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.
Brachiosaurus
"Brachiosaurus" is known only from the Morrison Formation of western North America (following the reassignment of the African species). The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons, and flat floodplains. Several other sauropod genera were present in the Morrison Formation, with differing body proportions and feeding adaptations. Among these were "Apatosaurus", "Barosaurus", "Camarasaurus", "Diplodocus", "Haplocanthosaurus", and "Supersaurus". "Brachiosaurus" was one of the less abundant Morrison Formation sauropods. In a 2003 survey of over 200 fossil localities, John Foster reported 12 specimens of the genus, comparable to "Barosaurus" (13) and "Haplocanthosaurus" (12), but far fewer than "Apatosaurus" (112), "Camarasaurus" (179), and "Diplodocus" (98). "Brachiosaurus" fossils are found only in the lower-middle part of the expansive Morrison Formation (stratigraphic zones 2–4), dated to about 154–153 million years ago, unlike many other types of sauropod which have been found throughout the formation. If the large foot reported from Wyoming (the northernmost occurrence of a brachiosaurid in North America) did belong to "Brachiosaurus", the genus would have covered a wide range of latitudes. Brachiosaurids could process tough vegetation with their broad-crowned teeth, and might therefore have covered a wider range of vegetational zones than for example diplodocids. Camarasaurids, which were similar in tooth morphology to brachiosaurids, were also widespread and are known to have migrated seasonally, so this might have also been true for brachiosaurids.