Travel Reference
In-Depth Information
And mammals, including us, have benefi ted from these growing numbers
of evolutionary opportunities.
Mammals arose from lineages of animals called therapsids, which were
common both before and after the Permo-Triassic event. Therapsid fossils
are plentiful, but until recently the fossil record of their mammalian descen-
dants that lived at the time of the dinosaurs was sparse. But over the past
twenty-fi ve years improved paleontological methods have revealed that the
mammals during the Age of Dinosaurs were almost as diverse as the dino-
saurs themselves. About 310 mammalian genera have been discovered dat-
ing from that time, more than half as many as the 547 genera of dinosaurs
currently known. The mammals from the Age of Dinosaurs ranged from
little mouse- and shrew-like animals to much larger freshwater swimmers
that looked like a cross between otters and beavers, along with fi erce little
wolverine-like creatures that could dine on baby dinosaurs.
Then, 65 million years ago, a meteorite or comet 10 kilometers in diam-
eter hit the Yucatan Peninsula. It is now generally concluded that it was this
impact that wiped out the dinosaurs and set the stage for the mammals.
The mammals were amply prepared for their big moment. Even though
they were for the most part small and shy, they were so diverse that ten dis-
tinct major lineages of them managed to survive the meteorite impact.
The surviving mammals inherited a grim, ash-covered world. They
would not have been able to repopulate that world so ef ectively if they had
not been able to draw on the diversity that had already been produced by
more than a hundred million years of earlier mammalian evolution. This
diversity explains why, less than ten million years after the impact, mammals
as dif erent as whales and bats had evolved to fi ll ecological niches left vacant
by the dinosaurs and seagoing reptiles.
Catastrophe in progress
During the past quarter of a billion years more species have appeared than
have gone extinct. There are many reasons for this increase in the number of
species. They include the emergence of new kinds of symbiotic relationships
