Excerpt from the Seminars on Science course:
The Link Between Dinosaurs and Birds: Evolution and Classification

Theropods Compared
When scientists encounter a new specimen, part of their research is to observe, describe, and identify it. In part, the identification process happens by following an extremely detailed checklist marking the presence or absence of particular characters. Examining the patterns of the appearance and disappearance of characters is the key to figuring out the relatedness of organisms.

Birds are living dinosaurs.
Research over the last couple of decades has shown that birds are indeed a kind of theropod dinosaur. In fact, theropod dinosaurs, like Velociraptor, are more closely related to modern birds than they are to almost any other kind of dinosaur you are likely to have heard of (including Tyrannosaurus, Triceratops, Diplodocus, and Stegosaurus that is, they share a more recent common ancestor). Just as we, as humans, are primates and mammals, chickens, as birds, are theropods, dinosaurs, and reptiles.

What is a theropod?
Theropod dinosaurs are a diverse group that first appeared over 230 million years ago and have lived on every continent. The early theropods look fairly similar to later ones—they were both bipedal and usually carnivorous. But other than being fearsome creatures, why are theropod dinosaurs so interesting? This dinosaur group includes not only spectacular animals like the giant predators Giganotosaurus, Tyrannosaurus , and Carcharodontosaurus, but also some of the largest-brained, or smartest, dinosaurs. Furthermore, the study of theropods has real relevance for understanding animals that are alive today. We can better study the evolutionary history of living birds now that we know they descended from theropods, and ultimately our definitions of both birds and dinosaurs will change.

Avian and Non-Avian Theropods Compared
Use this illustration to examine several skeletal features and make observations about the characteristics present in a typical advanced non-avian theropod (Deinonychus) and a typical modern bird (Gallus). Which characteristics are the same? Which characteristics are different?

Continue to the Illustration...

©2003 American Museum of Natural History