What are dinosaurs? What features characterize them? Who were their closest relatives? What were the times like when the dinosaurs first evolved? How did their appearance change the balance of diversity in terrestrial ecosystems? During the past decade we have come to know much more about these topics than ever before. What follows is only a brief review. For more extensive information, readers are referred to the chapters in Padian (1986a), particularly the Introduction, on questions of the Late Triassic - Early Jurassic transition; and to Gauthier (1984, 1986) on the characteristics of dinosaurs and their relations. Because this is meant to be a general account, like the other papers in this book, it is impossible to cite every worker, discovery, or relevant paper in the field, and some personal judgments must be exercised.

The title of this chapter makes a promise that cannot be kept: that somehow there could be summarized in its few pages all the paleoenvironments of dinosaur-bearing strata through time. Dinosaurs were –as far as can be determined – ubiquitious in the terrestrial realm. It would be impossible to summarize the Recent terrestrial settings of birds (avian dinosaurs), let alone 135 million years of terrestrial settings. For this reason, this paper will be restricted to generalizations about ancient environments, highlighting particularly interesting or productive dinosaur-bearing localities in North America.

Why plants? – Why include plants in a course on dinosaurs? They don't move, lack large teeth and sharp claws, and are non-sentient. The average student (who can spell Deinonychus correctly) hasn't a clue to the genera of even the most common plants. However, without plants, dinosaurs (or any other heterotroph) would not exist. Plants are the basis for global life, trapping the sun's energy in a form useful to consumers. Thus, the photosynthetic efficiency, palatability, and digestibility of plants define the total energy in the system available for use by consumers – and therefore the total biomass of consumers. Additionally, the morphology of individual plants determines the “packages” of food available to herbivores, and in the aggregate, the three-dimensional environment in which the herbivores and their carnivores live.

By the end of the last century, the assortment of dinosaur skeletons that had already accumulated were classified into two groups of approximately equal size based on the divergent plan of their hip bones: Saurischia, the “lizard-hipped” dinosaurs, and Ornithischia, the “bird-hipped” dinosaurs (Seeley, 1888). Today we continue to follow this early dinosaur classification, albeit for somewhat different reasons, and consider that it captures a fundamental, ancient bifurcation in dinosaur evolutionary history.

This important group of ornithischian dinosaurs has a history that extends from the Early Jurassic until the close of the Cretaceous Period. The group as a whole can logically be subdivided into three quite distinct lineages: the heterodontosaurs, the hypsilophodonts, and the iguanodonts. I shall discuss the attributes of these groups separately, so that direct comparisons can be drawn by the reader of the way in which ornithopods as a whole evolved and diversified during the latter part of the Mesozoic. However, before going on to deal with these individual groups of ornithopods in a little more detail, I shall briefly outline their history and degree of relatedness.

Explanatory scenarios for the evolution of dinosaur anatomy are often couched in terms of competitive advantage, with those animals of superior design overtaking those that are slower and less efficient. Many, if not the majority, of the most striking structural modifications, however, are the product of competition and display between conspecifics during courtship. Among dinosaurs, ornithischians evolved an elaborate array of bony crests on the skull. These often constitute the most diagnostic portions of the entire skeleton. Traditionally these bony head accessories, typically sheathed and extended by horn, were viewed as the defensive weaponry of quiescent herbivores. More recently they have been reinterpreted on the basis of living analogs (such as horned ungulates) as secondary sexual characteristics employed primarily during intraspecific rivalry (see Norman's contribution to this volume).

Prosauropods were small to large (maximum lengths approximately 2.5 to 11 m), bipedal to quadupedal, plant-eating dinosaurs with small skulls and elongate necks (Figures 1A, C). They are basal members of the Sauropodomorpha, which include the giant sauropods of the Jurassic and Cretaceous. The Prosauropoda as a group is usually regarded as paraphyletic, because the general trends toward large size, quadrupedality, and herbivory seem to continue into the Sauropoda proper. However, a rigorous phylogenetic analysis of this question has not been made, and some evidence suggests that the prosauropods represent a radiation independent of the lineage leading to the more derived Sauropoda.

The sauropod dinosaurs, those long-necked, long-tailed giants, burst onto the world scene in the Lower Jurassic period and lived on until the end of the Age of Reptiles at the close of the Cretaceous. From the shapes of their teeth it is clear that they were herbivorous, and what an enormous amount of plant material they must have continuously consumed to fuel their gigantic bulk. Colbert (1962) estimated the weight of an adult Apatosaurus (Brontosaurus) at 34 metric tons, and that of Brachiosaurus at 78 metric tons. Dale Russell (see Russell, Beland and McIntosh, 1980) regards these estimates as too high by perhaps a factor of two, but in any case their masses were immense. The sauropods were quadrupedal, and their limbs were oriented in a columnar position as in large mammals, and unlike those of other dinosaurs. From this fact and a bone histology more nearly like that of mammals than typical reptiles, Bakker (1975) and de Ricqlès (1968, 1980) have independently suggested that the sauropods may have been warm-blooded (endothermic). Many dinosaur experts concede that this may have been true of some of the smaller, faster-moving carnivores, but the majority seem unconvinced of endothermy in the sauropods. Instead, it seems more appropriate to regard them as inertial homeotherms: that is, their large mass ensured a fairly constant body temperature, as in large mammals. However, as in mammals, the story is probably more complex than a single fact or factor can explain (Thomas and Olson, 1980; Weaver, 1983).

Theropods have traditionally been portrayed as extinct bipedal predators built along the lines of such celebrated terrors as Tyrannosaurus, Deinonychus, and Allosaurus. The earliest theropods indeed fit that image, and all of them are decidedly extinct. However, it has become increasingly apparent that living birds trace their genealogy to those extinct theropods (Ostrom, 1976; Gauthier, 1986; Gauthier and Padian, 1985 and this volume), and that any adequate consideration of the evolutionary history of Theropoda must assess the lineage as a whole, instead of arbitrarily focusing on the Mesozoic forms alone. When the approximately 8,600 living avian descendants of the ancestral theropod are taken into account, together with the various extinct Mesozoic and Cenozoic taxa, theropods display a far greater range of size, form, behavior, and diet than we ever pictured in our traditional image.

Tyrannosaurus rex is unquestionably the most famous dinosaur, probably because of its awesome size and appearance, and its apparent strength, agility and prowess. In spite of its fame, the anatomy and relationships of Tyrannosaurus and its kin remain poorly understood. By Late Cretaceous times (about 65 to 100 million years ago), carnivorous dinosaurs (theropods) had reached their peak of diversity in the Northern Hemisphere, and there were many specialized species of all sizes.

One of the most salient advances in vertebrate paleontology in recent decades has been the settling of the question of the origin of birds, a problem that has vexed evolutionary biologists since well before Darwin. To be sure, the consensus is not unanimous, and many details of this branch of the phylogenetic tree are yet to be worked out, but we now have a much clearer picture of this problem than we had a decade ago. Less settled, but equally stimulating, has been the controversy over the origin of flight in birds and other flying vertebrates. Was there a gliding stage? Did flight begin from the ground up or from the trees down? Were birds initially arboreal? What selective pressures drove the ancestors of birds to take advantage of the aerial opportunity?

Conventional paleontological wisdom holds that there are two major categories of fossil evidence: body fossils (skeletal remains), and trace fossils (including tracks and traces). Ichnology, the study of trace fossils, requires a parallel taxonomy of scientific names (parataxonomy or ichnotaxonomy), like the form taxa of fossil plant remains. This ichnotaxonomy describes a large variety of traces attributable to invertebrates (Hantzschel, 1975) and vertebrates (Haubold, 1984; Leonardi, 1984; Leonardi et al., 1986).

The giant dinosaurs, including the biggest reptiles that ever lived, have long overshadowed (figuratively and literally) the other vertebrates with which they shared the Mesozoic Era. Indeed, there is such an overall size disparity between dinosaurs and their contemporaries that it is no wonder that studies on the other vertebrates have lagged in comparison: if you want to find them, you have to look harder, and you have to look specifically for them.

The demise of the dinosaurs some 66.4 million years ago (MYA) has come to embody for scientist and layman alike the concept of extinction, especially that of mass extinction. So pervasive has this idea become that no book, whether specializing on dinosaurs or representing a general overview of biology or geology, concludes without dealing with dinosaur extinction. In the context of the Phanerozoic history of life, this preoccupation with dinosaur extinction is probably not warranted because it may tell us little about extinction or evolutionary processes in general.

K.S. Thomson, 1985: p. 73

R.T. Bakker, 1986: p. 462

It is a now oft-repeated statement that we are in the Second Golden Age of dinosaur studies. This may at first seem to be yet another overstatement by dinosaur fanatics; in fact, it is substantiated on a number of fronts. Research activity is certainly at an all-time high, with resident dinosaur researchers on every continent (except Antarctica) and dinosaurs known from every continent (including Antarctica). This activity has resulted in a spate of discoveries, including not only new genera and species, but entirely new types of dinosaurs, such as the segnosaurs. Well-known groups are producing surprises, such as armored sauropods and sauropods bearing tail clubs. Good specimens of previously named genera are revealing unsuspected structural features that almost defy explanation, as in the skull of Oviraptor. However, dinosaur studies extend far beyond the traditional emphasis on dinosaur morphology, and encompass paleobiogeography, paleoecology, taphonomy, physiology, tracks, eggs, histology, and extinction, among others. In some cases, several of these studies can be applied to a single taxon or locality to give us a fairly detailed understanding of the paleobiology of some species.