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Re: Phytodinosauria status



In a message dated 6/7/01 0:43:23 AM EST, qilongia@yahoo.com writes:

<< All basal dinosaurs, whether ornithischian, sauropodomorphan,
 or theropodan, are basally obligate bipeds; faculative
 bipedality in basal thyreophores, marginocephalians, and
 iguanodontians is not considered implausible based on the weight
 of evidence that they split off separate and all evolved
 obligate quadrupeds. >>

I would disagree with this statement. The basalmost dinosaur could well have 
been an obligate semisprawling quadruped (and arboreal into the bargain). If, 
as I think, the forelimb evolved gradually into a functioning wing 
(progressively losing its outermost digits in the process) in a lineage of 
arboreal dinosaurs, then the much larger ground-dwelling offshoots of this 
lineage should display a trend away from quadrupedality toward bipedality: 
the closer the lineage comes to birds, the more "obligate" the bipedality of 
the cursorial dinosaurs that branch away from the lineage.

So: most primitive are the sauropods (still have the fifth toe), and also the 
most quadrupedal. Next, prosauropods and ornithischians (fifth toe reduced or 
absent, with mt V just a splint), then herrerasaurians, then other theropods, 
etc.

A lot of this evolution happened in the Middle Triassic or even earlier, 
among small forms (e.g., lagosuchians, which by their feet fit best between 
herrerasaurians and later theropods); by the Late Triassic all the major 
dinosaur branches were already well differentiated from one another. Fully 
erect stance and perforated acetabulum probably evolved convergently 2-3 
times within the lineages, the result of the hind limbs taking on a 
considerable burden. Perf acetabulum is also known in ornithosuchians, where 
it almost certainly appeared independently of dinosaurs, and probably for 
much the same reasons.