[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Phytodinosauria status



-----Original Message-----
From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
David Marjanovic
Sent: Thursday, June 07, 2001 11:58 AM
To: The Dinosaur Mailing List
Subject: 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???<<
*
* The fingers get in the way of feathers? I was talking to Phil Currie
recently and we were talking about the third digit in early birds and how he
thought that they would have been enclosed in skin. I told him that I've
seen two Confusisornis specimens that have the third digit perpendicular to
the other two fingers, in both wings. So, the feathers were over/on top of
the digits. Did I just argue against what I first said?

> 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.<<
*
* Ah yes, Dryosaurs, hypsilophonds were quadrapedal.
*

> 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...<<

So, what mammals regrew them? None, that I know of. Of course I don't really
know mammals that much, but I disagree with 're-evolution' of a complete and
functional digit.

>>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.<<
Or, that prosauropods and sauropods are sister groups and had nothing to do
with each other other than at the very beginning of dinosaurs. More research
is in the works on this : >

> 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<<
Ah, sarcasm.

> 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?<<

Then all the dinosauromorphs have been found and we know them all, right?
Wait till you see what's coming from Europe!!!
Tracy