Mosasaur
Mosasaurs (from Latin "Mosa" meaning the 'Meuse river', and Greek σαύρος "sauros" meaning 'lizard') are an extinct group of large marine reptiles containing 38 genera in total. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from an extinct group of aquatic lizards known as aigialosaurs in the Early Cretaceous. During the last 20 million years of the Cretaceous period (Turonian-Maastrichtian ages), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.
Ceratosaurus
All North American "Ceratosaurus" finds come from the Morrison Formation, a sequence of shallow marine and alluvial sedimentary rocks in the western United States, and the most fertile source for dinosaur bones of the continent. According to radiometric dating, the age of the formation ranges between 156.3 million years old (Mya) at its base, and 146.8 million years old at the top, which places it in the late Oxfordian, Kimmeridgian, and early Tithonian stages of the Late Jurassic period. "Ceratosaurus" is known from Kimmeridgian and Tithonian strata of the formation. The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons. The Morrison Basin stretched from New Mexico to Alberta and Saskatchewan, and was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins were carried by streams and rivers and deposited in swampy lowlands, lakes, river channels, and floodplains. This formation is similar in age to the Lourinhã Formation in Portugal and the Tendaguru Formation in Tanzania.
The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs. Other dinosaurs known from the Morrison include the theropods "Koparion", "Stokesosaurus", "Ornitholestes", "Allosaurus", and "Torvosaurus"; the sauropods "Apatosaurus", "Brachiosaurus", "Camarasaurus", and "Diplodocus"; and the ornithischians "Camptosaurus", "Dryosaurus", "Othnielia", "Gargoyleosaurus", and "Stegosaurus". "Allosaurus", which accounted for 70 to 75% of all theropod specimens, was at the top trophic level of the Morrison food web. Other vertebrates that shared this paleoenvironment included ray-finned fishes, frogs, salamanders, turtles like "Dorsetochelys", sphenodonts, lizards, terrestrial and aquatic crocodylomorphans such as "Hoplosuchus", and several species of pterosaurs such as "Harpactognathus" and "Mesadactylus". Shells of bivalves and aquatic snails are also common. The flora of the period has been revealed by fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Vegetation varied from river-lining forests of tree ferns and ferns (gallery forests) to fern savannas with occasional trees such as the "Araucaria"-like conifer "Brachyphyllum".