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Re: A few more...
In a message dated 95-09-29 16:47:24 EDT, swf@ElSegundoCA.ATTGIS.COM (Stan
>Actually, given the limited knowledge we have of dinosaur embryos,
>we cannot rule this out in the case of basal dinosaurs vs. sauropods.
>I do not find the reversal required there to be all that impossible.
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." 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 splint metatarsal remained
would be, in serial order, prosauropods, segnosaurs (pelvis opisthopubic),
and ornithischians (predentary bone developed). 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 the way, all this evolutionary change occurred initially and fairly
rapidly in small forms a meter or less (perhaps even much less) in total
length during the Middle Triassic. Horrible fossil record! Incidentally, I
suspect segnosaurs remained smallish facultatively bipedal animals until
their mini-radiation in the Cretaceous. This may explain some of their
convergent theropod features, and the lack of a Triassic-Jurassic fossil
record of the group.