Sinosauropteryx
Three specimens have been assigned to "Sinosauropteryx prima": the holotype GMV 2123 (and its counter slab [opposite face], NIGP 127586), NIGP 127587, and D 2141. Another specimen, IVPP V14202, was assigned to the genus but not to the only species by Zhang and colleagues. The assignment of an additional larger specimen to "S. prima", GMV 2124, was later found to be in error. All of the fossils were found in the Jianshangou or Dawangzhangzi Beds of the Yixian Formation in the Beipiao and Lingyuan regions of Liaoning, China. These fossil beds have been dated to 124.6–122 million years ago, during the early Aptian stage of the Early Cretaceous.
Gorgosaurus
Most specimens of "Gorgosaurus libratus" have been recovered from the Dinosaur Park Formation in Alberta. This formation dates to the middle of the Campanian, between 76.5 and 74.8 million years ago, and "Gorgosaurus libratus" fossils are known specifically from the lower to middle section of the formation, between 76.6 and 75.1 million years ago. The Two Medicine Formation and Judith River Formation of Montana have also yielded possible "Gorgosaurus" remains. At this time, the area was a coastal plain along the western edge of the Western Interior Seaway, which divided North America in half. The Laramide Orogeny had begun uplifting the Rocky Mountains to the west, from which flowed great rivers that deposited eroded sediment in vast floodplains along the coast. The climate was subtropical with marked seasonality, and periodic droughts sometimes resulted in massive mortality among the great herds of dinosaurs, as represented in the numerous bonebed deposits preserved in the Dinosaur Park Formation. Conifers formed the forest canopy, while the understory plants consisted of ferns, tree ferns and angiosperms. Around 73 million years ago, the seaway began to expand, transgressing into areas formerly above sea level and drowning the Dinosaur Park ecosystem. This transgression, called the Bearpaw Sea, is recorded by the marine sediments of the massive Bearpaw Shale.
The Dinosaur Park Formation preserves a great wealth of vertebrate fossils. A wide variety of fish swam the rivers and estuaries, including gars, sturgeons, sharks and rays, among others. Frogs, salamanders, turtles, crocodilians and champsosaurs also dwelled in the aquatic habitats. Azhdarchid pterosaurs and ornithuran birds like "Apatornis" flew overhead, while the enantiornithine bird "Avisaurus" lived on the ground alongside multituberculate, metatherian and eutherian mammals. A number of species of terrestrial lizards were also present, including whiptails, skinks, monitors and alligator lizards. Dinosaur fossils in particular are found with unrivaled abundance and diversity. Huge herds of ceratopsids roamed the floodplains alongside equally large groups of saurolophine and lambeosaurine hadrosaurs. Other herbivorous groups like ornithomimids, therizinosaurs, pachycephalosaurs, small ornithopods, nodosaurids and ankylosaurids were also represented. Small predatory dinosaurs like oviraptorosaurs, troodonts and dromaeosaurs hunted smaller prey than the huge tyrannosaurids; "Daspletosaurus" and "Gorgosaurus", which were two orders of magnitude larger in mass. Intervening predatory niches may have been filled by young tyrannosaurids. A "Saurornitholestes" dentary has been discovered in the Dinosaur Park Formation that bore tooth marks left by the bite of a young tyrannosaur, possibly "Gorgosaurus".