With such a wealth of habitats a whole new host of animals took over Big Bend. Crocodiles and turtles hung on from the Age of Reptiles, but nature’s evolutionary torch passed to the warm-blooded mammals who increased rapidly in numbers, size, and diversity. Remains of 29 species of early, extinct forest-dwelling mammals have been discovered near the Fossil Bone Exhibit site on Tornillo Flat. This same floodplain later accommodated a hippo-like plant-eater, a browsing collie-sized mammal, a panther-like cat, and the little ancestral horse, Eohippus. No bigger than a fox terrier, Eohippus had not yet developed the typical horse hoof and still had four toes on his front feet and three on his hind. He browsed among low forest plants, because nature hadn’t yet invented grass. Some 20 million years ago, when we get our next glimpse of the Big Bend, grasses were well established and the Earth began to witness the rapid rise of grazing animals. In the park, Castolon had a savanna-type environment, with a sub-humid to semi-arid climate. Rabbits and camel- and sheep-like mammals flourished. Plant eaters ranged in size from a tiny mouse to an enormous rhinoceros. This giant was about 3 meters (10 feet) long, stood 2 meters (7 feet) high at the shoulder, and had massive, bony horns on its head. There were also carnivores to fatten on the herbivores. In the middle of the Age of Mammals, Big Bend country became the seat of widespread and repeated volcanic disturbances, with lava flows, ash falls, and mountains bulging up like blisters as they filled with molten rock. Here, most of these events centered on the Chisos, where the signs can be seen in mountain peaks to this day. Much of what happened since has not even left a shadow; the record and the rocks have both been erased by millions of years of weathering and erosion. But you can see “living fossils” in the high Chisos canyons. These are the ponderosa pines, Arizona cypress, and Douglas-fir trees descended from the moist woodland species that populated this region during the last Ice Age. Occasional remains of the great Ice Age mammoths have also come from gravels in deeply eroded ravines. And you can see all around you the evidence of two latecomers who seem to have reached Big Bend around the same time: Man and the Desert. But here, deep in the river’s canyon, both are apparently as remote from you as your own daily world. [Illustration: University of Texas paleontologist Wann Langston excavates the sacrum of an extinct sauropod dinosaur. These bones were excavated near Tornillo Creek.] [Illustration: This huge cottonwood tree casts its shade along Terlingua Creek. Early settlers of Big Bend greenbelts used the cottonwood for roofbeams. Later, the trunks shored up mine shafts.] Along the Greenbelt and Among the Grasses