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Re: Phytodinosauria status



> << All basal dinosaurs, whether ornithischian, sauropodomorphan,
>  or theropodan, are basally obligate bipeds; [...] >>
>
> 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,

Here you IMHO run into the same problems as Feduccia -- why should the
forelimbs (and only they) evolve into wings in a quadruped, and why should
they lose digits in an arboreal animal???

> then the much larger ground-dwelling offshoots of this
> lineage should display a trend away from quadrupedality toward bipedality:

Apparently prosauropods and ornithischians display the opposite trend.

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

Losing the fifth toe is very easy, it seems. Considering how many times
mammals have done it... and that *Psittacosaurus* has very reduced fourth
and fifth fingers, unlike neoceratopsians...

However, I don't think that prosauropods are paraphyletic, because this
would require sauropods to re-evolve the fifth toes and most of the
respective metatarsals. *Isanosaurus* might support this by its age.

> A lot of this evolution happened in the Middle Triassic or even earlier,
> among small forms (e.g., lagosuchians, which by their feet

and, AFAIK, only 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.

This requires the existence of lots of undiscovered basal dinosaurs and more
basal dinosauromorphs with closed acetabula. AFAIK all basal dinosauromorphs
have partly open acetabula, haven't they?