Clades and Species of Theropoda Paleobiogeography and Evolutionary History of Theropoda Theropods as Living Animals Summary Discussion Questions Bibliography

Of all dinosaur clades, Theropoda is the most intensively studied and discussed by paleontologists. Based on their frequent appearances on

Why Study Theropods?

the covers of prestigious science journals and mentions in news reports of the past 20 years or so, theropods remain the most famous of dinosaurs. This fame is at least partially related to an enduring, perhaps vicarious fascination with large predatory animals in general, such as sharks, crocodiles, Komodo dragons, large cats, and grizzly bears. Theropods fit well in this group of meat-eaters but with one exception - some of their members were the largest carnivores that ever lived in terrestrial environments; a few species may have approached 8 metric tons in weight and 15 meters in length. Consequently, an informal competition for discovery of the largest land-dwelling carnivore focuses exclusively on members of the Theropoda, a sideline issue for paleontologists for more than 100 years. This contest began with the announcement by Barnum Brown in 1902 of the first specimen of Tyrannosaurus rex (Chapter 3) and continued more recently with Carcharodontosaurus of Morocco and Giganotosaurus of Argentina.

Of course, not all theropods were large predators. In fact, they were extremely diverse in size, form, and function during their 165-million-year history. For example, the majority of theropods, unlike most dinosaurs, were bipedal. This ple-siomorphic trait of dinosaurs was probably related to natural selection for longer hind limbs, shorter fore limbs, modification of the pes, and lightening of the skeletal structure. Increased mobility and speed were subsequent benefits of this evolution (Chapter 6). Indeed, some theropods have skeletal features that reflect most of the adaptive characteristics seen in modern mammals and large flightless birds capable of rapid movement. The most dynamic records of theropod bipedalism and other forms of movement are their tracks, and interpretations made from such features corroborate their activities. Theropod tracks, first described by Edward Hitchcock in the mid-nineteenth century (Chapter 3), are by far the most abundant of dinosaur tracks and constitute a considerably greater fossil record than theropod skeletal remains (Chapter 14).

The past 20 years of paleontological research have also witnessed better documentation of other theropod trace fossils. Examples include nests, toothmarks, coprolites, and rare examples of gastroliths (Chapter 14), which have been accompanied by body fossils such as juvenile remains, eggs, and embryos (Chapter 8). Examination of all these data has led to a more complete picture of theropod lifestyles than can be interpreted on the basis of sometimes fragmentary skeletal data alone. Through such integrated analyses, theropods have turned out to be more complex animals than originally thought, far beyond the "eat-and-run" killing machines so entrenched as fictional stereotypes.

Resurgent interest in theropods followed spectacular finds of an Early Cretaceous theropod (Scipionyx) in Italy (with partial preservation of its internal organs), as well as beautifully preserved feathered theropods from Cretaceous deposits in China (including one with feathers on all four limbs). Feathered theropods, in particular, provide even more evidence to support their evolutionary connection to birds (Chapter 15). Interpretations regarding the physiological implications of feathers or feather-like structures in theropods have also prompted much scientific discussion, especially relating to thermoregulation (Chapter 8). Moreover, any conventional dividing line between theropods and birds is obscure. For example, both a dinosaur capable of flight (Microraptor) and a bird not quite able to fly (Archaeopteryx) have been hypothesized. These hypotheses sometimes create semantic difficulties whenever paleontologists try to explain differences between descendants and ancestors in theropod lineages. Indeed, by cladistic reasoning, theropods have the longest history of all dinosaur clades, beginning with the first dinosaurs about 230 million years ago through to the birds of today. As a result, this long history justifies the inordinate amount of attention theropods receive.

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