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Re: Let's find the basal Ornithodire.



Dave Peters (davidrpeters@earthlink.net) wrote:

<The long arm is the last thing that shows up. After most everything else
is in place. But note the juvenile phase in Longisquama has relatively
longer arms and shorter legs, as in pteros.>

  I do not agree that a "juvenile phase" in *Longisquama* exists. The
highly eroded lower right edge of the holotypic slab, often showing entire
irregular sections missing (which Dave shows tracings overlying further
"fronds" extending), shows that if there was any material present
posterior to the dorsal series exposed, it would be within the slab, if
unprepared -- IF present. NO ONE has corroborated the increasing
"discoveries" of neonates, and apparently the entire absence of an
intermediate, initially ossifying, pterosaur non-adult. As has been shown
by others and has yet to be refuted, juveniles with NO trace of
ossification post hatching OR post birth DO NOT delay ossification of
their bones; they are NOT mobile without hardened bones to support their
musculature and weight. This is a simple biological fact, and Dave does
not regard this strongly enough in light of the "neonates."

<As in Longisquama. Not exposed in Sharovipteryx.>

  The sternal elements of *Longisquama* are not, in my opinion,
sufficiently oriented to show "broadening" as opposed to, say, a fully
terrestial lizard, with shorter forelimbs as hindlimbs. The prescence of a
lacertilian shoulder system, including a broad Y-shaped interclavicle.
*Megalancosaurus,* a dyed-in-the-wool arboreal or aquatic animal and
non-volant in any way, has a broader, if even MORE fixed shoulder
apparatus, and extremely elongated, broad sterna.

<As in Longisquama and Cosesaurus. Not exposed in Sharovipteryx.>

  I don't agree with this assessment. The "keel" I see is a
misinterpretation of a dislocated or slightly separated interclavicle in
*Longisquama,* and the "nature" of a keel in *Cosesaurus* is
uncorroborated except in vague bright-exposure tracings, in which the
majority of the pectoral region is obscured beneath the holotype's
anterior dorsals and ribs.

<covered above. shows up in juvie Longi.>

  My refutation of this is above and elsewhere.

<by this you mean solidification of the ventral pelvis? If so, shows up in
Cosesaurus + L + S. by this you mean expansion of the anterior process of
the ilium? If so, ditto the above.>

  By the expansion of the sacrum, I referred to expansion of the sacrum,
i.e., incorporation of further central elements from the dorsal or caudal
series, from the general 1 or 2, up into 3, or four, as present in basal
pterosaurs.

<not sure if anything in the dorsal spine is locked up in basal pteros.
Curious about this though. Please explain.>

  Rigidification of the dorsal series, aka, "locking up" occurs through
orientation of zygapophyses, development of addition vertebral processes
(zygosphenes, hyposphenes, xenarthrans, etc.,) -- this endorses reduction
of the flexibility of the spine (useful in a terrestrial biped or aquatic
animal with an elongated dorsum [if for instance that *Megalancosaurus* is
aquatic]). In basal pterosaurs, vertebrae become small, centra smaller --
they become essentially additional intervertebral articulations in the
style of the zygapophyses, and they bear condyle-in-socket intercentral
surfaces. This reduces much intervertebral movement. So far, such
vertebrae are unknown in proposed pterosaur ancestors. They retain
elongated "lizard-like" spines, with the sole exception of drepanosaurids,
in which the anterior dorsals are expanded in such a fashion to prevent
dorsiflexion. Such dorsiflexion (as well as latiflexion= turning to the
side) is a hazzard to a pre-pterosaur learning to fly. If the arms are
already elongated in *Eudimorphodon* or *Preondactylus,* it stands to
reason that, along with the modifications to the vertebrae mentioned
above, the ancestor should have this, and as in birds, this may have
occured BEFORE the arms become effective aerial locomotors.

<They don't close up in basal pterosaurs. See Peters 2001. Historical
Biology 15(4).>

  These DO close up in pterosaurs: they become fixed, locked elements that
serve a function: to prevent the wrist from moving in an entire plane,
normal in terrestrial animals for locomotion. Since pterosaurs rotate the
effective posture of the humerus lateral or dorsal 90 degrees to its
position in other terrestial animals, the "dorsal" side of the manus faces
medially, allowing flexion at the wrist ONLY inboard of the ulna. Birds,
as pterosaurs, fuse or lock their wrists into effective "blocks,"
increasing as the pterosaurs got larger (i.e., towards Pterodactyloidea)
into a proximal and a distal block, instead of a set of distinct, but
"locked" elements of the ulnare, radiale, and all distal supra-metacarpal
carpals.

<Actually its the elongation of the fourth metacarpal relative to I-III.
That's in L + S. And are there such things as proximal metacarpals? Did
you mean proximal carpals? If so, that's probably present in Longisquama,
but it's a bit messy around the wrist. And tiny.>

  My next question should have clarified this, but I do admit I was vague.
I mean reduction in mean size, reduction in diameter, circumference, area,
etc.

<In L + S. See pterosaurinfo.com>

  Not to the extent that ANY lizard doesn't get a long fourth : third
digit ratio. However, both *Longisquama* and *Sharovipteryx* do NOT
preserved osseous or even "impressed" material that ANY ONE ELSE has been
able to verify.

<Am I the Dave you are referring to?>

  Yes.

<If so, can lizards draw the humerus close to the trunk? If so, then
Longisquama and Sharovipteryx could.>

  Indeed, the former is preserved in such a position, *Sharovipteryx* is
... equivocal ... depending on how much gratitude one gives to some of the
tracings I have seen on this, as well as my own observation: I have been
unable to find a complete antebrachium or humerus in the material though
what seems to be present is _typical_ for a quadrupedal lacertilian [and
I've seen a few].

  My argument is that Dave has stated elsewhere that pterosaurs held their
humeri parallel to the trunk. I argue that this is NOT possible for the
majority of pterosaurs given the lateral orientation of the glenoid with a
limited caudal component, preventing the humerus from turning caudally in
the position Dave so often positions it in.

  BTW, as a clarification, I write David Marjanovic for _that_ David, but
"Dave" for Dave Peters. Sorry for the confusion.

<Ah, Luke...look beyond the obvious. Start with the toes. Do your
cladogram. Let PAUP guide you. Sharovipteryx even has a long neck. It's
going off in another direction. Still, PAUP says it's a sister taxon.
Closer than almost anything else known to exist.>

  I don't have the ... patience ... to mess with PAUP* for the purpose of
plugging in material from elements gleaned from grain-sized structures in
slabs like that in the VERY SMALL *Sharovipteryx* holotype. My
examination, though brief, of a cast of the *Longisquama* holotype showed
that these slab structures are so ... irregular and massive in proportion
to the clear impressions that observations of other structures are
artefactual, not actual. This is a bias, I admit.

<More importantly, if a taxon fulfilled ALL of your requirements, guess
what, it would be a pterosaur!  You have to allow for something to evolve
from "not a pterosaur" to "a pterosaur".  Think about the theropod-bird
connection and make the parallels in your mind work to accept this. Or try
a cladogram.>

  It may also be a bird. *Confuciusornis* satisfies these criteria. Not
that I hold birds and pterosaurs as sister-groups, but this supports that
these features would refutably infer the progression of a terrestrial or
aquatic animal into a volant, actively flighted animal. Since Heilmann, so
far every single "pro-avian" model has essentially conformed to dinosaurs.
Since Heilmann, no such progression has occured to produce a pre-pterosaur
intermediate between pterosaurs and ANY reptilian group, "ornithodiran,"
"protorosaurian," "prolacertiform," or otherwise. While I admire PAUP* for
the tool it is, I DO strongly stress that it is a tool, and not a means by
deriving an end-all hypothesis to be thrown up in support of any
relationship it may "support." As David said, one's data put into the
"machine" will get you your result. If you put a juvenile animal in it,
prepare to be surprised when it finds a juvenile grouped with adults of
the same species. Test this by using neotenic salamanders, hominids with
growth series, birds, cats, etc.

 ... Indeed, this response does help, but I fear it does not clarify my
position much clearer than has been said, and aside from my other post and
eventual response on neoteny/paedomorphism, I will not go further unless
more data is to be offered.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


                
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