Archaeopteryx

The first Archaeopteryx ("ancient wing") fossil to be discovered was a single feather found in the Solnhofen Limestone Formation of Bavaria in 1860, during quarrying operations. Since then seven skeletons have been found, the most complete of which are the British Museum specimen (discovered in 1862) and the Berlin specimen (discovered in 1877). Although most of the specimens seem to belong to the same species, Archaeopteryx lithographica, a case has been made that the seventh specimen represents a different and smaller species, Archaeopteryx bavarica. The Solnhofen Limestone was deposited in hypersaline lagoons that formed along the northern margin of the Tethys Sea during the Late Jurassic, and the finegrained limestones that formed in the lagoons contributed to the exceptional preservation of soft-tissue structures, including feathers. Archaeopteryx may have lived around the lagoons in bushes and shrubs, as there is no evidence of trees, and occasionally may have been blown out into the lagoons during storms, becoming waterlogged and sinking into the fine carbonate sediment. Archaeopteryx was a small animal, about the size of a pigeon, and is considered to be a bird, as it has feathers, with those on the wings specialized for flight. However, many of its skeletal characteristics are dinosaurian, and if the feather impressions had not been preserved it would undoubtedly have been identified as a small theropod or carnivorous dinosaur. In particular, the skull has openings characteristic of dinosaurs, and toothed jaws are present rather than a beak. The arms still terminate in separate clawed fingers, unlike the fused wing of a bird, and its pelvis and long bony tail are similar to those of a theropod. All modern birds have a fused mass of vertebrae called the pygostyle rather than a long bony tail. However, Archaeopteryx does have a furcula, the wishbone of birds, which acts as a spring during flight; a broad platelike sternum for the attachment of the flight muscles; and feathers, which probably developed initially from epidermal scales as insulation and were only later adapted for flight.

Significance of Archaeopteryx
The status of Archaeopteryx as the oldest known bird has made it extremely significant in ongoing discussions about the origin of birds and the origin of flight. At the time of its discovery, shortly after the publication of Charles Darwin'sOnthe Origin of Species (1859), it seemed to be a perfect "missing link" between birds and reptiles, and was proposed as such by Thomas Henry Huxley. However, the apparent lack of a furcula was used to discredit this view, and rival views suggesting that birds were most closely related to crocodiles, basal archosaurs (dinosaur ancestors), and even mammals became more popular. This changed in the 1970's when John Ostrom cataloged numerous similarities between Archaeopteryx and theropod dinosaurs, particularly the advanced dromaeosaur Deinonychus. Subsequent analyses using cladistics, a method in which the distribution of advanced characteristics in related species can be assessed by computer, have fully supported this view. Analyses of the wings of Archaeopteryx have shown that the feathers are adapted for flight and that the animal was probably capable of fast cruising flight, although it would not have been very manouverable. Specimens of feathered dinosaurs and early birds from the Lower Cretaceous of China have shown that feathers probably originated as insulating structures that were subsequently adapted for flight in animals such as Archaeopteryx.Howthat happened is still a matter for debate, but the two main possibilities are the arboreal hypothesis, which suggests that flight originated in gliders that started to flap, and the cursorial hypothesis, which suggests that flight developed in animals that were running and jumping after prey.