460 CHAPTER 17diversity of fishes took the place of the great Mesozoic marine reptiles. Late in
the Cenozoic, modern coral reefs became prevalent in the tropics. Today they are
the marine equivalent of tropical rainforests because of their extraordinarily rich
diversity of fishes, sponges, and other animals—and they are just as threatened
by human activities.
On land, many of the modern families of angiosperms and insects had become
differentiated by the late Cretaceous, and many more evolved in the Paleocene or
Eocene. Many fossil insects of the late Eocene and Oligocene belong to genera that
still survive. The savannahs that developed in the Oligocene because of the more
arid climate were populated by grasses (Poaceae), which underwent a major adap-
tive radiation at this time, and herbaceous plants, many groups of which evolved in
the Paleogene from woody ancestors. Among the most important of these groups
is the family Asteraceae, which includes sunflowers, daisies, ragweeds, and many
others. It is one of the two largest plant families today.
The most dramatic biotic change between the Cretaceous and the Paleogene is
the utter absence of the great dinosaurs and other archosaurs that had ruled the
world, and in their stead, the rapid proliferation of even more diverse birds and
mammals. Calibrated phylogenies of living birds indicate that most of the orders
(e.g., pigeons, pelicans, owls) originated in the Paleocene [42, 73], and many liv-
ing orders and families of birds are recorded from the Eocene (56–33.9 Mya) and
Oligocene (33.9–23 Mya). The songbirds (Passeriformes), which account for half
of the living species, first displayed their great diversity in the Miocene (23–5.3
Mya). Another great adaptive radiation was the snakes, which began an exponen-
tial increase in diversity in the Oligocene. Snakes today feed on a great variety of
animal prey, from worms and termites to bird eggs and wild pigs, and they include
marine, burrowing, and arboreal forms. Some can even glide between trees.The adaptive radiation of mammals
Because you may be more familiar with mammals than with other animals or
plants (and because you are a mammal), we describe their Cenozoic ups and
downs in a little more detail. Although the marsupial and placental mammals
originated in the Cretaceous, most of the fossils that can be assigned to mod-
ern orders occur after the K/Pg boundary (66 Mya). It has often been suggested
that the extinction of the large nonavian dinosaurs at the end of the Cretaceous
relieved the mammals from competition and predation, and allowed them to
undergo adaptive radiation—but this overlooks the great diversity of extinct
groups of mammals that coexisted with dinosaurs (see Figure 17.28). More-
over, two research groups that used multiple fossils to calibrate the rate of DNA
sequence evolution concluded that the stem lineages leading to many of the living
orders of mammals originated in a burst of diversification at least 80 Mya, during
the Cretaceous (FIGURE 17.30) [10, 64]. They found no evidence that the rate of
mammal diversification increased after the dinosaurs’ demise.
The marsupial families that include kangaroos, wombats, and other living
Australian marsupials evolved in the Eocene and Oligocene. Marsupials probably
arose in Asia: they are known as fossils from all the continents, including Antarc-
tica. Today they are restricted to Australia and South America (except for the North
American opossum, which evolved from South American ancestors). In South
America, marsupials experienced a great adaptive radiation; some resembled kan-
garoo rats, others saber-toothed cats (FIGURE 17.31A,B). Most South American
marsupials became extinct by the end of the Pliocene.
In addition to marsupials, many groups of placental mammals evolved in South
America during its long isolation from other continents. These mammals included
an ancient placental group, the Xenarthra (or Edentata), which includes the giant17_EVOL4E_CH17.indd 460 3/22/17 1:37 PM