Dinosaurs first appeared about 210 million years ago and shared basic reptilian adaptations for life out of water, including dry, scaly, water-tight skin and eggs suited for development on land with membranes and shells to protect the embryo. The reptiles that gave rise to dinosaurs had two openings in their cheek bones, allowing jaw muscles to bulge outward when the jaw shut for better biting and chewing. Dinosaurs were distinguished especially by their hip bones and tails; the modified hip allowed many to become the first animals able to stand on two legs, using the tail as a counterbalance and moving the legs faster and more efficiently. With these modifications, dinosaurs increased in size, numbers, and diversity of forms and dominated the earth for 140 million years before disappearing 65 million years ago for reasons still uncertain. Their legacies are petrified bones, fossil footprints, and birds. Apatosaurus and Tyrannosaurus are among the most familiar dinosaurs, but they did not live at the same time: Apatosaurus died out over 40 million years before Tyrannosaurus appeared. Neither made the Dinosaur Valley footprints, which were instead made by Pleurocoelus, a smaller, later relative of Apatosaurus, and Acrocanthosaurus, a smaller, earlier relative of Tyrannosaurus. Acrocanthosaurus lived in the Early Cretaceous 140 to 100 million years ago in Oklahoma and Texas, reached 40 ft in length and 7 US tons in weight, and belonged to the spinosaurid family of carnosaurs. Pleurocoelus lived in the Early Cretaceous 140 to 100 million years ago from Maryland to Montana and Texas, reached 50 ft in length and 27 US tons in weight, and belonged to the brachiosaurid family of sauropods. Tyrannosaurus lived in the Late Cretaceous 100 to 65 million years ago from Montana to Texas, reached 50 ft in length and 8 US tons in weight, and belonged to the tyrannosaurid family of carnosaurs. Apatosaurus lived in the Late Jurassic 160 to 140 million years ago from Montana to Oklahoma to Baja California, reached 75 ft in length and 33 US tons in weight, and belonged to the diplodocid family of sauropods.