Saturday, April 19, 2008

Centrosaurus

I've broken my routine and drew a Centrosaurus instead of a theropod. I couldn't decide what theropod I wanted to draw but I was getting a bit bored of theropods.

Anyway, I guess this would be Centrosaurus apertus Lambe, 1904 because of the cranial ornamentation. C. apertus and C. brinkmani Ryan et Russell, 2005 differ in their cranial ornamentations with C. apertus having larger parietal ornamentations. C. brinkmani is also restricted to the Oldman Formation of southern Alberta.

I don't really know enough about ceratopsians to write anything interesting (I just think they look great!) but I might as well comment on the forelimb posture...for the longest time, there has been a debate regarding the forelimb posture of ceratopsians. Some argued for an erect parasaggital posture while others argued for a sprawling posture with the humerus averted laterally. Anatomically, the sprawling posture seems to make sense. However, this left a discrepancy with ceratopsian trace fossils that had shoulder-width manual trackways. The manual impressions are also directed laterally unlike sprawling reptile manual impressions that are directed medially.

As Paul and Christiansen (2000, Paleobiology 26: 450-465) pointed out, this bipolar reconstruction of erect versus sprawling posture is misleading, as "fully erect" is suggestive of columnar elephantine limbs, while most large mammals don't even have this posture. "The issue of forelimb posture in ceratopsians is primarily a question of whether the humerus operated in a largely parasagittal fashion or employed significant mediolateral rotation during locomotion" (Paul and Christiansen 2000, p.451). Manual trackways would enforce the hands to be directly beneath the shoulder - the humerus likely operated in a largely parasagittal fashion but with the elbows slightly averted, instead of a "sprawling" manner. The elbows were probably flexed, so that takes care of the anatomy.

The problem, as I've heard from a friend who's worked on ceratopsian forelimb posture, is as mentioned above that the manual impressions are directed outward but most reconstructions force the hands to face forwards. Now, anatomically, that would rotate the whole forelimb laterally hence the sideways averted humerus and sprawling posture. But if the hands were to face sideways, as the trackways suggest, then the whole forelimb rotates so that it is pretty much underneath the body in an "erect" posture but with the elbows flexed, not the elephantine columnar fully erect posture.

So to summarise, I believe ceratopsians held their forelimbs pretty much underneath their shoulders with their elbows flexed and hands facing slightly outwards.

2 comments:

Zach said...

Makes sense to me. I imagine they adopted such a strange forelimb posture to hold up those giant heads. I like your picture, too, although there's just something off about the area between the head and the pelvis...

But I can't really complain. Ceratopsians are viciously hard to get "right." Besides ankylosaurs, I find them the most difficult dinosaurs to restore.

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