The Two Medicine Formation is famous for the “good mother lizard,” Maiasaura peeblesorum, but records a much more extensive fauna in sediments deposited for 14 million years, from about 84 million years ago to 70 million years ago. This coincides roughly with the entire span of the Campanian Age of the Late Cretaceous. The Two Medicine Formation is exposed in the U.S. state of Montana and Canadian province of Alberta and is roughly coeval with the Judith River Formation, deposited to the east, and lies below the Hell Creek Formation, which records the last few million years of the Mesozoic in the same area.
The Two Medicine Formation is largely made up of sandstones and mudstones deposited by lakes and rivers flowing from west to east from the proto-Rocky Mountains to the Western Interior Cretaceous Seaway, a shallow sea that split North America in two and joined the Gulf of Mexico with the Arctic Ocean and was present for much of the Mesozoic. The Two Medicine Formation and Judith River Formation to the east represent a wedge of terrestrial deposits and are separated by the Sweetgrass Arch (an erosional feature).
The Two Medicine Formation lies above the marine Virgelle Sandstone and lies below the marine Bearpaw Shale. Both the Virgelle Sandstone and Bearpaw Shale were deposited at times of high sea-level and the Two Medicine-Judith River wedge and later Hell Creek Formation were deposited at times of low sea level. When seas were relatively high during the Early and Late Campanian the Two Medicine Formation represented a forested near-shore plain, but during the middle portion of the Campanian at the time of lowest sea-levels, the eastern Judith River Formation was a near-shore plain and the Two Medicine Formation represented more of an upland environment. Overall, the combined Two Medicine Formation and Judith River Formation would be quite similar to the modern coastal plains of U.S. states of Georgia and the Carolinas.
A single genus of pterosaur is known from the Two Medicine Formation, the azhdarchoid Montanazhdarcho minor. This 2.5-meter wingspan pterosaur was originally considered to be an azhdarchid, but more recent research suggests it could be a member of a different azhdarchoid family, possibly a tapejarid or thalassodromid.
The bipedal carnivorous dinosaurs were quite common in the Two Medicine, with smaller species being particularly abundant. All known Two Medicine Formation theropods were coelurosaurs, those theropods closer to birds than Allosaurus. The only large theropods were the tyrannosaurids Daspletosaurus torosus and Gorgosaurus libratus. The deinonychosaurs are represented by the dromaeosaurids Bambiraptor feinbergorum, Dromaeosaurus albertensis, and Saurornitholestes langstoni, and the troodontid Troodon formosus. The oviraptorosaur Caenagnathus sternbergi and the enantiornithine bird Avisaurus gloriae were also present.
Two species of armored ankylosaurs were present in the Two Medicine Formation. The include the nodosaurid Edmontonia rugosidens which bore large shoulder spikes, and the ankylosaurid Scolosaurus cutleri which had a club at the end of its tail.
The Two Medicine Formation has several known species of ceratopsian dinosaurs. Two species of leptoceratopsid are present, Cerasinops hodgkissi and Prenoceratops pieganensis. Four species of centrosaurine ceratopsid are also present, Brachyceratops monanensis, Einiosaurus procurvicornus, Rubeosaurus ovatus, and Achelousaurus horneri. Einiosaurus, Rubeosaurus, and Achelousaurus are found in progressively higher locations within the formation and have been proposed to form a single lineage ancestral to the stratigraphically younger genus Pachyrhinosaurus.
Ornithopod dinosaurs were among the most abundant and common dinosaurs found in the Two Medicine Formation. The small, possibly burrowing thescelosaur Orodromeus makelai is known from several skeletons from a variety of different ages. The large-bodied duck-billed hadrosaurids were abundant and include the brachylophosaurin saurolophines Acristavus gagslarsoni and the famous Maiasaura peeblesorum known from eggs, nests, babies, and individuals from several different age groups. Other saurolophines include the kritosaurin Gryposaurus latidens and the saurolophin Prosaurolophus maximus. The hollow-crested lambeosaurine Hypacrosaurus stebingeri is also known from eggs, nests, babies, and individuals from a several different age groups.