However, the supertree shows that the placental mammals had
already split into these sub-groups by 93 million years ago, long
before the space impact and at a time when dinosaurs still ruled the
After the origin of these sub-groups - or orders - the rate of
mammal evolution fell and remained low again until the Eocene Epoch,
55 million years ago.
The start of the Eocene was marked by rapid global warming and
an explosion in the diversity of mammal lineages.
"The [supertree] is a new way of showing all the mammal species
on the planet, starting with a common ancestor. Species
relationships can be inferred from morphological characteristics and
genetic sequences," explained Dr Jones.
"If we had done this from scratch, we would have had to get
molecular and morphological data for 4,000 different species.
"What we did instead was use already published information from
hundreds of researchers around the world. We used a new technique
called supertree construction which allows us to get all the
information that's out there, re-code it and re-analyse it as if
it's all part of one dataset."
'Straw man' theory
The composition of rocks and marine sediments laid down at the
boundary between the Palaeocene and Eocene epochs show that global
temperatures rose by around six degrees Celsius in less than 1,000
years - an event known as the thermal maximum.
Dr Rob Asher, an expert on mammalian phylogeny at the University
of Cambridge, said: "Palaeontologists have known for over a hundred
years that not all modern placental mammal groups appear right after
the K-T boundary.
"Most orders of placental mammals - what I mean by that is cats
and bats and whales and people - appear at the Eocene. On the
flipside, not all dinosaurs disappear at the end of the Cretaceous.
"There was a period of several million years at the end of this
period which witnessed several extinctions of non-avian dinosaurs.
So the old textbook idea that at the K-T boundary dinosaurs
disappeared and mammals appeared is a bit of a straw man."
But the idea that mammal fossils from the Cretaceous represent
ones ancestral to today's mammals was a controversial question, said