Dromaeosaurian theropods

These bird-like dinosaurs exhibit a number of interesting
anatomical changes to the basic theropod body plan. Some changes
are quite subtle, but others are less so.
One notable feature is the ‘thinning’ of the tail: the tail becomes
very narrow and stiffened by bundles of long, thin bones, the only
flexible part being close to the hips (Figure 16, top). As argued
earlier, this thin, pole-like tail may well have been valuable as a
dynamic stabilizer to assist with the capture of fast-moving and
elusive prey. However, this type of tail dramatically changed the
pose of these animals because it was no longer a heavy, muscular
cantilever for the front half of the body. If no other changes had
been made to its posture, such a dinosaur would have been
unbalanced and constantly pitch forward on to its nose!
To compensate for the loss of the heavy tail, the bodies of these
theropods were subtly altered: the pubic bone, which marks the
rearmost part of the gut and normally points forward and
downward from each hip socket in theropods, was rotated
backwards so that it lay parallel to the ischium (the other lower
hip bone). Because of this change in orientation, the gut and
associated organs could be swung backwards to lie beneath the
hips. This change shifted the weight of the body backwards, and
compensated for the loss of the heavy counterbalancing tail. This
layout of hip bones, with the pubis rotated backward, is seen in
living and fossil birds as well as maniraptoran theropods.
Another equally subtle way of compensating for the loss of the
counterbalancing tail would be to shorten the chest in front of the
hips, and this is also seen in these bird-like theropods. The chest
also shows signs of being stiffened, and this probably reflects the
predatory habits of these animals. The long arms and three-clawed
hands were important for catching and subduing their prey and
needed to be very powerful. The chest region was no doubt
strengthened to help securely anchor the arms and shoulders to
withstand the large forces associated with grappling and subduing
prey. Birds also have a short, and greatly stiffened, chest region to
withstand the forces associated with anchoring the powerful flight
muscles.

At the front of the chest, between the shoulder joints, there is a
V-shaped bone (which is in fact the fused clavicles, or collar bones –
Figure 17) that acts as a spring-like spacer separating the shoulders,
it also helped to anchor the shoulders in place while these animals
were wrestling their prey. Birds also exhibit fused collar bones; they
form the elongate ‘wish bone’, or furcula, that similarly acts as a
mechanical spring that separates the shoulder joints during
flapping flight.

The joints between the bones of the arm and hand were also
modified so that they could be swung outward and downward with
considerable speed and force to strike at prey in what has been
called a ‘raking’ action. When not in use, the arms could be folded
neatly against the body. The leverage for this system was also of
considerable advantage to these creatures, because the arm muscles
that powered this mechanism were located close to the chest and
operated long tendons that ran down the arm to the hand (rather
than having muscles positioned further out along the arm); this
remote control system kept the weight of the body closer to the hips
and helped to minimize the delicate problem of balance in these
theropods. The arm-striking and arm-folding mechanism is closely
similar to that employed by birds when opening and closing their
wings during and after flight.

Archaeopteryx
The early bird-like fossil Archaeopteryx (Figure 16, bottom) exhibits
many maniraptoran theropod features: the tail is a long and very
thin set of vertebrae that anchored the tail feathers on either side;
the hip bones are arranged with the pubis pointing backward and
downward; at the front of the chest there is a boomerang-like
furcula; the jaws are lined with small, spiky teeth, rather than a
more typical bird-like horny beak; the arms are long, jointed so that
they can be extended and folded just as in theropods, and the hands
are equipped with three sharply clawed fingers that in their
arrangement and proportions are identical to those seen in
maniraptoran theropods.

Chinese wonders
During the 1990s, explorations in quarries in Liaoning Province
in north-eastern China began to yield some extraordinary, and
extraordinarily well preserved, fossils of Early Cretaceous age. At
first, these comprised beautifully preserved early birds such as
Confuciusornis, and the skeletons included impressions of feathers,
34. Restoration of the living Archaeopteryx
beaks, and claws. Then in 1996, a complete skeleton of a small
theropod dinosaur, very similar in anatomy and proportions to
the well known theropod Compsognathus (Figure 14), was
described by Ji Qiang and Ji Shu’an. They named the dinosaur
Sinosauropteryx. This dinosaur was remarkable because there was
a fringe of filamentous structures along its backbone and across its
body, suggesting some sort of covering to the skin that was akin to
the ‘pile’ on a roughly made carpet; there was also evidence of soft
tissues in the eye socket and in the region of the gut. It was clear
that some small theropods had some type of body covering. These
discoveries led to concerted efforts to find more such fossils at
Liaoning; they began to appear with increasing regularity and
ushered in some truly breathtaking revelations.
Shortly after Sinosauropteryx was discovered, another skeleton
was revealed. This animal, named Protoarchaeopteryx, was the
first to show the presence of true bird-like feathers attached to its
tail and along the sides of its body, and its anatomy was much
more similar to that of dromaeosaurians than Sinosauropteryx.
Another discovery revealed an animal that was extremely similar
to Velociraptor, but this time named Sinornithosaurus (again,
apparently covered in a ‘pile’ of short filaments). Newer discoveries
have included Caudipteryx, a large (turkey-sized), rather
short-armed creature noted for a pronounced tuft of tail feathers
and shorter fringes of feathers along its arms; smaller, heavily
feathered dromaeosaurians; and in the spring of 2003 a quite
remarkable ‘four-winged’ dromaeosaurian, Microraptor, was
unveiled to the world. This latter creature was small and classically
dromaeosaur-like, with the typically long, narrow tail, bird-like
pelvis, long, grasping arms, and sharp rows of teeth lining its
jaws. The tail was fringed by primary feathers and its body
covered in downy ones. However, what was singularly impressive
was the preservation along the arms of flight feathers forming
Archaeopteryx-like wings and, very unexpectedly, similar wing-like
fringes of feathers attached to the lower parts of the legs – hence the
name ‘four-wing’.