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Re: Rotund Triceratops
And now it begins...
Although I believe BHI is sincere in their work, I think the poster was a
classic example of presenting selective data and arm-waving.
Coracoids: It has been discussed more than once on the list that coracoids
were closer to one another than older mounts have shown, but there are
developmental reasons why they cannot litterally meet in a bone to bone
contact. Also, while the poster dismissively refers to a "ghost" cartilagenous
pre-sternum, it should be noted that the anterior surface of ceratopsian
sternals are indeed pitted and rugose in a manner consistent with a
cartilagenous extension to the sternum. Likewise the AMNH Centrosaurus
sternals show clear evidence of attachment of cartilagenous xiphisterhal ribs.
Should we ignore them because they don't ossify?
Ribcage: As for the ribcage, there are many good and well documented
ceratopsian ribcages (e.g. the AMNH Centrosaurus), and none of them look like
the one reconstructed by BHI. The poster noted correctly that many of these
specimens are laterally conpressed, but failed to note that the Leptoceratops
specimens they showed were obviously dorsally-ventrally compressed. In fact,
one of the specimens appeared to have overlapping coracoids, which probably
wasn't their position in life! Perhaps if they had provided a quarry map it
would have cleared up the issue of distortion in their triceratops specimen,
but instead we were given a bunch of photos of the mount and told to take their
word for it (I was in fact actually told this, though not by Pete Larson).
While on the topic of rib cages, it's worth looking at Psittacosaurus
specimens. Some have been preserved on their stomachs in a "sleeping"
position, and it is worth noting that the ribcage is always unnaturally
distended laterally in those specimens. I'd hazard to guess that the same
thing happened in the BHI Triceratops.
This isn't to say that I think they got it all wrong. Having mounted a
score of skeletons in Thermopolis, I'd agree that the hindlimbs of Triceratops
(and all dinosaurs) swung out as they swung forward, although I'm not sure that
the enlargement of the anti-trochanter in some ceratopsians is functionally
related to this. Also, I think the general shape of the ribgace is correct,
with the widest part in the middle and the fairly sharp narrowing towards the
pectoral girdle. I just think their specimen is distorted in a manner that
over emphasizes this shape.
As for scapular mobility...for the moment I have no official opinion,
although one talk st SVP clearly showed scapular moblity in extant crocodilians
far in excess of what anyone has previously assumed, so I'd hesitate to rule it
out in dinosaurs a priori.
That's my 2..er 3...ok 5 cents worth.
Zoology & Physiology
University of Wyoming
Laramie, WY 82070