synthesising, and in the wake of this Avian dinosaurs.
disaster, 80% of all terrestrial animals
became extinct, including the
It is well established amongst
dinosaurs, which until then had ruled zoologists that birds are simply a
the land.
lineage of dinosaurs – the avian
dinosaurs, with the rest of the
Indeed, the Mesozoic Era (from 245- Dinosauria being known as non-avian
66 million years ago, and encompass- dinosaurs. Hundreds of shared traits
ing the Triassic, Jurassic and
in behaviour, reproduction, the
Cretaceous Periods) is commonly
skeleton and soft tissues clearly
referred to as the ‘Age of the
reveal that birds and dinosaurs have a
Dinosaurs’, since during this time
close evolutionary relationship.
dinosaurs dominated the planet,
occupying a wide variety of
Fossil and comparative morphologi-
ecosystems. Yet this mighty reign
cal evidence indicates that birds
came to an abrupt end when they
evolved from a theropod dinosaur of
were completely wiped out by the
the Maniraptora clade; most likely a
consequences of the meteorite strike. small dromaeosaur. These creatures
Or were they? Remarkably, one
are commonly known as ‘raptors’,
lineage of dinosaurs not only
which only evokes confusion as this
survived, but prospered to the extent term is also used to describe modern
that they are still the most species-
birds of prey (owls, eagles, hawks
rich and arguably successful group of and falcons). Dromaeosaurs were
vertebrates on this planet. In fact,
popularised in Jurassic Park as
look out of the window and you’ll
fierce, agile, intelligent dinosaurs
probably see one!
with grasping hands and huge slash-
ing talons on their feet. Skull fossils
reveal a large brain case, indicating
that, like modern birds, dromaeosaurs
were probably highly intelligent.
They had long, flexible forelimbs,
most likely an adaptation to swiftly
seize prey, however this turned out to
be a handy exaptation to produce the
powerful flight stroke of birds today.
walled bones. The latter trait was
co-opted during evolution and
exapted by birds for flight, given the
need to reduce body weight to
become airborne. Their indisputable
membership of the therapod clade
also means that birds are related to
arguably the most infamous and
fearsome of dinosaurs, the giant
hyper-predatory Tyrannosaurus rex!
Numerous other common traits
include: aspects of the skeletal
structure in the pubis, wrist, scapula,
clavicles (which are fused to form a
‘wishbone’) and vertebrae; muscula-
ture around the arms and the pectoral
girdle; microstructure of the eggshell;
and a reduced, stiffened tail. Many of
these characteristics are exaptions
that were to predispose these animals
to a life on the wing. Further exam-
ples include expanded sinuses in the
skull (thereby reducing weight), and a
secondary bony palate, which allows
for more efficient respiration and
therefore also a highly active life-
style. Large orbits (eye openings) are
also an exaption for navigating a
complex, three-dimensional, aerial
environment.
And there is one other important
common feature….feathers. We tend
to think of birds being the only
feathered animals, yet literally
thousands of fossils have been
Shared Features.
discovered of bone fide dinosaurs
clearly sporting feathers of some kind
Birds are united with theropod
– definitive proof of the dinosaur-bird
dinosaurs by having a bipedal posture evolutionary relationship.
and associated skeletal modifications
of the legs and ankles; weight is
Feathered dinosaurs.
borne on only three digits, the first
and fifth toes having become reduced
and lost respectively. They also have Archaeopteryx has for a long time
been considered the ‘first bird’; a
only three main digits in the manus,
‘missing link’ between dinosaurs and
or hand, with the fourth and fifth
birds that fully validated Charles
becoming highly reduced. Another
Darwin’s theory of evolution,
diagnostic feature of the bird-
published in On the Origin of Species
maniraptor clade are hollow, thin-
only two years before the discovery
of the first Archaeopteryx in 1861.
Known initially only from a single
feather, fossils of Archaeopteryx have
been found in deposits dated at being
150 million years old. As well as
feathers, fossilised specimens exhibit
clearly reptilian features such as a full
set of teeth and a long bony tail. But
in light of new fossil evidence, the
current consensus is that other taxa