FIGURE 1.2 Geologic time scale used as a standard by geologists and paleontologists worldwide. Largest units of geologic time are eons, followed (in order of most inclusive to least inclusive) by subdivisions eras, periods, and epochs. Figure is not scaled according to amounts of time.

drowned while attempting to swim across lakes or streams (Chapter 7). However, dinosaurs did have some reptilian contemporaries, pterosaurs and plesiosaurs, which flew and swam, respectively (Chapter 6). These were not dinosaurs, although all three groups had common ancestors. Furthermore, no convincing evidence has revealed that dinosaurs lived underground because no dinosaur has ever been found in a burrow, nor have any burrows been attributed to them. Some anatomical evidence indicates that a few small dinosaurs were capable of climbing trees (Chapter 9), but no skeletal remains have been found in direct association with a fossil tree. Dinosaurs appear to have been well adapted to living in the many environments associated with land surfaces, which obviously worked very well for them during their 165-million-year existence.

Dinosaurs are well known for the enormous size of some individual species, in comparison to modern land-dwelling animals. Indeed, some dinosaurs were the largest land animals that ever left footprints on the face of the Earth (Chapter 10).

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