306 Evolution? The Fossils Say YES!
just a few years ago. By the Late Cretaceous, both marsupials and placentals were evolving
very fast, and most of the archaic groups of Mesozoic mammals were gone.
Radiation in the Aftermath
The placental or eutherian mammals comprise about twenty living orders and sev-
eral extinct ones. The morphological and adaptive range of this group is extraordi-
nary; diversification has produced lineages as varied as humans and their primate
relatives, flying bats, swimming whales, ant-eating anteaters, pangolins, and aard-
varks, a baroque extravagance of horned, antlered and trunk-nosed herbivores
(ungulates), as well as the supremely diverse rats, mice, beaver and porcupines of the
order Rodentia. Such adaptive diversity, and the emergence of thousands of living
and fossil species, apparently resulted from a radiation beginning in the late Meso-
zoic between 65 and 80 million years ago. This explosive radiation is one of the more
intriguing chapters in vertebrate history.
—Michael J. Novacek, “The Radiation of Placental Mammals”At the end of the Cretaceous 66 million years ago, the nonavian dinosaurs vanished from
the planet. One group of scientists argue that the impact of a big rock from space did
them in, while another group points out that there were too many survivors who could
not have outlived such an extreme catastrophe. Instead, they suggest that the extinction
was due to more gradual changes, and this is supported by evidence of the fossil record
(for a review of the topic, see Prothero 2016). Whatever the cause, by the early Paleocene,
the terrestrial realm was devoid of large animals, and there were plenty of vacant ecologi-
cal niches for any opportunistic creature to occupy. Within a million years after the end
of the Cretaceous, mammals began an explosive evolutionary radiation, with many new
groups appearing for the first time in the fossil record, and the mostly shrew-sized Meso-
zoic mammals evolving into much larger dog-sized and even cow-sized animals. By the
middle Eocene, only 15 million years after the nonavian dinosaurs had vanished, almost
all the living orders of mammals (rodents, rabbits, bats, whales, carnivores, primates, and
so on) had appeared, although they were very primitive members of those families that
look nothing like their living descendants. Paleontologists frequently point to the evo-
lutionary radiation of Cenozoic mammals as a classic example of what life can do when
competition is suddenly removed and there are many new ecological resources and adap-
tive zones left vacant.
Deciphering this evolutionary explosion has been one of the major challenges for pale-
ontologists for over a century. The major problem is that for a very long time the fossil record
of mammals was very poor in Cretaceous and Paleocene rocks, so paleontologists had only
fragmentary teeth and jaws to work with, and only from a few places such as North America
and Europe. Complete skeletons were extremely rare, skulls were scarce and often badly
distorted or damaged because deposits this old are usually crushed under the weight of
millions of tons of rocks that were deposited on top of them and often deformed by later
mountain-building events as well. Paleontologists did the best they could by matching
the patterns of teeth from the Paleocene with those from the Cretaceous and attempting