_Uintatherium_ 6-horned, saber-toothed plant eater _Stylinodon_ gnawing-toothed mammal _Palaeosyops_ early titanothere _Helaletes_ primitive tapir _Sciuravus_ squirrel-like rodent _Smilodectes_ lemurlike monkey _Trogosus_ gnawing-toothed mammal _Hyrachyus_ fleet-footed rhinoceros _Ischyrotomus_ marmotlike rodent _Homacodon_ even-toed hoofed animal _Orohippus_ ancestral horse _Patriofelis_ large flesh eater _Mesonyx_ hyenalike mammal _Helohyus_ even-toed hoofed mammal _Metacheiromys_ armadillolike edentate _Machaeroides_ saber-toothed mammal _Hyopsodus_ clawed, plant-eating mammal _Saniwa_ monitorlike lizard _Crocodilus_ crocodile _Echmatemys_ turtle Other lakes formed in response to similar crustal movements in nearby places. One such lake, _Grand Valley Lake_ (fig. 49), formed about 25 miles southwest of Teewinot Lake; both contained sediments with nearly the same thickness, composition, appearance, age, and fossils. Although these two lakes are on opposite sides of the Snake River Range, the ancestral Snake River apparently flowed through a canyon previously cut across the range and provided a direct connection between them. Development of mammals The Cenozoic Era is known as the “Age of Mammals.” Small mammals had already existed, though quite inconspicuously, in Wyoming for about 90 million years before Paleocene time. Then about 65 million years ago their proliferation began as a result of the extinction of dinosaurs, obliteration of seaways that were barriers to distribution, and the development of new and varied types of environment. These new environments included savannah plains, low hills and high mountains, freshwater lakes and swamps, and extensive river systems. The mammals increased in size and, for the first time, became abundant in numbers of both species and individuals. The development and widespread distribution of grasses and other forage on which many of the animals depended were highly significant. Successful adaptation of _herbivores_ (vegetation-eating animals) led, in turn, to increased varieties and numbers of predatory _carnivores_ (meat-eating animals).