The vegetation of the Paleocene is of essentially the same kind as that of the latest Cretaceous, the difference between the two being principally a matter of species. Among the marine invertebrates, which are still very incompletely known, the most important feature to be noted is a negative one; namely, the almost universal and complete absence of the characteristic Mesozoic types, Ammonites, Belemnites, and the like. Of the land animals, the more or less aquatic reptiles allied to the Rhynchocephalia, the Choristodera, survived, but all the other Mesozoic orders of reptiles vanished in what appears to be a startlingly sudden way; no trace of the Dinosaurs, Pterosaurs, Ichthyosaurs, Plesiosaurs, Mosasaurs, etc., has been found in the Paleocene. On the contrary, the Mammals have greatly increased in numbers and importance, but are still extremely primitive and very like those of the late Cretaceous; with one, or perhaps two, exceptions, they all belong to extinct orders and are of small or moderate size, not a single large one having been found. The close relation with the Mesozoic is seen in the presence of several of the Multituberculata; the last and largest of the order, Polymastodon and Ptilodus, the latter a survival from the Laramie, are the common genera.

The primitive flesh-eaters, Creodonta, and primitive hoofed animals, Condylarthra and Amblypoda, are abundant, and the very curious extinct orders of the Tillodonta and Tceniodonta, and primaeval Lemuroids, and perhaps Insectivora, complete the list. The assemblage is an extremely archaic one and notable for the absence of the characteristically Tertiary groups; there are no Monkeys, Rodents, Bats, true Carnivores, Artiodactyls, Perissodactyls, or any of the other higher orders of mammals.