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.
Marshosaurus
The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such as "Camarasaurus"," Brachiosaurus", "Barosaurus", "Diplodocus", and "Apatosaurus". Dinosaurs that lived alongside "Marshosaurus" included the herbivorous ornithischians "Camptosaurus", "Dryosaurus", "Stegosaurus" and "Othnielosaurus". Predators in this paleoenvironment included the theropods "Saurophaganax", "Torvosaurus", "Ceratosaurus", "Stokesosaurus", "Ornitholestes" and "Allosaurus", which accounted for 70 to 75% of theropod specimens and was at the top trophic level of the Morrison food web. "Stegosaurus" is commonly found at the same sites as "Allosaurus", "Apatosaurus", "Camarasaurus", and "Diplodocus". Early mammals were present in this region, such as docodonts, multituberculates, symmetrodonts, and triconodonts. 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".