A study published in the journal Nature announced
the finding that the extinction of dinosaurs 65 million years ago
had little effect on the evolution of mammals.
One theory had suggested that the rise of the mammals
was directly linked to the disappearance of the dinosaurs. The
evidence to challenge the connection comes from the most complete
family tree to have been compiled for mammals. It shows how different
groups, such as primates and rodents, are related and when they
An international team compiled the mammalian 'supertree'
from existing fossil data and from genetic analyses. Throughout the
Cretaceous period, when dinosaurs walked the planet, mammals were
relatively few in number, and were prevented from diversifying and
evolving in ecosystems which were dominated by reptiles.
According to the established view, the extinction of
the dinosaurs removed this constraint, allowing mammals to diversify
and flourish, and placing them on course for achieving their present
position of dominance on Earth. Under this model, placental mammals
split into major sub-groupings, which originated and rapidly diversified
after the mass extinction event - thought to have been caused by an
asteroid or comet striking Earth 65 million years ago (a point in time
recorded in rocks and referred to by geologists as the K-T boundary).
Co-author of the report, Kate Jones, from the Zoological
Society of London, confirmed the traditional view of mammalians rising
from the extinction of the dinosaurs. However, she went on to point out
the new findings - that the supertree shows that placental mammals had
already split into these sub-groups by 93 million years ago, long before
the space impact and at a time at which dinosaurs still ruled the planet.