Summary On Prehsitoric Life

Here we look at the overall sweep of non-avian dinosaur evolution. Factoring in time intervals of a poor geological record, in which preservation is artifically low, dinosaurs as a group increased markedly in number and diversity, particularly during the Late Jurassic-through-latest Cretaceous time interval. This increase is attributable to ceratopsian and ornithopod herbivores, and theropods.

The global pattern of dinosaur evolution from the Late Triassic to the Late Cretaceous, is one of generally increasing endemism, likely attributable to the increasing separation of continental masses. Late Triassic and Early Jurassic dinosaur faunas shared their terrestrial world with a variety of other vertebrates; and global vertebrate faunas were relatively homogeneous. Distinct among all the Late Triassic and Early Jurassic vertebrates, however, dinosaurian herbivores were the first to be able to reach, and thus add to their diets, tall foliage.

By Middle Jurassic time, dinosaurs likely consolidated their dominance in the terrestrial realm, even though terrestrial deposits from this time interval are comparatively rare. This fact is especially unfortunate for understanding the details of dinosaur evolution, since many of the groups that became so abundant and diverse in the Cretaceous had their roots in the Middle Jurassic. The Late Jurassic has been called the "Golden Age of

Dinosaurs," with the abundance of many familiar forms including very large theropods and sauropods.

The Cretaceous was a truly astounding time in dinosaur evolution. Aside from the wholesale dominance of new forms (in particular, ornithopods and ceratopsians, as well as a wide range of theropods), many of the spectacular adaptations that we've seen, such as advanced chewing, evolved in the Cretaceous. A driving force in all this evolutionary ferment may have been the rise of flowering plants; yet what we know of dinosaur diets suggests that the fibrous gymnosperms constituted the bulk of the nutrition. Processing such plants may have been the driving force in the development of sophisticated modes of chewing. It can certainly be said that, as plants evolved effective methods for dispersal and colonization, dinosaurs apparently hitched a ride, increasing markedly in number and diversity as they took advantage of the radiation of vascular plants.

The end of the Cretaceous (explored in greater detail in Chapter 15) was of course the end of non-avian dinosaurs. Overall diversity trends show no gradual decrease from some previous high point; rather, non-avian dinosaurs increase in diversity throughout the Mesozoic and then abruptly, at 65.5 Ma, disappear from the fossil record.

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