Daspletosaurus - Paleoecology

Paleoecology

All known Daspletosaurus fossils have been found in formations dating to the middle to late Campanian stage of the Late Cretaceous Period, between 77 and 74 million years ago. Since the middle of the Cretaceous, North America had been divided in half by the Western Interior Seaway, with much of Montana and Alberta below the surface. However, the uplift of the Rocky Mountains in the Laramide Orogeny to the west, which began during the time of Daspletosaurus, forced the seaway to retreat eastwards and southwards. Rivers flowed down from the mountains and drained into the seaway, carrying sediment along with them that formed the Two Medicine Formation, the Judith River Group, and other sedimentary formations in the region. About 73 million years ago, the seaway began to advance westwards and northwards again, and the entire region was covered by the Bearpaw Sea, represented throughout the western United States and Canada by the massive Bearpaw Shale.

Daspletosaurus lived in a vast floodplain along the western shore of the interior seaway. Large rivers watered the land, occasionally flooding and blanketing the region with new sediment. When water was plentiful, the region could support a great deal of plant and animal life, but periodic droughts also struck the region, resulting in mass mortality as preserved in the many bonebed deposits found in Two Medicine and Judith River sediments, including the Daspletosaurus bonebed. Similar conditions exist today in East Africa. Volcanic eruptions from the west periodically blanketed the region with ash, also resulting in large-scale mortality, while simultaneously enriching the soil for future plant growth. It is these ash beds that allow precise radiometric dating as well. Fluctuating sea levels also resulted in a variety of other environments at different times and places within the Judith River Group, including offshore and nearshore marine habitats, coastal wetlands, deltas and lagoons, in addition to the inland floodplains. The Two Medicine Formation was deposited at higher elevations farther inland than the other two formations.

The excellent vertebrate fossil record of Two Medicine and Judith River rocks resulted from a combination of abundant animal life, periodic natural disasters, and the deposition of large amounts of sediment. Many types of freshwater and estuarine fish are represented, including sharks, rays, sturgeons, gars and others. The Judith River Group preserves the remains of many aquatic amphibians and reptiles, including frogs, salamanders, turtles, Champsosaurus and crocodilians. Terrestrial lizards, including whiptails, skinks, monitors and alligator lizards have also been discovered. Azhdarchid pterosaurs, and neornithean birds like Apatornis flew overhead, while the enantiornithiform bird Avisaurus and several varieties of multituberculate, marsupial and placental mammals scurried beneath the feet of Daspletosaurus and other dinosaurs.

In the Oldman Formation, Daspletosaurus torosus could have preyed upon hadrosaurs like Brachylophosaurus and Hypacrosaurus, small ornithopods like Orodromeus, ceratopsians like Centrosaurus, pachycephalosaurs, ornithomimids, therizinosaurs and possibly ankylosaurs. Other predators included troodonts, oviraptorosaurs, the dromaeosaur Saurornitholestes and possibly an albertosaurine tyrannosaur (genus currently unknown). The Dinosaur Park and Two Medicine Formations have faunas comparable to the Oldman, with the Dinosaur Park in particular preserving an unrivaled array of dinosaurs. The albertosaurine Gorgosaurus lived alongside species of Daspletosaurus in the Dinosaur Park and Upper Two Medicine environments. Young tyrannosaurs may have filled the niches in between adult tyrannosaurs and smaller theropods, which were separated by two orders of magnitude in mass. A Saurornitholestes dentary has been discovered in the Dinosaur Park Formation that bore tooth marks left by the bite of a young tyrannosaur, possibly Daspletosaurus.