Birds from dinosaurs: an evolutionary commentary

The implications of these new discoveries are truly fascinating. It
has already been argued, with logic and some force, that small
theropod dinosaurs were highly active, fast-moving, and
biologically ‘sophisticated’ animals. On this basis, they seemed
reasonable candidates as potential endotherms; in a sense, our
inferences about their way of life suggested that they had most to
benefit from being endothermic. The Liaoning discoveries confirm
that many of these highly active, bird-like dinosaurs were small
animals. This is a crucial point, as small size puts greatest
physiological stress on endotherms because a large percentage of
internally generated body heat can be lost through the skin surface;
so small, active endotherms would be expected to insulate their
bodies to reduce heat loss. Small theropod dinosaurs, therefore,
evolved insulation to prevent heat loss because they were
endotherms – not because they ‘wanted’ to become birds!
Liaoning discoveries indicate that various types of insulatory
covering developed, most probably by subtle modifications to the
growth patterns of normal skin scales; these ranged from hair-like
filaments to full-blown feathers. It may well be that genuinely
bird-like flight feathers did not evolve for the purposes of flight,
but had a far more prosaic origin. Several of the ‘dinobirds’ from
Liaoning seem to have tufts of feathers on the end of the tail (rather
like a geisha’s fan) and fringes of feathers along the arms, on the
head, or running down the spine. Clearly preservational biases may
also play a part in how and on which parts of the body these may
be preserved. But for the present, it seems at least possible that
feathers evolved as structures linked to the behaviour of these
animals: providing recognition signals, perhaps, as in living birds,
or being used as part of their mating rituals, long before any
genuine flight function had developed.

In this context, gliding and flight, rather than being the sine qua
non of avian origins, become later, ‘add-on’ benefits. Obviously,
feathers have the potential for aerodynamic uses; just as with
modern birds, the ability to jump and flutter may well have
embellished ‘dinobird’ mating displays. For example, in the case of
the small creature Microraptor, a combination of fringes of feathers
along the arms, legs, and tail would have provided it with the ability
to launch itself into the air from branches or equivalent vantage
points. From just this sort of starting point, gliding and true
flapping flight seem a comparatively short ‘step’ indeed.


We should not, however, get too carried away with the scenario
outlined above. Although the Liaoning discoveries are indeed
incredibly important, offering, as they do, a richly detailed window
on dinosaurian and avian evolution in the Cretaceous, they do not
necessarily provide all the answers. One crucial point that must be
remembered is that the quarries of Liaoning are Early Cretaceous in
age, and their fossils are therefore considerably younger (by some
30 Ma at least) than the earliest well-preserved feathered dinosaur
with highly developed and complex wings, Archaeopteryx.
Whatever the path that led to the evolution of the first flying
dinosaurs, and ultimately to birds, it was emphatically not via the
extraordinary feathered dinosaurs from Liaoning. What we see at
Liaoning is a snap shot of the evolutionary diversification of avian
theropods (and some true birds), not the origin of birds: bird
origins are still shrouded by sediments of Middle or possibly even
Early Jurassic age – before Archaeopteryx ever fluttered to Earth.
Everything that we know to date points to a very close relationship
between theropod dinosaurs and early birds, but those crucial Early
or Middle Jurassic theropods that were ancestral to Archaeopteryx
are yet to be discovered. It is to be hoped that in future years some
spectacular discoveries will be made that fill in this part of the
story.

Chapter 5 concluded with the view that dinosaurs lived at a time in
Earth history that favoured large-bodied, highly active creatures
that were able to maintain a stable, high body temperature without
most of the costs of being genuinely endothermic. The ‘dinobirds’
from Liaoning suggest that this view is wrong – small, insulated
theropods simply had to be endothermic and their close
relationship to birds, which we know are endothermic, simply
reinforces the point.

My response to this is: well, yes and no. There is now little doubt
that bird-like theropod dinosaurs were endotherms in a true sense.
However, I do think that the arguments suggesting that the
majority of more traditional dinosaurs were inertial homeotherms
(their large body size enabled stable internal temperature) still hold.
There is some evidence in support of my view to be found among
living endotherms. Elephants, for example, have a much lower
metabolic rate than mice – for exactly these reasons. Mice are small,
lose heat rapidly to the environment, and have to maintain a high
metabolic rate to replenish the heat loss. Elephants are large
(generally dinosaur-sized) and have a stable internal body
temperature due to their size, not just because they are
endothermic. Indeed, being a large endotherm is, in part at least, a
physiological challenge. For example, elephants suffer problems if
they move around too quickly: their postural and leg muscles create
a great deal of extra chemical heat, and they need to use their large,
‘flappy’ ears to help them to radiate heat rapidly to prevent fatal
overheating.

Dinosaurs were on the whole super-large and their bodies would
have been capable of maintaining a constant internal temperature;
extrapolating from the elephant, it would not have been in
dinosaurs’ interests to be genuine endotherms, in a world that
was in any case very warm. Having evolved physiologically as
mass-homeotherms (having a stable internal body temperature
that was made possible by large body size), the only group of
dinosaurs that bucked the general dinosaurian trend toward large
size and evolved into a small-bodied group were the
dromaeosaurian theropods.

It is clear, from their anatomy alone, that dromaeosaurians were
highly active and would have benefited from homeothermy, and
their relatively large brains would have demanded a constant supply
of oxygen and nutrients. Paradoxically, homeothermy cannot be
maintained at small body size without an insulatory covering
because of the unsustainable heat loss through the skin. The choice
was stark and simple: small theropods had to either abandon their
high-activity lifestyle and become conventionally reptilian, or boost
internal heat production and become properly endothermic,
avoiding heat loss by developing skin insulation. So, I propose that
it is not a case of ‘all or nothing’; most dinosaurs were basically
mass-homeotherms that were able to sustain high activity levels
without the full costs of mammalian or avian styles of endothermy;
however, the small, and in particular the dromaeosaurian,
theropods (and their descendants, the true birds) were obliged to
develop full-blown endothermy and the associated insulatory
covering.