She is
completely obsessed with dragons,
having collected hundreds of models, trinkets and whatnot throughout the years.
Since we are both paleontology students, naturally, we discussed the potential phylogenetic position the dragon
lineage would occupy in the tree of life, were they real. I had recently been
thinking about that, and was leaning toward them being an offshoot of the Rauisuchia – an extinct group of
land-living relatives of modern crocodiles, though much larger. Like dinosaurs,
birds and mammals, the rauisuchians had an erect
limb posture – a feature also depicted in many dragons, and characteristic
of highly active predators or grazers. I will not dwell into details here, but
an erect limb posture is associated with a more efficient respiratory
(breathing) system, which enables the animal to use energy more efficiently to
sustain a high-rate metabolism, something that likely is needed for a creature
capable of breathing fire. In other words, the similar limb postures, and the
sense this similarity makes in terms of
its associated function, hints toward a connection between dragons and
rauisuchians.
An artist’s impression of the rauisuchian Postosuchus. Image from http://www.bbc.co.uk/nature/life/Postosuchus
Now, do not
forget that there are other animals with an erect limb posture. However, there
are other morphological similarities that make the Rauisuchia a good candidate
for the ancestors of dragons. Most dragons have scaly skin, which rules out a mammalian origin, though not the
therapsids (advanced members of the Synapsida, more colloquially called
mammal-like reptiles, since they are a sort of transition group between
reptiles and mammals), which also had an erect limb posture, though no fur. The
skull morphology of most dragons is
very reminiscent of that of modern crocodiles, with the anteriorly flattened
snout and anteriolateral nostrils. In addition, dragons are often depicted with
conical teeth, just like those of modern crocodiles.
Left: an artist’s impression of a dragon; right: a
modern crocodile. Notice the similarities in snout morphlology. Pictures from –
left: http://www.chrisscalf.com/Sci%20Fi%20Portfolio/SciFi%20Html/Dragon%20Fire.html
;
right: http://ozmagic.homestead.com/australiancrocodileaborigine.html
right: http://ozmagic.homestead.com/australiancrocodileaborigine.html
That skull form
is close to that of rauisuchians, although their skulls are deeper and flatter,
but also similar to that of some carnivorous therapsids, in particular the
gorgonopsids.
Left: a model of the head of the rauisuchian Postosuchus; right: a gorgonopsid
therapsid, a creature not far from the first true mammals, and another possible
precursor or dragons, based on superficial skull similarities. Pictures from –
left: http://www.cmstudio.com/postosuchus.html ; right: http://wonjae.files.wordpress.com/2010/05/gorgonopsid_2.jpg
However, Shirin
made a good point: some dragons are depicted with feathers, which has only been found in the dinosaur lineage. The
rauisuchians diverged from the dinosaur ancestral lineage at the very base of
the archosaur group, which unifies dinosaurs, pterosaurs and crocodylians,
around the early Triassic period, about 20 million years before the dinosaurs
first appeared; the therapsids are separated from the dinosaurs by around 100
million years of evolution. Feathers are thus unlikely to be found in
descendants of either of these groups, since the relation to the feathered
dinosaurs is remote.
Dragons must
then have come from dinosaurs, Shirin argued. The reason why I was doubtful
about that is because of the quadrupedal
stance of dragons – dragons are definitely carnivorous, or perhaps
omnivorous, but the only four-legged dinosaurs were (more or less strictly)
herbivores – no one was adapted for a meat-based diet. Dragons ought to have
emerged from the theoropod group, if
from any. When I later thought about it, the idea makes sense in many ways: the
Theropoda is where most known feathered dinosaurs belong to; they had highly
pneumatised bones – hollowed-out with air holes – making them very
light-weighted, something important for flight; it gave rise to flight at one
definite point, so it had the potential for possibly generating another. However,
the theropods are strictly bipedal, and show no signs of reversal to the (not
directly) ancestral quadrupedal posture.
After some more
thinking, I thought of the spinosaurid
family, a rather odd type of theropods with crocodile-like snouts bearing conical
teeth and rather long, powerful arms. The snout and tooth shapes drew my mind
to dragons, and the well-developed forelimbs could be a start to developing a
four-legged stance. What characterises at least Spinosaurus is the long sail on its back, supported by greatly
elongated upward projections from the dorsal (back) vertebrae. Could the
development of this membrane between roughly parallel bone outgrowths be the
forerunner of the flight membrane on the dragon wings? Could those long
vertebral processes have been to anchor strong back muscles used for flight? It
may seem vague, but there might be a connection here.
Leaving the
dragon riddle unresolved for now, let us turn to another one of our discussions:
Shirin’s hypothesis that hippogriffs
– majestic beasts with the hind part of the body (trunk, tail and hindlimbs)
being of a horse and the front, covered in feathers, resembling an eagle’s –
are close relatives of the
platypus, a modern member of a primitive mammalian group – Monotremata –
which has fur, but a duck’s bill and lays eggs. Half of the platypus is a
mammal, and half is a bird; the same goes for the hippogriff.
An interesting
observation, but the respective mammals and birds they are a chimera of are not
closely linked at all. Ducks and eagles are possibly remotely related – I’m not
sure of bird evolution, but it is not an unreasonable assumption that they are
only distantly related – and the mammalian part of the platypus – a large,
elongated trunk with short, stubby limbs sticking out from the side of the body
and with webbed feet – is not reminiscent of any other mammal I have ever seen,
and certainly not to a horse. If the hippogriff and platypus are sister taxa,
one would expect at least some similarity in the animal groups their respective
body parts mimic.
Left: the platypus; right: an artist’s impression of
the hippogriff. Images from – left: http://troghead.blogspot.co.uk/2012/04/platypus.html#
; right: http://hogwartsrpg.wikia.com/wiki/File:Hippogriff_drawing.jpg
Still, Shirin’s
important observation deserves an explanation, and it can be found in convergent evolution – when unrelated
organisms develop similar traits as a result of adapting to similar conditions.
The main problem with this idea is that the platypus is amphibious (living both
in water and on land), while the hippogriff flies: they have adapted to two
completely different media – water and air, respectively. An analogy to how improper
an argument of convergent evolution between the platypus and the hipogriff can
be found in comparing the fish fins with bird wings.
These are the
kind of things that go through our heads…
Admit that it is awesome! Hahaha
Admit that it is awesome! Hahaha
