462 CHAPTER 17survived through the most recent glaciation, until about 10,000 years ago, and two
genera (the African and Indian elephants) persist today.
In the Eocene, a group of archaic hoofed mammals gave rise to a great radiation
of carnivores and hoofed mammals (ungulates). The Carnivora proliferated on land
and gave rise to the marine seals. Among the ungulates, the Perissodactyla, or odd-
toed ungulates, were very diverse from the Eocene to the Miocene, then dwindled
to the few extant species of rhinoceroses, horses, and tapirs. The artiodactyls (order
Cetartiodactyla) are first known in the Eocene as rabbit-sized animals, but other-
wise bore little similarity to the pigs, camels, and ruminants that appeared soon
afterward. In the Miocene, the ruminants began a sustained radiation, mostly in the
Old World, that is correlated with the increasing prevalence of grasslands. Among
the families that proliferated are the deer, the giraffes and relatives, and the Bovi-
dae, the diverse family of antelopes, sheep, goats, and cattle. During the Eocene, an
artiodactyl lineage related to today’s hippopotamuses became aquatic and evolved
into the cetaceans: the dolphins and whales (see Figure 20.3).
Rodents (Rodentia) are recorded first from the late Paleocene. They became the
most diverse order of mammals, in part because of an extraordinary proliferation
of rats and mice within the last 10 My. They are related to one of the oldest and, in
many ways, structurally most primitive placental orders: the Primates. The earliest
fossils assigned to this order are so similar to early placental mammals that it is
Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_1703.ai Date 11-02-2016(B)(B)(A) (C)(D)FIGURE 17.31 Some extinct Cenozoic mammals. (A) Thylacosmilus
was a Miocene to Pliocene South American marsupial, one of at least
seven lineages of mammals that independently evolved the saber-
tooth form. (B) Another was the famous saber-toothed cat Smilodon,
from the Pleistocene in North America. (C) The armored, armadillo-
like Glyptodon and (D) this giant ground sloth, Megatherium, are
Pleistocene representatives of the Xenarthra.17_EVOL4E_CH17.indd 462 3/22/17 1:37 PM