About 150 million years ago during the late Jurassic, 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 that time. Diplodocus lived alongside Apatosaurus, Brachiosaurus, Camarasaurus, Stegasarus, and primitive birds and mammals. Groups of juvenile Diplodocus found in Montana appear to have become stuck in mud while crossing rivers, suggesting these giant dinosaurs were very social, similar to birds, the descendants of dinosaurs. Sauropods were preyed on 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, and could reach speeds of 19 to 34 mph when hunting. Allosaurus teeth with saw-like edges are often found with Diplodocus fossils, and Allosaurus has also been found in groups, suggesting they too were 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 bordering the southern edge of the shallow inland sea, with sandstone layers laid down by rivers alternating with mudstone lakes that began as river flood plains. Most exposures of the formation in Montana are around the edges of mountain ranges such as the Crazy Mountains to the northwest. The inland sea was populated by marine reptiles like Ichthyosaurus and long-necked plesiosaurs. Diplodocus was not only the largest known dinosaur in North America during Jurassic times but also one of the most common fossils found in the upper Morrison Formation. Although some paleontologists speculate that Allosaurus hunted in packs, fossils show breaks, punctures, and other lesions that may have occurred when the animals were attacked by other Allosaurus.