About 150 million years ago during late Jurassic times, much of Montana lay underwater as part of an arm of an inland sea extending southward from the Arctic Ocean, while a semi-arid climate with distinct wet and dry seasons supported conifer trees, cycads, and ferns on land. This was the age of sauropods, gigantic herbivorous dinosaurs that browsed trees and ferns, and in Montana the most common was Diplodocus, one of the longest dinosaurs, with full-grown adults averaging 100 feet in length and long necks well adapted to feeding on the vegetation of the time. Diplodocus lived alongside Apatosaurus, Brachiosaurus, Camarasaurus, Stegasarus, and primitive birds and mammals. Groups of juvenile Diplodocus found in Montana became stuck in mud while crossing rivers, suggesting these giant dinosaurs were very social, similar to birds, the descendants of dinosaurs. Sauropods were preyed upon by Allosaurus, a large meat-eating dinosaur that may have stood at the top of the late Jurassic food chain; it was an active predator that pursued and killed its quarry, averaged 28 feet in length, could reach speeds of 19 to 34 mph when hunting, and had saw-like teeth often found with Diplodocus fossils. Allosaurus also has been found in groups, suggesting that it too was very social. Jurassic fossils in Montana occur in the Morrison Formation, sandstones and mudstones deposited about 155 to 150 million years ago, when this part of Montana was a vast coastal plain along the southern edge of the shallow inland sea, with river-laid sandstone layers alternating with mudstone lakes formed from river flood plains. Most exposures of the formation in Montana lie around the edges of mountain ranges, such as the Crazy Mountains to the northwest. The inland sea was populated by marine reptiles including Ichthyosaurus and long-necked plesiosaurs, and although some paleontologists speculate that Allosaurus hunted in packs, fossils also show breaks, punctures, and other lesions that may indicate attacks by other Allosaurus.