Defining Dinosaurs

Summary Discussion Questions Bibliography

Because this book is about dinosaurs, probably the most appropriate way to start is by defining them. This is not an easy task, even for

Definition of "Dinosaur"

dinosaur experts, so here is a preliminary attempt:

A dinosaur was a reptile- or bird-like animal with an upright posture that spent most

(perhaps all) of its life on land.

The term "reptile-like" is applied because dinosaurs evolved from reptilian ancestors, yet they were clearly different from present-day reptiles such as crocodiles, alligators, and lizards. Hence these modern animals are not "living dinosaurs," nor were their ancient counterparts. Therefore, anatomical distinctions and differing lineages separate modern reptiles and dinosaurs, even though both groups had common ancestors. However, dinosaurs had many features similar to those of modern reptiles, which warranted their original classification as such (Chapters 3 and 5). Yet some dinosaurs also had anatomical and attributed behavioral characteristics similar to modern birds (Chapter 15). So dinosaurs would appear as a diverse group of organisms that were transitional between certain ancestral reptiles and modern birds, although these relations will be expanded upon, clarified, and corrected later.

Upright posture, also known as an erect posture, is important when defining dinosaurs. "Upright" means that an animal stands and walks with its legs directly underneath its torso. This posture is distinguished from sprawling or semi-erect postures, where the legs project outside the plane of the torso. Sprawling postures are seen in most modern amphibians and reptiles (Fig. 1.1). With only a few exceptions, dinosaurs were among the first animals to be bipedal, or habitually walk on two legs. This is indicated by both the anatomy and tracks of early dinosaurs or dinosaur-like animals (Chapter 6). A bipedal stance that is not upright does not result in effective movement. Four-legged (quadrupedal) dinosaurs also had an upright posture, as can be seen from their anatomy and tracks (Chapters 5 and 14). In the nineteenth century, dinosaurs were interpreted as large lizards, so older illustrations depict sprawling, reptile-like stances (Chapter 3). Nowadays, modern museum mounts of dinosaurs and better-informed illustrators reconstruct nearly all dinosaurs with their legs underneath their torsos. Why dinosaurs developed an upright posture is not yet fully understood, but current evidence points toward the evolution of more efficient movement on land (Chapter 6).

The land-dwelling habit of dinosaurs is also important in their definition. Based on all information to date, dinosaurs that preceded the evolution of birds did not fly as part of their normal lifestyle, although some may have been gliders (Chapters 9 and 15). Likewise, no conclusive evidence indicates that dinosaurs swam, although a few of their tracks suggest swimming abilities (Chapter 14). Their remains in deposits from ancient aquatic environments suggest that they sometimes i

FIGURE 1.1 Differences in postures of a dinosaur and a large modern reptile. (A) Skeleton of the Late Cretaceous ornithopod Edmontosaurus annectus from Alberta, Canada. Posterior view of the rear limbs leaving a trackway, showing the typical dinosaurian trait of legs held underneath its body (erect posture). Specimen in the Royal Ontario Museum of Toronto, Ontario. (B) American crocodile, Crocodylus acutus, in Costa Rica, showing a sprawling posture and also leaving a trackway. This same typical reptilian posture can change to a semi-erect posture by the crocodile standing up or walking. Photo by Nada Pecnik, from Visuals Unlimited.

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