Ostrom and Archaeopteryx the earliest bird

Having described Deinonychus, Ostrom continued to investigate the biological properties of dinosaurs. In the early 1970s a trifling discovery in a museum in Germany was to bring him right back to the centre of some heated discussions. While examining collections of flying reptiles, Ostrom noticed one specimen, collected from a n quarry in Bavaria, that did not belong to a pterosaur, or flying |

reptile, as its label suggested. It was a section of a leg including the j thigh, knee-joint, and shin. Its detailed anatomical shape reminded Ostrom of that of Deinonychus. On closer inspection, he could also = make out the faintest impressions of feathers! This was clearly an unrecognized specimen of the fabled early bird Archaeopteryx (Figure 13). Excited by his new discovery, and naturally puzzled by its apparent similarity to Deinonychus, Ostrom began carefully restudying all the known Archaeopteryx specimens.

The more Ostrom studied Archaeopteryx, the more convinced he became of the extent of the anatomical similarity between this creature and his much larger predatory dinosaur Deinonychus (Figure 16). This led him to reassess the monumental and then authoritative work on bird origins that had been written by ornithologist and anatomist Gerhard Heilmann in 1926. The sheer number of anatomical similarities between carnivorous theropod dinosaurs and early birds drove Ostrom to question Heilmann's conclusion in that work that the similarities could only have been due to evolutionary convergence.

17. Comparison of the clavicles of (a) early theropod dinosaurs, (b) Archaeopteryx (clavicles are fused together), and (c) modern birds

Armed with more recent discoveries of dinosaurs around the world, Ostrom was able to show that a number of dinosaurs did actually possess small clavicles, removing at a stroke Heilmann's big stumbling block to a dinosaurian ancestry for birds. Encouraged by this discovery and his own detailed observations on theropods 2 and Archaeopteryx, Ostrom launched a comprehensive assault on ยก5 Heilmann's theory in a series of articles in the early 1970s. This led a to the gradual acceptance of a theropod dinosaur ancestry of birds by the great majority of palaeontologists, and would no doubt have pleased the far-sighted Huxley and deeply irritated Owen.

The close anatomical, and therefore biological, similarity between theropods and the earliest birds added fuel to the controversy concerning the metabolic status of dinosaurs. Birds are highly active, endothermic creatures; perhaps the theropod dinosaurs might also have possessed an elevated metabolism. The once clear dividing line between feathered birds, with their distinctive anatomy and biology which merited them being separated off from all other vertebrates as a discrete class, the Aves, and other more typical members of the class Reptilia (of which the dinosaurs were just one extinct group) became worryingly blurred. The extent of this blurred line has become even more pronounced in recent years (as we shall see in Chapter 6).

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