ManyMesozoic Erabird-like creatures have been found, some which are clearly dinosaurs. There are many similarities between birds andtheropoddinosaurs, including the number of openings in the skull (they'rediapsids), secondary palate structure, leg and foot structure and proportions, upright stance, oviparous birth (laying eggs), bone structure (bones interlaced with vessels), and, in some instances, feathers.
FEATHERED, BIRD-LIKE DINOSAURS In the last few years, many fossils of feathered dinosaurs have been found near Yianxin, in Liaoning Province, China. Two new Chinese feathered dinosaurs dating from between 145 and 125 million years ago (during the late Jurassic and early Cretaceous periods) have been found,Protarchaeopteryx robustaandCaudipteryx zoui. Their features are more dinosaur-like than bird-like, and they are considered to betheropoddinosaurs. Their feathers were symmetrical, which indicate that they could not fly (flightless birds have symmetrical feathers while those that fly have asymmetrical ones).
These finds, along with the feathered dinosaurSinosauropteryx, found a few years ago, also in the same region of China, and the bird-likeUnenlagiain Argentina, reinforce the theory that birds are descended from dinosaurs.
The theropod (meaning "beast-footed") dinosaurs are a diverse group of bipedal saurischian dinosaurs. They include the largest terrestrial carnivores ever to have made the earth tremble. What most people think of as theropods (e.g., T. rex, Deinonychus) are extinct today, but recent studies have conclusively shown that birds are actually the descendants of small nonflying theropods. Thus when people say that dinosaurs are extinct, they are technically not correct. Still it's not as exciting seeing a sparrow at your birdfeeder as it would be to see a Tyrannosaurus rex there.
Using proper terminology, birds are avian dinosaurs; other dinosaurs are non-avian dinosaurs, and (strange as it may sound) birds are technically considered reptiles. Overly technical? Just semantics? Perhaps, but still good science. In fact, the evidence is overwhelmingly in favor of birds being the descendants of a maniraptoran dinosaur, probably something similar (but not identical) to a small dromaeosaur. What is this evidence?
If we look back into the history of the issue, it is apparent that many comparative anatomists during the 16th through 19th centuries noticed that birds were very similar to traditional reptiles. In 1860, shortly after the publication ofCharles Darwin'sinfluential workOn the Origin of Species By Means of Natural Selection, a quarry worker in Germany spotted an unusual fossil in the limestone of theSolnhofen Formation(lateJurassicperiod). This fossil turned out to be the famous 'London specimen' ofArchaeopteryx lithographica. It was a beautiful example of a "transitional form" between two vertebrate groups (traditional reptiles and birds); just what Darwin expected would eventually be found.Archaeopteryx, generally accepted as being the oldest known bird, is an important link between birds and other coelurosaurs that has helped to illuminate the evolutionary history (phylogeny) of the group. It is now widely held to be the ancestor of all living birds; this is a common misconception. In fact, recent expeditions in China, Mongolia, Madagascar, Argentina, and elsewhere may uncover dinosaurs that usurp the "urvogel" status ofArchaeopteryx.
Some researchers today do not agree that dinosaurs gave rise to birds, and are working to falsify this theory, but so far the evidence for the theory has swamped their efforts. If they were to conclusively establish that birds are more likely descended from another group (Crocodylomorpha, the group containing crocodiles, has been suggested), that would be a major upheaval in our knowledge of phylogeny. One single well-preserved fossil bird unequivocably of Triassic age might shed some doubt on the theory of the maniraptoran affinities of birds. That would be a major find. Some bird-like fossils have been presented as Triassic birds, but so far have not held up under peer review. Such is the dynamic nature of science.
Harvard University's Arkhat Abzhanov, associate professor of organismic and evolutionary biology, and Bhart-Anjan Bhullar, a Ph.D. student in Abzhanov laboratory and the first author of the study, did just that and found evidence that the evolution of birds is the result of a drastic change in how dinosaurs developed. Rather than take years to reach sexual maturity, as many dinosaurs did, birds sped up the clock (some species take as little as 12 weeks to mature), allowing them to lock into their baby dinosaur look.
"What is interesting about this research is the way it illustrates evolution as a developmental phenomenon," Abzhanov was quoted as saying in a press release. "By changing the developmental biology in early species, nature has produced the modern bird –- an entirely new creature –- and one that, with approximately 10,000 species, is today the most successful group of land vertebrateson the planet."
One of the most intriguing questions of science is whether birds evolved from the dinosaurs. The dispute is not over whether there are evolutionary relationships between birds and dinosaurs. On that point, all paleontologists agree. The birds and the dinosaurs are closely related. The question is, how are they related? In one scenario, birds are dinosaurs. The birds represent a branch of the dinosaur lineage that survived the Cretaceous crisis and radiated into the forms we know today. In another scenario, birds and dinosaurs had a common ancestor that gave rise to both groups. Birds were never dinosaurs, but they are the closest living group to those extinct reptiles.