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

Dinosaur palaeobiology: a new beginning

It was not until the 1960s and early 1970s that the study of fossils
began to re-emerge as the subject of wider and more general
interest. The catalyst for this re-awakening was a younger
generation of evolutionarily minded scientists eager to demonstrate
that the evidence from the fossil record was far from being a
Darwinian ‘closed book’. The premise that underpinned this
new work was that while evolutionary biologists are obviously
constrained by working with living animals in an essentially
two-dimensional world – they are able to study species, but they do
not witness the emergence of new species – palaeobiologists, by
contrast, work in the third dimension of time. The fossil record
provides sufficient time to allow new species to appear and others to
become extinct. This permits palaeobiologists to pose questions
that bear on the problems of evolution: does the geological
timescale offer an added (or different) perspective on the process of
evolution?; and, is the fossil record sufficiently informative that it
can be teased apart to reveal some evolutionary secrets?
Detailed surveys of the geological record began to demonstrate
rich successions of fossils (particularly shelled marine creatures) –
considerably richer than Charles Darwin could ever have imagined,
given the comparative infancy of palaeontological work in the
middle of the 19th century. Out of this work emerged observations
and theories that would challenge the views of biologists over the
modes of biological evolution over long intervals of geological
time. Sudden massive, worldwide extinction events and periods
of faunal recovery were documented which could not have been
predicted from Darwinian theory. Such events seemed to reset the
evolutionary timetable of life in a virtual instant, and this prompted
some theorists to take a much more ‘episodic’ or ‘contingent’ view
of the history of life on Earth. Large-scale, or macroevolutionary,
changes in global faunal diversity over time seemed to be
demonstrable; these again were not predicted from Darwinian
theory and required explanation.

Most notably, however, Niles Eldredge and Stephen Jay Gould
proposed the theory of ‘punctuated equilibrium’. They suggested
that modern biological versions of evolutionary theory needed to be
expanded, or modified, to accommodate patterns of change seen
repeatedly among species in the fossil record. These consisted of
prolonged periods of stasis (the ‘equilibrium’ period) during which
relatively minor changes in species were observable, and contrasted
with very short periods of rapid change (the ‘punctuation’). These
observations did not fit well with the Darwinian prediction of slow
and progressive change in the appearance of species over time
(dubbed ‘evolutionary gradualism’). These ideas also prompted
palaeobiologists to question the levels at which natural selection
might function: perhaps it could operate above the level of the
individual in some instances?

As a consequence, the whole field of palaeobiology became more
dynamic, questioning, and also outward-looking; it was also
prepared to integrate its work more broadly with other fields of
science. Even highly influential evolutionary biologists such as
John Maynard Smith, who had had little truck with fossils at all,
were prepared to accept that palaeobiology had valuable
contributions to make to the field.
While the general field of scientific palaeobiology was
re-establishing its credentials, the mid-1960s was also a time
of important new dinosaur discoveries; these were destined to
spark ideas that are still important today. The epicentre of this
renaissance was the Peabody Museum at Yale University, the
original workplace of ‘bone-fighter’ Othniel Charles Marsh.
However, this time it was in the person of John Ostrom, a young
professor of palaeontology with a strong interest in dinosaurs.