The mere suggestion of a possible linked lineage between the feather-laden creatures we know today as birds and the extinct, reptilian dinosaurs would have once been deemed entirely ludicrous; however, arguably the most important paleontological finding in establishing this very notion occurred in the late 19th century with the discovery of the first Archaeopteryx lithographica fossil – dubbed the oldest bird. This significant finding prompted biologist and anatomist Thomas Huxley to first conceptualise a radical and controversial new theory; that the modern bird is descendant from dinosaurs. In the subsequent years, many intriguing specimens have been unearthed, hinting at striking physiological and behavioural similarities between birds and dinosaurs such as the feather, skeletal structure and nesting behaviour. Yet, in spite of the multitude of supportive evidence, embryological and anatomical proof has emerged that possibly discredit this bird-dinosaur link.
In 1861 the first Archaeopteryx fossil was found in Germany embedded within Jurassic Solnhofen limestone. Upon analysis, Huxley proposed that this original avian bore striking similarities to theropod dinosaurs bearing features such as pneumatic bones, free trunk vertebra and lacking keratin covered premaxilla. However it wasn’t until the work of John Ostrom of Yale University in the 1970s that Huxley’s hypothesis was deemed credible (Flannery 1999, p.701). After a systematic skeletal study of all five Archaeopteryx fossils available to him at the time, Ostrom argued that the Archaeopteryx was indeed a bird with theropod-like features. He asserted that if it weren’t for the presence of feather imprints, the specimens “would be identified unquestionably as coelurosaurian theropods… They are remarkably similar in a number of details to those of certain small theropods, namely Ornitholestes, Deinonychus, Velociraptor, Chirostenotes” (Ostrom 1976, p.109). Ostrom found ample skeletal evidence in favour of Huxley’s original theory: the wrist bones of the Archaeopteryx are reduced, facilitating a wing-folding movement; the vertebrae extend from the back of the skull, as evident in coelurosaurian theropods, rather than from beneath as in modern birds; additionally, there are striking similarities in the forelimb and hind limb bones as well as in the pelvic and pectoral girdles (Flannery 1999, p.703). The unique, mosaic-like qualities of the Archaeopteryx have rendered the discovery of this Jurassic bird critical in the pursuit of understanding the evolution of the modern bird.
More recently, China’s rural Liaoning Province was the discovery site of a new bird-dinosaur fossil link; dubbed a close relative of Compsognathus, a small dinosaur belonging to the carnivorous coelurosaurian clade, this dinosaur is a theropod, the very group that Ostrom maintained to be the predecessor of modern birds. A dark line appeared to run down the length of the neck and backbone of the fossil, which upon closer inspection revealed short, downy, feather-like features with a central stem. Yet, following further investigation, the specimen was deemed too recent to have given rise to the 150-year-old Archaeopteryx. In spite of this, the presence of the feathers suggests this dinosaur to be a descendant of the earliest bird ancestor (Gibbons 1996, p.720).
Possibly the most critical factor in establishing this bird-dinosaur connection is the evolution of the feather which Palaeontologists have time and again scrutinised in the hope of illuminating the true evolutionary path of the bird. “It is unlikely that a feature as unusual as feathers evolved twice,” concurs Peter Wellnhofer of the Paleontological Museum, Munich (Gibbons 1996, p.720). And while some – including the likes of Martin and Feduccia – maintained that feathers were solely linked to flight (Feduccia et. al. 2005, p.125-166), advocates of the bird-dinosaur theory reason that the feathers found on some flightless theropods...
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