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Re: A few more...




On Fri, 29 Sep 1995 Dinogeorge@aol.com wrote:



> 
> Specifically in this situation (prosauropods and sauropods), there is no need
> to postulate a reversal, merely the independent rudimentation of the fifth
> digit in Phytodinosauria and Aves. To me, this is more "parsimonious."

Actually both scenarios are equally parsimonious, involving two 
evolutionary steps. The only option to resolve the issue is to check 
congruence with other characters, this overwhelmingly comes out in 
support of a Dinosauria composed of Saurischia + Ornithischia with the 
"lagosuchians" as a paraphyletic outgroup. 

 The
> common ancestral phytodinosaur would have resembled a small, tree-climbing
> thecodontosaurid with five well-developed (grasping?) pedal digits (not to
> mention five well-developed manual digits, including a thumb with a powerful
> ungual phalanx). Descendant forms that retained the five pedal digits became
> sauropods; descendant forms in which the fifth digit "vestigialized" through
> the progressive loss of the phalanges until only a . Rudimentation of the 
> fifth
> digit occurred independently in the theropod-avian subclade of Dinosauria
> (Theropodomorpha--or you can simply call it Aves at this point, if you like).
> The most primitive-known theropodomorphs, such as lagosuchians, _Eoraptor_,
> and herrerasaurians, had only a splint metatarsal V, not to mention a lot of
> other theropod-like features, so I assert that it would have taken quite a
> string of reversals to derive something like a sauropod from within this
> region of the dinosaur cladogram.
> 
By assuming that "lagosuchians" (I am assuming you include Lagerpeton 
and Marasuchus) are more closely related to theropods than to other 
dinosaurs you imply that: the 9 synapomorphies linking Marasuchus to 
Dinosaurs s.s.  evolved convergently in both theropodomorphs and 
phytodinosaurs; 11 dinosaurian synapomorphies evolved convergently in 
"higher" theropodomorphs (those above and including Eoraptor) and 
phytodinosaurs and 
6 Saurishian synapomorphies evolved convergently in "higher" theropodomorphs 
and non-ornithischian phytodinosaurs. Is all this convergence really more 
likely than the reversal of a reduced fifth metatarsal? Particularly if 
it may have been embryologically present all along (just like the hoatzin 
claws).

Adam Yates.