12.26b: Field Museum, page 2

*Tyrannosaurus Rex*. Late Cretaceous theropod.

*Triceratops*. Late Cretaceous marginocephalian.

*Parasauropholus*. hadrosaur (ornithopod). Cretaceous.

The K-T extinction: 65 mya, a 10-km asteroid hit the earth at what is now Chixculub on the coast of the Yucatan Peninsula of Mexico. Perhaps the most lethal aspect of this impact was not the monster tidal wave that swept for thousands of kilometers overland, but rather the spray of molten droplets of silica that rained back down into the atmosphere and raised the temperature–briefly–by several hundred degrees. No land animal above about 25 kg survived. As in the early Triassic after the Permian extinction, it took millions of years for larger animals to evolve and fill niches that had long been vacant. Furthermore, the earth itself was changing with the emergence and spread of grasslands. Theropods survived only as birds, and the vertebrates which emerged as the new mid-size and large animals are mammals.

Early horse: *Hyracotherium.* Paleocene (65-55mya).

*Titanothere* Eocene (55-34mya). One of the first large land mammals. They were leaf-browsers, not grazers; that may be why they declined.

*Rodhocetus* (early whale) Eocene (55-50mya)

Early boar: *Archaeotherium*, Oligocene (34-24mya)

Early horses: *Mesohippus*, Oligocene (34-24mya) & *Pliohippus*, Miocene (24-5mya)

Early camelids: *Oxydactylus* and *Chalicothere.* Miocene (24-5mya).

*Teleoceras* (early rhino). Miocene (24-5mya).

*Andalgalornis* (terror bird), Pliocene (5-1.8mya).

*Nimravid* (carnivoran) and *Thylacosmilus* (saber-toothed marsupial).

*Arctodus* (short-faced bear). Pleistocene.

Mastodon. Similar to the Wooly Mammoth, but Mastodon is a browser of tree leaves, not a grazer of grass.

*Megatherium* (Giant Sloth), Pleistocene (1.8M-10K years ago)

*Megatherium* (Giant Sloth), hips and lower legs.

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