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Birds: The Late Evolution of Dinosaurs

Recently, fossils of early birds and their most immediate predecessors have been collected at an unprecedented rate from Mesozoic-aged rocks worldwide. This wealth of new fossils has settled the century-old controversy of the origin of birds. Today, we can safely declare that birds evolved from a group of dinosaurs known as maniraptoran theropods-generally small meat-eating dinosaurs that include Velociraptor of Jurassic Park fame.

Evidence that birds evolved from the carnivorous predators that ruled the Mesozoic ecosystems is plentiful and it comes from disparate lines of evidence. Traditionally, the prime source of evidence in support of this scientific view was the similar shape of the bones of birds and a variety of maniraptorans but spectacular new discoveries have added other lines of evidence to the table. One of these involves a suite of features from the eggs of these dinosaurs. A host of fossils have shown that not only did maniraptoran dinosaurs resemble birds in the way they laid their eggs but that these eggs also looked like the eggs of birds. Another line of evidence involves the handful of snapshots that tell us about the behavior of the maniraptoran theropods. Fossils of animals in brooding poses or in resting postures also show a startling similarity with the behaviors we see among living birds. Yet, perhaps the most compelling new line of evidence comes from the discovery of soft tissues associated with the skeletons of these predatory dinosaurs, many fossils of these creatures are now known to have been covered by plumage. All this evidence has highlighted the fact that many features that were previously thought to be exclusively avian-from feathers to a wishbone-have now been discovered in the immediate dinosaur predecessor of birds. Even flight is likely to have been an attribute inherited by birds from their dinosaurian forebears! If the new wealth of fossils has clarified the old controversy of the origin of birds, many other fossils have provided a vivid testimony of the early phases of avian evolution. Hidden in these fossils are the clues to how birds perfected their flying abilities and how they evolved warm bloodedness.

As we know it today, the known history of birds starts with the spectacular Archaeopteryx, a jay-sized creature with toothed jaws, a long lizard-like tail, and flight feathers. Archaeopteryx lived 150 million years ago in today's southern Germany. Although Archaeopteryx stands alone in the fossil record of birds of the end of the Jurassic period, within the last decade a large number and variety of birds have been found in Cretaceous rocks ranging from 130 to 115 million years ago. Some of these fossils show that a great diversity of birds with long bony tails preceded the evolution of birds with the familiar short bony tail.

Birds of the early Cretaceous blossomed in a range of shapes and sizes. The crow-sized, stout-beaked Confuciusornis sported enormous claws in its wings while the contemporaneous Sapeornis had very long and narrow wings like those of an albatross. These two birds were much larger than the sparrow-sized Eoenantiornis and Iberomesornis, which like most early birds had toothed jaws similar to those of Archaeopteryx. The different design of skulls, teeth, wings, and feet indicate that already at this early phase of their evolutionary history, birds had specialized into a variety of ecological niches, including seed-feeders, sap-eaters, insect-feeders, fish-eaters, and meat-eaters. At the same time, a host of novel features of the wings and ribcages suggests that soon after Archaeopteryx, birds evolved flying abilities not very different from the ones that amaze us today.

As the rocks of the Cretaceous period become younger, the fossil record includes a great number of bird species with even more diverse lifestyles. The hesperornithiforms-large, flightless, foot-propelled divers-made their debut around 100 million years ago. A few million years later, these supreme fish-eaters would be crowned kings of the aquatic birds with the tiny-winged, 4-foot long, Hesperornis. The hesperornithiforms swam the waters of a warm sea dissecting North America from the Gulf of Mexico to the Arctic. On the shore of this shallow sea, over herds of duck-billed dinosaurs, soared the tern-sized Ichthyornis. Its large head with sharp teeth was designed to catch fish.

Not all the birds that lived during the Mesozoic, the Age of Large Dinosaurs, may have looked as unfamiliar as Archaeopteryx, Confuciusornis, and Hesperornis. The early representatives of today's lineages of birds can also be traced back to this remote era of our geological past. In several continents, rocks from the last part of the Cretaceous period have started to provide the remains of early shore-birds, ducks, and other more familiar birds. Their descendants are the true heirs of the magnificent dinosaurs that ruled the Earth tens of millions of years ago.