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Coelurosauravus: glider? or bluffer?
Hailed in Science and elsewhere as the the earliest known example of a gliding
reptile, Coelurosauravus, seems to fall flat when looked at with a critical eye.
Frey, Sues and Munk (1997) reported that Coelurosauravus was different than
other gliders and flyers throughout prehistory because its gliding membrane and
its internal supports were entirely dermal in origin. No traditional skeletal
parts were used in the making of this wing.
Therein lies the problem.
Without mechanical pathways (I-beams or bones) leading back to the vertebral
column, these membranes are going to fold up. Draco uses ribs. Icarosaurus and
Kuehneosaurus use ?ribs (well, weâ??ll get into that later), flying squirrels,
flying lemurs, birds, bats and pterosaurs use limbs. Only Coelurosauravus uses
spars/rods that are dermal in origin to supposedly support itself in the air.
The limbs are free.
But think about it. Wouldnâ??t such a membrane simply fold up like an inverted
umbrella when filled with air during a fall? Images of archival film footage of
those magnificent men in their flying machines and their predecessors also come
Frey, Sues and Munk reported that all of the spars originated in the arm pit.
My reconstruction indicates that about half of the 23 spars originated on the
first dorsal rib and the other half bore a one-to-one correspondence with the
remaining ribs. So there _is_ a rib connection in Coelurosauravus. The only
problem is each rib connection is a universal joint, a single point that is
capable or rotating in any direction. And every spar is the shape of a strand
of angel-hair spaghetti. Not much support here, engineering-wise.
Maybe the ribs in Coelurosauravus simply spread out in order to make this
skinny little bluffer appear much bigger than it was, similar in concept to the
extradermal membranes in the Australian frilled lizard. Draco volans also
spreads its ribs to impress â??the ladiesâ??. And â??the ladiesâ?? spread their
ribs to show disinterest. The head crest is a sign that Coelurosauravus was
into intimidation, whether intraspecific or otherwise. It also has a pretty
substantial anterior process of the ilium, which permitted some form of bipedal
configuration, whether running or standing. So we can picture Coelurosauravus
standing bipedally and spreading its spars. Pretty impressive.
A one-shot wonder?
Maybe Icarosaurus and Kuehneosaurus are descendants of this bluffer. Previously
the spars in the spreading membranes of Kuehneosauridae were considered true
ribs because they were reported to connect to hyperelongated transverse
processes. But is that the case?
A closer look at Icarosaurus shows that the hyperelongated transverse processes
are actually laterally-oriented ribs fused to traditional transverse proceses.
They just look like giant transverse processes. Unfused samples are in the
anterior ribcage, between the scapulae. So that makes the membrane spars
_dermal_ in origin, exactly the same as in Coelurosauravus. The major
difference in the Kuehneosauridae is the connection between rib tip and
proximal spar. Itâ??s a vertical cylinder, and both the spar and the rib have
become I-beams. That makes wing folding as easy as a Navy jet with all the
strength needed to glide.
Donâ??t go by the Colbert (1970) reconstruction. He forgot to rotate the plane
of the transverse processes/ribs into the vertical plane where they belong, but
instead left them as they appeared in situ, like spread-out playing cards. His
reconstructed ribs canâ??t fold posteriorly, although he knows they did fold
because his paper demonstrates their range of motion.
No one has made the connection between Coelurosauravus and the Kuehneosaurids
before for good reason. The skulls of Icarosaurus and Coelurosauravus are quite
different. The latter has a naris displaced from the anterior of the skull, but
that is a primitive character shared with Huehuecuetzpalli mixtecus, a basal
lizard, and Protorosaurus, a basal protorosaur. Coelurosauravus has a long neck
not shared with Kuehneosaurids, again a primitive character going back to early
diapsids. Icarosaurus has a deep pelvis with no anterior ilial process. It also
has no crest and a short tail and many fewer spars, but all of these can be
accounted for by a change in lifestyle from bluffing to gliding over 20 million
years. One thing that hasnâ??t changed is the unique metapodial configuration
in which mtII is the longest in the set. Yes, thereâ??s a match.
So, if valid, somewhere between Coelurosauravus and Icarosaurus, a sister taxon
with the beginnings of I-beams for spars jumped and survived the fall only to
give rise to a new generation of ever improving gliders. But grandpa
Coelurosauravus probably never got off the ground -- IMHO.
Colbert E.H. 1970. The Triassic gliding reptile Icarosaurus. Bulletin of the
American Museum of Natural History 143(2): 85-142.
Frey, Sues and Munk 1997. Gliding Mechanism in the Late Permian Reptile
Coelurosauravus, Science, Vol 275, Issue 5305, 1450-1452 , 7 March 1997
Reynoso V_H. 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate
(Reptilia) from the Early Cretaceous of Tepexi de RodrÃµ Ã?guez, Central Mexico
Phil. Trans. R. Soc. Lond. B (1998) 353, 477-500.