 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 . 
