THE HiSToRy oF LiFE 457
lineages have living representatives: the monotremes (the egg-laying platypus and
echidnas), the marsupials, and the placental (eutherian) mammals.
The end of the Cretaceous is marked by the best-known mass extinction, caused
by the great environmental disruption that resulted from the impact of an asteroid
or some other extraterrestrial body. The site of this impact, the Chicxulub crater,
has been discovered off the coast of Mexico’s Yucatán Peninsula. There was surely
a staggering explosion on impact, probably followed by a worldwide cloud of dust
and vapor that blocked sunlight and created a long-lasting winter. This event was
formerly called the K/T extinction, using the abbreviations for Cretaceous and Ter-
tiary, but is now called the K/Pg extinction, with Pg referring to Paleogene. The K/
Futuyma Kirkpatrick Pg event extinguished the last nonavian dinosaurs, many mammalian groups, and Evolution, 4e
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Evolution4e_17.26.ai Date 01-18-201712457DROMAEOSAURIDAETYRANNOSAUROIDEABRACHIOSAURIDAESTEGOSAURINAEANKYLOSAURINAELAMBEOSAURINAECERATOPSINAEORNITHISCHIA SAURISCHIAArchaeopteryx63FIGURE 17.26 The great diversity of dinosaurs. The two great clades of dinosaurs, Ornith-
ischia (left) and Saurischia (right), are curved downward to fit the page. The names of most
lineages have been omitted, for simplicity. The exceptions are for the few pictured dino-
saurs. These are the ornithischian Lambiosaurinae (here, Parasaurolophus), Stegosaurinae,
Ankylosaurinae, and Ceratopsinae, which includes Styracosaurus (shown) and Triceratops
(not shown). Among the Saurischians are the sauropods, represented here by Brachiosauri-
nae, and the theropods, illustrated by Tyrannosauroidea (Tyrannosaurus), Dromaeosauridae
(Deinonychus), and Archaeopteryx, a close relative of the ancestors of birds. All ornithis-
chian lineages are extinct. (From [82].)17_EVOL4E_CH17.indd 457 3/22/17 1:37 PM