eerie footnote regarding lessons learned from the extinction of dinosaurs, and other animals that bit the dust after the asteroid hit. Any group of animals that is under a prolonged period of decline can be wiped out, should there be catastrophic event.
Sakamoto said, "This means that if some major catastrophe hits, then it is highly possible that whole groups of animals could be completely wiped off the face of the Earth."