| A Cape Hyrax, the closest living relative of the African and Asian elephants. Their common ancestor was one of the “archaic” mammals. (Picture courtesy: Richard Grenyer, Royal Botanic Gardens, UK)
New Delhi, March 28: Challenging a textbook theory, the first-ever evolutionary tree of known mammals unveiled today has shown that the extinction of dinosaurs did not result in the diversification of the ancestors of modern mammals.
The “super tree” of 4,510 mammals prepared by an international research team overturns the longstanding idea that the demise of the dinosaurs 65 million years ago left empty ecological niches for the ancestors of modern mammals to diversify.
The findings, to be published in the journal Nature on Thursday, show that modern mammals such as the primates, rodents and hoofed animals did not diversify until at least 10 to 15 million years after the mass extinction.
Geological evidence has indicated that the mass extinction of species 65 million years ago was triggered by the crash of a large asteroid on Earth.
“The super tree provides us the clearest picture yet of how mammals have evolved on this planet,” Andy Purvis, a senior biologist at the Imperial College, London, told The Telegraph. “It shows us our own place in nature.”
The analysis has pointed to two pulses of mammalian diversification — the first one about 93 million years ago, and the second 55 million years ago. “For the first 10 or 15 million years after the dinosaurs were wiped out, other types of mammals were running the show. It’s not the death of the dinosaurs but a bout of natural global warming later that kick-started today’s (mammal) diversity,” Purvis said.
“This tells us that the success story of modern mammals is not the result of dinosaurs going extinct as had been widely believed,” said Robin Beck, a palaeontologist at the University of New South Wales in Australia.
“Darwin had predicted 150 years ago that one day we would have a tree of evolution. This (tree of mammals) is a big step towards that goal,” Beck told The Telegraph over the phone.
His role in the project was to focus on the relationships between different mammalian “orders” — the major groups of mammals such as rodents, bats, and primates.
The super tree of mammals — which contains 4,510 of the known 4,554 mammalian species — indicates that some of the older or archaic mammals that emerged during the first pulse did indeed diversify after the mass extinction of dinosaurs.
However, all of those mammals have either died or have declined in diversity. An aggressive wolf-like cow, for instance, has gone extinct, while the diversity of sloths and armadillos has plummeted.
Some archaic mammals went extinct at the same time that the modern mammals began to radiate — a process of evolutionary diversification in which the number of species increases rapidly — about 55 million years ago.
Scientists believe that it is possible the ancestors of modern mammals were simply better adapted to the world at that time and emerged as “survivors” in a competition between archaic and modern mammals.
“Before the mass extinction, mammals were small…. There were no large herbivores like rhinos or elephants, no large carnivores like tigers, no large aquatic mammals like whales,” Beck said.
“Immediately after the mass extinction, we see a rapid appearance of larger and more diverse mammals which took over the niches previously filled by dinosaurs, but the mammals that diversified belonged to the archaic mammals,” he said.
The diversification of mammals about 10 to 15 million years after the mass extinction coincides with high global temperatures — a period called the cenozoid thermal maximum during which average temperatures rose by five degrees within 20,000 years and stayed high for 100,000 years.
“There is not enough evidence to say whether there is a direct link between this warming and the mammalian diversification,” Beck said.
The decade-long effort to produce the first nearly complete super tree of mammals by combining fossil evidence and genetic data was led by Olaf Bininda-Emonds at the Technical University of Munich, Germany. Scientists from Canada and the US also participated in the project.
Their finding reconciles differences between genetic data and fossil evidence that have long puzzled scientists.
“Molecular data indicates that our common mammalian roots have to go back to 90 million years, if not more, but many palaeontologists have been dubious of this claim given the lack of ancestral-looking fossils until about 55 million years ago,” said Ross MacPhee, a team member at the American Museum of Natural History.
The new study suggests that the absence of fossils of archaic mammals may be because there just weren’t enough of them.