This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
The Dinosaurs continue in even greater profusion than in the Jurassic; they are, of course, much commoner and better preserved in continental deposits than in marine, and hence are best known from the base and the summit of the system. Many of the genera were the largest land animals that ever lived, and the size of the bones is astonishing. The Wealden of Europe has yielded some magnificent Dinosaurs; especially the genus I guano-don, of which many complete skeletons have been found in Belgium. Dinosaurs are much less common in the marine Upper Cretaceous, but the green sands of New Jersey have yielded Hadrosaurus, a herbivorous Dinosaur much like Igua-nodon, and some carnivorous types also. The Laramie and Denver beds have preserved many fine specimens, which show that the Dinosaurs flourished in almost undiminished variety till the end of the Cretaceous. The erect, herbivorous type is represented in these beds by Monoclonius and Diclonius (Fig. 302), which are nearly related to Hadrosaurus. Triceratops(Fig. 301) and Torosaurus are huge, quadrupedal reptiles, with three large horns on the head and an extraordinary frill-like extension of the skull over the neck.
Carnivorous Dinosaurs likewise continued, such as Lcelaps, Tyrannosaurus and Ornithomi-mus, the latter with hind limbs which are especially birdlike in structure. The Birds of the Cretaceous are much more abundant and advanced than the known Jurassic birds. In the Upper Cretaceous of Kansas, and probably of England also, are found two remarkable birds, Hesperornis and Ichthyornis. In- the former, which was nearly 6 feet high, the wings were rudimentary, while Ichthyornis, a much smaller bird, had powerful wings. Both of these genera possessed teeth, like Archceopteryx, but except in that feature and in certain minor details of structure, they are entirely like modern birds. Bird bones like the corresponding parts of the Cormorants and Waders have been found in the green sands of New Jersey, but it is not known whether they had teeth.
Fig. 301. - Skull of Triceratops fiabellatus Marsh, from the side, 1/30. (Marsh).
Fig. 302. - Skull of Diclonius mirabihs Cope, from above, 1/19. (Cope).
Cretaceous Mammals are more numerous and varied than those of the Jurassic, but they continue to play a very modest role, and are nearly all of minute size. In America they have been found only in the uppermost Cretaceous, and in Europe they are not known as yet, though doubtless they existed in that part of the world. The mammals of the Laramie already begin to show affinities with the forms which are to succeed them in the Tertiary. The Multituberculata are represented by two genera, Meniscoessus and Ptilodus, while other mammals of doubtful affinities are Didelphops, Pediomys, and Cimolestes. Many others are known, but they are too imperfect for reference. With one exception, Thlceodon, which is of moderate size, all these mammals are exceedingly small.
In brief, Cretaceous life is still typically Mesozoic, but a change toward Cenozoic conditions is already manifest, especially in the Plants, the Gastropods, the Teleostean Fishes, and the Birds. There is still a gap between the life systems of the two eras, but it is not so wide as it was once believed to be, and it may be hoped that future discoveries will bridge it entirely.