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Re: polarity of bipedality in dinosaurs (real long--TOO long)
In a message dated 96-09-26 18:26:59 EDT, email@example.com (Jonathan R.
> I still have yet to see a well reasoned, detailed argument
> for the monophyly of the "Phytodinosauria". Bakker did not, as far
> as I could tell, provide one.
And I have yet to see a well reasoned, detailed argument for the monophyly of
"Saurischia." Benton (in _The Dinosauria_) lists 10 putative synapomorphies
uniting Sauropodomorpha with Theropoda, but on closer inspection these
characters are so minor that they could easily have arisen several times
(e.g., lateral overlap of quadratojugal on to the caudal process of the
jugal); or wrong (e.g., presence of accessory intervertebral articulations
[hyposphene-hypantrum] in dorsal vertebrae; manus more than 45 percent of the
length of the humerus and radius together); or present in Ornithischia (e.g.,
heavy pollex with a very broad metacarpal); or too vague (e.g., elongate
caudal cervicals giving a relatively long neck).
For a cladogram uniting sauropodomorphs with ornithischians, see
Cooper, M. R., 1985. "A Revision of the Ornithischian Dinosaur Kangnasaurus
coetzeei Haughton, with a Classification of the Ornithischia," Annals of the
South African Museum 95(8): 281-317.
Cooper calls the clade Cohort Ornithischiformes and notes these dental
characters as synapomorphies:
Laterally compressed, leaf-shaped teeth bearing marginal denticles to the
cutting edge; dentition heterodont with non-recessed, marginal cheek teeth.
Let me note that the teeth of the earliest prosauropods and ornithischians
are virtually indistinguishable and isolated specimens are frequently
mistaken for each other. A recent paper by Gauffre notes numerous additional
dental characters common to prosauropods and ornithischians:
Gauffre, F.-X., 1993. "The prosauropod dinosaur Azendohsaurus laaroussii from
the Upper Triassic of Morocco," Palaeontology 34(4): 897-908.
To these I would add: ilium with well-developed presacral anterior process
embracing caudal dorsal vertebrae and/or ribs (uniting anchisaurid
Prosauropoda, Segnosauria, and Ornithischia) and pendent 4th trochanter
(uniting anchisaurid Prosauropoda and Ornithischia; lost in known
Segnosauria). Bakker gives one further character, the "double breast bone,"
or sternum comprising two unfused elements. In thecodontians the sternum was
apparently entirely or largely cartilaginous (as it is in modern crocs),
whereas in theropods it is variably expressed, being absent or cartilaginous
in some, highly developed with a keel in others (such as birds). Only in
sauropodomorphs and ornithischians is it "doubled."
Bakker evidently feels the relationship between ornithischians and sauropods
(phytodinosaurs) is too obvious to require writing a paper on it. Indeed, if
sauropodomorphs are placed at the base of "Saurischia" between Ornithischia
and Theropoda, then (most parsimoniously) the common dinosaurian ancestor was
herbivorous and theropods were secondarily carnivorous.
> >The current BCF reason for manual digital loss is to accommodate the
> >increasing size of the wing feathers, which extended backward across digits
> >IV and V. The presence of the extra digits hampers the airfoil and, as you
> >note below, adds unnecessary weight to the wing.
> Excess digits don't seem to hamper bats, and pterosaurs just shoved
> theirs up front, without losing them. Seems to me it would make more sense
> to increase the outermost digit (ala pterosaurs and like the hindlimbs of
> many clmbing quadrapeds), first for climbing, then for strengthening the
> wing. Primitive birds which required grasping hands retained the minimum
> number of fingers needed to preserve a grasping function, but an ancestor
> had shed the rest, serially, for whatever reason.
Certainly not, if your wing design includes feathers or featherlike
structures that extend backward from digit II over digits III-V. Bats and
pterosaurs did not have feathered wings; they had membraneous wings, which
makes quite a bit of difference to the shape of the airfoil and the digits
involved in it.
> The third finger was already reduced before flight began, possibly
> on the way to being lost as it is in Tyrannosaurs, and so could not
> form the center of the outer portion of the wing, and was fused into
> the carpometacarpus to strengthen the manus. If they were reducing
> the number of fingers to get out of the way of the feathers, why not
> just eliminate that finger and expand digit II? Why not just build
> you wing from digit V and move the other digits up front to form a
> grasping battery as in pterosaurs? I'm afraid that it makes more
> sense that three fingers were what primitive birds started out with.
It's evolutionarily not very easy to get rid of unneeded body parts. This
goes for digits as well as the appendix. Three fingers are indeed what
primitive birds started out with, if by primitive birds you mean
_Archaeopteryx_ and its immediate kin. But when "flight began," that is, when
the earliest theropodomorphs began gliding and otherwise controlling their
trajectories in the trees, they still had all five manual digits.
> Actually, many, if not most, adaptions involving changing numbers of
> serial elements in vertebrates (eg. vertebrae, fingers, phalanges,
> etc) occur in serial (length changes do too, with notable
> exceptions: I believe Walrus flippers are way weird this way...).
> Serial loss of digits suggests that birds are vertebrates.
Don't be a wise guy. You know I mean serially V, then VI, then III, instead
of symmetrically about the axis of the hand, I and V reducing together.
> Why are so many of the Mongolian Cretaceous forms smaller than North
> American ones?
This is almost certainly because the small forms preserve better in Mongolia
(different geology and taphonomy), not because there were more small
dinosaurs in Mongolia than in North America during the Cretaceous. Mongolian
ankylosaurs and tyrannosaurs are fully comparable in size with their North
American counterparts, and the ornithomimids are bigger. Plus, there are
Mongolian sauropods bigger than any North American Late Cretaceous dinosaurs.
> Uh... *I*N*SU*L*A*T*I*O*N* comes to mind...
Yeah, well, somebody needs to demonstrate that dinosaurs were endothermic and
>required< insulation to retain and regulate body temperature. Be my guest...