By Anna Salleh for ABC Science Online
The death of the dinosaurs was not the catalyst for modern mammalian evolution that many people think, a new study shows.
According to an international team of researchers, mammals evolved much slower than people believe. And it was not until 10 million years after the dinosaurs went extinct that there was a rise in the rate of new mammal species.
"[The research] challenges this idea about the dinosaurs suppressing mammal evolution," says evolutionary biologist, Dr Marcel Cardillo, part of team that publishes its results today in the journal Nature.
Dr Cardillo, a visiting fellow at the School of Botany and Zoology at the Australian National University, and colleagues constructed the largest, most complete family tree of modern mammals.
This involved mapping the relationship between 99 per cent of the roughly 4,500 mammalian species that exist today.
"It's the first time anyone has put together an essentially complete evolutionary tree for mammals," Dr Cardillo said.
The phylogenetic 'supertree' reveals an evolutionary pattern that contradicts conclusions based on the fossil record.
The fossil record suggests there was one big burst in mammal evolution immediately after the dinosaurs died out, Dr Cardillo says.
But the new supertree shows that mammals diversified into major groups such as primates, rodents and carnivores, 100 to 85 million years ago, during the age of the dinosaurs.
The tree also shows the rate of evolution of mammals remained fairly constant after the dinosaurs died out 65 million years ago.
And it was not until 10 million years later that the rate of mammalian evolution started to pick up again.
By 35 million years ago, most of the modern day families of mammals, such as the dog and cat families, had evolved, Dr Cardillo says.
He says the fossil record is very "patchy" which makes it difficult to reach reliable conclusions about evolution based on it alone.
Quite often fossils are mere fragments of an animal's skeleton, making it very hard to identify whether it's a mammal or not, he says.
Building the supertree
To create their phylogenetic supertree, Dr Cardillo and colleagues combined data from 2,500 different family trees, based on fossil, molecular and morphological data.
Molecular studies compare similarities in the DNA of living species to find evolutionary relationships between them. Morphological studies compare differences in the features of living species.
The researchers used a large computer to analyse the data and to determine the branches of the tree.
For each section of the tree they calculated the length of the branches, which represent the time elapsed between divergent groups, using a known rate of DNA mutations.
They then used fossil data to peg absolute dates on the tree.
The research challenges the short-fuse 'burst' model of mammalian evolution, which assumes mammals rapidly diversified into the number of species we see today.
Instead, say the researchers, it was a much slower process.
Dr Cardillo says the recent findings suggest there needs to be a rethink about the relationship between dinosaurs and mammals.
The two groups may have found ways to exploit different ecological niches, or may not have interacted to the extent that we think they did, he says.
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