83 and 74 million years ago. Achelousaurus specimens are found in the highest levels of the formation, probably closer to the end of that timeframe, 74 million years ago. The Two Medicine Formation is typified by a warm semiarid climate. Its layers were deposed on the east coast of the Laramidia island continent (which consisted of western North America). The high cordillera in the west, combined with predominantly western winds, would have caused a rain shadow, limiting annual rainfall. Rain would mainly have fallen during the summer, when convection storms flooded the landscape. The climate would thus also have been very seasonal, with a long dry season and a short wet season. Vegetation would have been sparse and a little varied. In such conditions, horned dinosaurs would have been dependent on oxbow lakes for a continuous supply of water and food – the main river channels tending to run dry earlier – and perished in them during severe droughts when the animals concentrated around the last watering holes, causing bone beds to form. The brown paleosol in which the horned dinosaurs were found – a mixture of clay and coalified wood fragments – resembles that of modern seasonally dry swamps. The surrounding vegetation might have consisted of about high conifer trees. Achelousaurus ate much smaller plants, though: a 2013 study determined that ceratopsid herbivores on Laramidia were restricted to feeding on vegetation with a height of or lower. More or less contemporary dinosaur genera of the area included Prosaurolophus, Scolosaurus, Hypacrosaurus, Einiosaurus and tyrannosaurids of uncertain classification. As proven by tooth marks, horned dinosaur fossils in the Landslide Butte Field Area had been scavenged by a large theropod predator, which Rogers suggested were Albertosaurus. The exact composition of the fauna Achelousaurus was part of is uncertain, as its fossils have not been discovered in direct association with other taxa. Its intermediate anagenetic position suggests that Achelousaurus shared its habitat with forms roughly found in the middle or at the end of the time range of its formation. As with horned dinosaurs, Horner assumed he had found transitional taxa in other dinosaur groups of the Two Medicine Formation. One of these was a form in between Lambeosaurus and Hypacrosaurus; in 1994 he would name it Hypacrosaurus stebingeri. Today, Hypacrosaurus stebingeri is no longer seen as having evolved through anagenesis because autapomorphies of the species have been identified. Horner saw some pachycephalosaur