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Re: neoflightless dinosaurs
Guy Leahy (firstname.lastname@example.org) wrote:
<Another feature which could be interpreted as supporting
neoflightlessness in oviraptorosaurs is the extensive postcranial
pneumatization. Oviraptorosaurs exhibit fully pneumatic vertebrae, from
the axis all the way to the middle of the tail. To the best of my
knowledge, oviraptorosaurs are the only theropods which possess pneumatic
foramina in the centra of caudal vertebrae. The only other theropod which
displays any pneumatic features of the caudals is *Acrocanthosaurus*, and
here the pneumatization is superficial, is limited to the neural spines
and does not invade bone. The degree of pneumatization seen in
oviraptorosaurs is more extensive than seen in *Archaeopteryx*, and
oviraptorosaurs appear to be more like extant birds in this feature. The
occurance of fully pneumatic caudal vertebrae also suggests that
oviraptorosaurs possessed abdominal air sacs, which may indicate a more
avian-like air sac lung system in oviraptorosaurs than *Archaeopteryx*.
The occurrance of a more avian-like ribcage (large sternal plates,
ossified uncinate processes) is also consistent with the idea that the
oviraptorosaur respiratory system may have been more like modern birds
Well, I have a few problems in this regard. But let me say that I agree
with most of what Greg said earlier, some of it has a distinct feel of
gross similarity in some derived forms that the basal forms do not express
a relation to. I do not know how either Greg or Guy regard segnosaur
relationships, but nearly all the pneumatic vertebral features
oviraptorosaurs possess, so do segnosaurs. Oddly enough, *Caudipteryx*
does not seem to have them, and this may be a measure of reduction in
Caudi, or convergence between the two clades (ovis and segnos). Something
else, though ... oviraptorosaurs do not have posterior dorsals with
pneumatic openings, though they are camarate in section. The only
theropods with remotely caudally extensive pneumatization in the dorsal
series are basal tetanurans, rather far removed from bird diversity.
*Acrocanthosaurus* does, however, possess pneumatic caudals, as do some
other various theropods, including *Bagaraatan*, where *Acrocanthosaurus*
and *Carcharodontosaurus* have large pleurocoels in the centra that are
broad and pit-like rather than foramen-like.
As for abdominal airsacs, I suggest you read Carrier and Farmer: crocs
have abdominal air sacs, too. They would be expected in most dinosaurs,
and the authors posit that these structures drove the abdominal anatomy of
most dinosaurs with their lattice-work gastalia in most thyeropods and
reduction in sauropods and ornithischians, and reduction in birds, along
with pelvic aspiration.
Jaime A. Headden
Little steps are often the hardest to take. We are too used to making leaps
in the face of adversity, that a simple skip is so hard to do. We should all
learn to walk soft, walk small, see the world around us rather than zoom by it.
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