Archaeopteryx is the earliest and most primitive bird known. Archaeopteryx lived in the late Jurassic Period around 150–145 million years ago, in what is now southern Germany during a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now. Similar in size and shape to a European Magpie, Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with small theropod dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest homeothermy), and various skeletal features. These features make Archaeopteryx the first clear candidate for a transitional fossil between dinosaurs and birds. Thus, Archaeopteryx plays an important role not only in the study of the origin of birds but in the study of dinosaurs.

Dakosaurus is an extinct genus within the family Metriorhynchidae that lived during the Late Jurassic and Early Cretaceous. It was large, with teeth that were serrated and compressed lateromedially (flattened from side to side). The genus was established by Friedrich August von Quenstedt in 1856 for an isolated tooth named Geosaurus maximus by Plieninger. Dakosaurus was a carnivore that spent much, if not all, its life out at sea. The extent of its adaptation to a marine lifestyle means that it is most likely that it mated at sea, but since no eggs or nests have been discovered that have been referred to Dakosaurus, whether it gave birth to live young at sea like dolphins and ichthyosaurs or came ashore like turtles is not known. The name Dakosaurus means "tearing lizard", and is derived from the Greek Dakos- ("to tear") and -sauros ("lizard").

Hesperornis is an extinct genus of flightless aquatic birds that lived during the Santonian to Campanian sub-epochs of the Late Cretaceous (89-65 mya). Hesperornis was a large bird, reaching up to 2 meters (6.5 feet) in length. It had virtually no wings, and swam with its powerful hind legs. The toes were probably lobed rather than being webbed, as in today's grebes; like in these, the toes could rotate well, which is necessary to decrease drag in lobed feet but not in webbed ones such as in loons, where the toes are simply folded together. Like many other Mesozoic birds such as Ichthyornis, Hesperornis had teeth in its beak which were used to hold prey (most likely fish). In the hesperornithiform lineage they were of a different arrangement than in any other known bird (or in non-avian theropod dinosaurs), with the teeth sitting in a longitudinal groove rather than in individual sockets, in a notable case of convergent evolution with mosasaurs.

Leedsichthys problematicus was a giant pachycormid (an extinct group of Mesozoic bony fish) that lived in the oceans of the Middle Jurassic period. The closest living relative of the pachycormids is the bowfin, Amia calva, but this is only very distantly related. Like the largest fish today, the whale sharks and basking sharks, Leedsichthys problematicus derived its nutrition using an array of specialised gill rakers lining its gill basket to extract zooplankton from the water passing through its mouth and across its gills. The name Leedsichthys means "Leeds' fish", after the fossil collector Alfred Nicholson Leeds, who discovered it before 1886 near Peterborough, England. The fossils found by Leeds gave the fish the species epithet problematicus, because the remains were so fragmented that they were extremely hard to recognize and interpret.

Repenomamus is the largest mammal known from the Cretaceous period of China, and it is the mammal for which there is the best evidence that it fed on dinosaurs. It is not possible to determine if Repenomamus actively hunted live dinosaurs or scavenged dead dinosaurs. Repenomamus was probably not a fast runner. The humerus and femur left their joints at a somewhat splayed angle, and the legs were relatively short compared to the body. The feet were plantigrade. Repenomamus' behavior and overall body shape may have resembled those of modern day Tasmanian devils. Repenomamus was carnivorous. A specimen of R. robustus has been discovered with the fragmentary skeleton of a juvenile Psittacosaurus preserved in its stomach. This the strongest evidence that Mesozoic mammals fed on dinosaurs, and this created interest in the popular press. There were, however, earlier indications that Mesozoic mammals fed on dinosaurs.

Xianglong zhaoi, was found at the Zhuanchengzi locality near Yizhou in the Liaoning Province of China. From the Late Triassic the fossil is 15.5cm long and shows the whole skeleton as well as imprints of the animal’s skin. The most striking feature are eight elongated ribs, that helped to spread the wing-like patagium to the left and right of the animal’s body. Based on the shape and size of the patagium the authors conclude that Xianglong possessed great maneuverability. The feet and claws of Xianglong show adaptations for climbing indicating that the lizard lived in trees. The authors suggest that the rib-supported patagium of different lizard species arose by convergent evolution.

Xiphactinus ("Sword-ray") was a large, 4.5 to 5 m (15 to 20 feet) long predatory bony fish that lived in the Western Interior Sea, over what is now the middle of North America, during the Late Cretaceous. When alive, the fish would have resembled a gargantuan, fanged tarpon (to which it was, however, not related). Skeletal remains of Xiphactinus have come from Kansas, Alabama, and Georgia; Europe and Australia.