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DINOSAURS and BIRDS

Birds probably evolved from the maniraptors, a branch of bird-like dinosaurs . This idea has been hotly debated for over a hundred years. New fossil evidence is reinforcing this theory, which is now accepted by most scientists.

In order to determine what animals birds evolved from, scientists use fossil evidence to trace the emergence of bird-like traits. Many Mesozoic Era bird-like creatures have been found, some which are clearly dinosaurs. There are many similarities between birds and theropod dinosaurs, including the number of openings in the skull (they're diapsids), secondary palate structure, leg and foot structure and proportions, upright stance, oviparous birth (laying eggs), bone structure (bones interlaced with vessels), and scales (modified in birds and some dinosaurs to be feathers).

Recently, scientists have reorganized the groups in which many animals have been classified using a system called cladistics. Since birds are descended from dinosaurs, they are in the same group, dinosauria. So the national symbol of the United States is actually a dinosaur (the bald eagle).

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 robusta and Caudipteryx zoui. Their features are more dinosaur-like than bird-like, and they are considered to be theropod dinosaurs. 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 dinosaur Sinosauropteryx, found a few years ago, also in the same region of China, and the bird-like Unenlagia in Argentina, reinforce the theory that birds are descended from dinosaurs.

THE OLDEST-KNOWN BIRDS
The Archaeopteryx is one of the most famous and oldest-known fossil birds, and dates from the late Jurassic period (about 150 million years ago). It is now extinct. Although it had feathers and could fly, it had similarities to dinosaurs, including its teeth, skull, and certain bone structures. Some paleontologists think that Archaeopteryx was a dead-end in evolution and that the maniraptors led to the birds.

The first Archaeopteryx fossilized feather impression was found in 1860 in a limestone quarry in Germany. A year later, a much more complete fossilized Archaeopteryx was found at the same quarry. Impressions of its feathers and bone structure were quite clear. Many more have been found since, for a total of seven.

In 1868, Thomas Henry Huxley interpreted the Archaeopteryx fossil to be a transitional bird having many reptilian features. Using the fossils of Archaeopteryx and Compsognathus, a bird-sized and bird-like dinosaur, Huxley argued that birds and reptiles were descended from common ancestors. Decades later, Huxley's ideas fell out of favor, only to be reconsidered over a century later (after much research and ado) in the 1970's. In 1986, J. A. Gauthier looked at over 100 characteristics of birds and dinosaurs and showed that birds belonged to the clade of coelurosaurian dinosaurs. [Gauthier, J.A., 1986. Saurischian monophyly and the origin of birds, in The Origin of Birds and the Evolution of Flight, California Academ of Sciences Memoir No. 8]

Bird fossils are rare because bird bones are hollow and fragile, but Jurassic, Cretaceous, Eocene and Miocene-Pliocene bird fossils have been found.

BIRD-LIKE ANIMALS
In the chain of creatures leading from dromaeosaurid dinosaurs (advanced theropods) to birds, Sinosauropteryx is the earliest bird-like dinosaur. For now, the bird-like animals include (in chronological order):

• Protoavis (meaning "first bird") is an extinct diapsid from the late Triassic period (80 million years earlier than Archaeopteryx). Its partly toothless jaw and keel-like breast bone were like those of birds. It also had a tail, dinosaur-like rear legs, and hollow bones. There is some dispute about whether this animal was a bird or a dinosaur; the answer depends partly on whether the Protoavis fossil belongs to one or two different genera. Fossils have been found in Texas, USA.
• Archaeopteryx - The oldest known bird had asymmetrical feathers - it could probably fly short distances and was the size of a crow. This bird was probably an evolutionary dead-end. (from Germany, 150 million years ago).
• Sinosauropteryx - Sinosauropteryx had a coat of downy, feather-like fibers that are perhaps the forerunner of feathers. This ground-dwelling dinosaur had short arms, hollow bones, a three-fingered hand, and was about the size of a turkey. (from China, 121-135 mya).
• Protarchaeopteryx - Long, symmetrical feathers on arms and tail, but it probably could not fly. It was the size of a turkey (from China, 121-135 mya).
• Caudipteryx - a small, very fast runner covered with primitive (symmetrical and therefore flightless) feathers on the arms and tail, with especially long ones on the tail. It was about the size of a turkey. (from China, 121-135 mya)
• Iberomesornis (meaning "Iberian=Spanish intermediate bird") was a small, early, toothed bird that lived during the early Cretaceous period. It was capable of powered flight. It had tiny, spiky teeth in its beak and was the size of a sparrow. Its hip was primitive compared to modern birds; its ilium, ischium, and pubis were all parallel and directed backward. Iberomesornis was named by paleontologists Sanz and Bonaparte in 1992. Fossils were found in Spain. The type species is I. romeralli.
• Unenlagia - a much larger ground-dwelling theropod about 4 feet (1.2 m) tall and 8 feet (2.4 m) long. It had flexible arm movement (up and down movements were possible, like that which a bird uses in flying). (from Argentina, 90 mya).
• Patagonykus (meaning "Patagonia claw") was a lightly-built meat-eater with a single, clawed finger on each hand. It was about 6.5 ft (2 m) long. It had long legs, a long tail, and short arms. Patagonykus lived during the late Cretaceous period, about 90 million years ago. Patagonykus was either a bird-like dinosaur (an advanced theropod, or a primitive bird; it possessed qualities of both groups of animals, and there is much scientific debate over which it is. Patagonykus was similar to Mononykus. Fossils were found in Patagonia, a region of southern Argentina. The type species is P. puertai. Patagonykus was named by paleontologist F. Novas in 1996
• Velociraptor - a larger, ground-dwelling carnivore with a swiveling wrist bone (this type of joint is also found in birds and is necessary for flight). About 3 feet tall (1 m). (from Mongolia, 85 - 80 mya).
• Mononykus (meaning "single claw") was a small, insect-eater from the Late Cretaceous period, about 72 million years ago. Mononykus was either a bird-like dinosaur (an advanced theropod, or a primitive bird; it possessed qualities of both groups of animals, and there is much scientific debate over which it is. Mononykus had short arms with one long, thick clawed finger on each hand (hence its name). It was lightly built, had long, thin legs, and a long tail. Mononykus was roughly 28 inches (70 cm long). A fossil was found in SW Mongolia in 1923 (and originally called Mononychus). Mononykus was named by Perle, Norell, Chiappe, and Clark in 1993. The type species is M. olecranus.
• Hesperornis (meaning "western bird") was an early, flightless bird that lived during the late Cretaceous period. This diving bird was about 3 feet (1 m) long and had webbed feet, a long, toothed beak, and strong legs. Although it couldn't fly, Hesperornis was probably a strong swimmer and likely lived near coastlines and ate fish. Fossils have been found in North America .
• Ichthyornis (meaning "fish bird") were 8 inch (20 cm) long, toothed, tern-like, extinct bird that date from the late Cretaceous period. It had a large head and beak. This powerful flyer is the oldest-known bird that had a keeled breastbone (sternum) similar to that of modern birds. It lived in flocks nesting on shorelines, and hunted for fish over the seas. Ichthyornis was originally found in 1872 in Kansas, USA, by a member of paleontologist Othniel C. Marsh's Yale University expedition. Fossils have been found in Kansas and Texas, USA and Alberta, Canada. (Subclass Odontornithes, Order Ichthyornithiformes)
• Eoalulavis (from Spain) - the earliest bird that had good maneuverability while flying, even at low speeds (this extra flight control is obtained from a tuft of feathers on the thumb called the alula - it also helps in takeoffs and landings).

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