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
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
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. 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
and Archaeopteryx, Ostrom launched a comprehensive assault on
Heilmann’s theory in a series of articles in the early 1970s. This led
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