Birds are dinosaurs just as snakes are reptiles and horses are mammals. The most prominent feature that distinguishes birds from other dinosaurs is their ability to fly. Last of the Dinosaurs uses the term non-avian dinosaur to refer to those dinosaurs that did not fly and are thus not considered true birds. The basal Avialae, or primitive birds, described in this chapter are defined as maniraptorans closer to birds than to Deinonychus, a dromaeosaurid. Some of these creatures flew and some did not, but all evolved from flying ancestors. All were part of an evolutionary trajectory leading to the anatomical and physiological traits now seen in birds.
Archaeopteryx is considered to be the first true bird, yet it retained many anatomical features that were not seen in later birds, such as teeth, a long and bony tail, and underdeveloped wings. Archaeopteryx also dates from the Late Jurassic, about 150 million years ago—about 25 million years before other known fossil specimens of early birds. That considerable gap in the fossil record has only recently begun to be filled with evidence of other stages in the evolution of small coelurosaurs into birds. Paleontologist Kevin Padian, one of the leading researchers on bird origins, explains that looking for fossil evidence of the earliest birds can best be done by looking for traits found in Archaeopteryx that are shared with later birds but which are not known in other vertebrates, and searching for such traits in non-avian theropods. If one were to seek evidence of other stages in the evolution of small coelurosaurs into birds, then one should look in Early and Middle Jurassic sediments, since the first known bird (Archaeopteryx) is Late Jurassic in age.
There is more than a superficial resemblance between dinosaurs and birds. Fossil evidence strongly supports an evolutionary transition in which birds were descended from dinosaurs. The discoveries described in these pages constitute a few of more than 120 anatomical features that were originally found in small meat-eating dinosaurs and inherited by birds, including:
• Lightweight skeletons with hollow bones.
• A furcula, or wishbone, found in many theropod groups, including the oviraptorosaurs Ingenia ("for Ingeni-Khobur"), Oviraptor ("egg thief"), and the dromaeosaur Bambiraptor ("Bambi thief").
• Teeth. Many early birds, including Archaeopteryx, had teeth like their dinosaur ancestors.
• Pygostyle-like structures at the tail tips of some birdlike dinosaurs, including oviraptorosaurs and therizinosaurs.
• Feathers or feather-like filaments ("protofeathers") in most coelurosaur groups, including compsognathids (Sinosauropteryx), tyrannosauroids (Dilong), therizinosaurs
(Beipiaosaurus), oviraptorosaurs (Caudipteryx, Protarchaeopteryx), troodontids (Jinfengopteryx), and dromaeosaurids (Microraptor, Sinornithosaurus, and probably Velociraptor).
• The ability to walk on two legs, supported by three weight-bearing toes.
• Dinosaurlike claws on birds such as Archaeopteryx, which had a dromaeosaurlike sickle-claw on the second toe of each foot.
• Birdlike shoulder and breast bones in some dinosaurs that allowed the arms to fold and move in the same fashion that primitive birds moved their wings. This trait was most pronounced in dromaeosaurs such as Unenlagia ("half-bird") from South America and Bambiraptor from Montana.
• Long arms in small theropods, comparable in length to the bones of bird wings. For instance, dromaeosaurs had relatively long arms compared with other theropod dinosaurs. Specifically, their forelimbs were nearly as long as, or possibly longer than, their hind limbs.
• The enlargement of the forebrain in non-avian theropod dinosaurs was a trend continued in birds, whose brain capacity is greater than that found in most dinosaurs.
• Egg laying and brooding behaviors. Dinosaurs and birds laid eggs, and there is evidence that some dinosaurs watched over their nests and young—a behavioral trait observed in modern birds.
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