How to blow up a pterosaur

Pick up comparably sized bones of a
bird and a mammal, and the latter
will weigh a lot more than the
former. Both actually have hollow
bones, but although the major bones
of the tetrapods (the amphibians,
reptiles, birds and mammals) have a
hollow core, in birds the walls of the
bone are very thin and the hollow
spaces inside are much greater
compared with other tetrapods. As a
result, even the bone of a dead bird is
proportionally lighter than the
equivalent of a mammal or lizard,
there's simply not much of it there.
The difference is quite obvious, but
oddly enough in life the discrepancy
is even greater.
In life, the hollow spaces of the bone
of non-avian animals is filled with
bone marrow - the tissue that
produces red blood cells - which
means that despite their being
"hollow", bones are pretty heavy
structures. The bones of birds on the
other hand, are full of air.
Extensions of the lungs called air sacs
invade the bones through holes in
their surface and fill the internal
cavity, making the bones literally air-
filled.
Like a great many features of birds
(feathers, reversed pubis, enlarged
sternum), pneumatic bones appeared
in dinosaurs long before the birds
were on the scene and numerous
dinosaurs have similarly pneumatic
bones. So too do the members of not-
dinosaurs-but-close-to-dinosaurs
group, the pterosaurs. In fact this is
one of the features that suggests
pterosaurs are likely close relatives
of dinosaurs.
As it happens, some of the sauropod
dinosaurs (including familiar names
like Diplodocus and Brachiosaurus )
may have had the most extreme
pneumatisation of any animals. Some
of their enormous neck bones are
almost entirely air by volume (over
90%) though they maintain relatively
solid limbs. The pterosaurs on the
other hand (much like birds in fact)
tried to pneumatise all their bones
and had a pretty decent go of it, as
shown by a new paper by my former
student Ross Elgin and myself.
We describe a small pterosaur from
the Cretaceous of Brazil where
erosion has stripped off the surface
of many of the bones allowing us to
see which are pneumatic and to what
degree. Combining this with various
bits of data from other pterosaur
specimens and the list of pneumatic
bones is quite impressive.
A single rib of a Cretaceous pterosaur
showing the hollow core now filled with
grey mudstone. The scale at the bottom is
5mm across. Photograph: Dave Hone
In addition to the expected vertebrae
and major elements like the humerus
and femur and parts of the wing,
we've found evidence of pneumaticity
in two major components of the
pelvis (the ischium and pubis), in the
sternum and even the ribs. Not only
that, but the degree to which the bones
are pneumatic is pretty impressive.
The main animal under study had a
wingspan of perhaps a metre and a
half, (which is sizeable, though not
large by the standards of the really
big pterosaurs) and so had some
pretty large bones. The sternum, for
example, (which supports much of the
major flight muscles) is around 5cm
across and is a large flat plate.
However, the bone wall on this is as
little as 0.06mm! To call this "paper
thin" would be a serious
understatement, it's barely even
there. At the other end, some of the
ribs are only around 1mm in
diameter, and yet these have been
invaded by airsacs and pneumatised,
with one being over 75% air by
volume.
It certainly speaks to the degree that
pterosaurs have modified their
skeletons and allowed their lungs to
effectively run riot over their skeletal
anatomy. On top of all this, the
specimen is of a young animal and
the extent of the pneumaticity likely
increased as pterosaurs grew, so we
might expect the adults to be even
more extreme with other bones
invaded, or the current set to increase
the level of pneumaticity. Even so, the
levels of weight savings might have
been tiny - those ribs would have
weighed very little to begin with so to
pneumatise them (something I don't
think even birds have managed,
though admittedly their ribs are fairly
flat structures) is incredible.
The solutions that evolution can find
to problems is quite wonderful and in
cases like this where there is
convergent similarity in both huge
dinosaurs and the birds allows us to
potentially make some interesting
comparisons given the very different
problems faced by terrestrial giants
and flying animals.