locally semiarid environment with distinct wet and dry seasons. The Morrison Basin, where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan; it was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins were carried by streams and rivers and deposited in swampy lowlands, lakes, river channels, and floodplains. This formation is similar in age to the Lourinhã Formation in Portugal and the Tendaguru Formation in Tanzania. Apatosaurus was the second most common sauropod in the Morrison Formation ecosystem, after Camarasaurus. Apatosaurus may have been more solitary than other Morrison Formation dinosaurs. Fossils of the genus have only been found in the upper levels of the formation. Those of Apatosaurus ajax are known exclusively from the upper Brushy Basin Member, about 152–151 mya. A.louisae fossils are rare, known only from one site in the upper Brushy Basin Member; they date to the late Kimmeridgian stage, about 151mya. Additional Apatosaurus remains are known from similarly aged or slightly younger rocks, but they have not been identified as any particular species, and thus may instead belong to Brontosaurus. The Morrison Formation records a time when the local environment was dominated by gigantic sauropod dinosaurs. Dinosaurs known from the Morrison Formation include the theropods Allosaurus, Ceratosaurus, Ornitholestes, Saurophaganax, and Torvosaurus; the sauropods Brontosaurus, Brachiosaurus, Camarasaurus, and Diplodocus; and the ornithischians Camptosaurus, Dryosaurus, and Stegosaurus. Apatosaurus is commonly found at the same sites as Allosaurus, Camarasaurus, Diplodocus, and Stegosaurus. Allosaurus accounted for 70–75% of theropod specimens and was at the top trophic level of the Morrison food web. Many of the dinosaurs of the Morrison Formation are of the same genera as those seen in Portuguese rocks of the Lourinhã Formationmainly Allosaurus, Ceratosaurus, and Torvosaurusor have a close counterpartBrachiosaurus and Lusotitan, Camptosaurus and Draconyx, and Apatosaurus and Dinheirosaurus. Other vertebrates that are known to have shared this paleo-environment include ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans, and several species of pterosaur. Shells of bivalves and aquatic snails are also common. The flora of the period has been evidenced in fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Vegetation varied from river-lining forests of tree ferns with fern understory (gallery forests), to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum. References External links Batuman, Elif.