Re: the headless juvenile Pterodaustro

["Jaime A. Headden" <qilongia@yahoo.com>]

david peters (davidrpeters@earthlink.net) wrote:

<2. Why would a skull and cervical series decay or disappear more rapidly than
the rest of the skeleton? After all, this is a split slab-counterslab
specimen.>

  Why would we not find sauropod skulls? Of the hundreds of specimens of
sauropod, 15 different sauropod skulls are known. The most prevalent bones
found are caudal vertebrae and limb bones? Why? Or more to the point, why so
some *Archaeopteryx* specimens come intact and complete (Berlin, Eichstätt),
some disarticulated but mostly there (London, München), some intact and
articulated but not all there (Solnhofen, Teyler/Haarlem), and some just
utterly broken and scrambled (Maxberg, Ninth)? Or why do some "dinobirds" from
the same formation in Liaoning Province have integument, and others don't? Even
of the same taxon? Why so many *Triceratops* skulls and less so body skeletons?

  The prevalence and artifactual nature of inconsistent preservation in
taphonomic processes are replete with such "whys" and these usually have
environmental processes connected to them. Animal scavenging, differential
substrate embedding for some sauropods, the acidity of the lagoon or lake
bottom in which a body drops corroding all non-osseous material before burial,
etc. Collection also plays a small part in the questions raised above. Making a
meaningful why out of these has been the task of paleontology for decades. In
this case, however, loss of a juvenile's skull can be explained (as I
suggested, using the disarticulated vertebral column but intact limbs as a
clue) to scavenging.

  In Dave's tracing, he identifies a broad semi-circular shape as a "dewlap",
but given the mess of lines used, this broad uninteresting region to one side
of a mass of squiggly lines can be explained just as easily as a smooth region
surrounded by the irregular splitting of a slab. Or maybe a frontoparietal. Or
maybe irregular splitting of a slab. I have previously commented, as has Chris
Bennett onlist, about the funny condition that happens when you break a slab
into part and counter-part, this splitting occuring as a weak-point between the
cementing of two layers of rock due to the presence of an inclusion. 1) the
whole inclusion is usually, but not always, in the same plane; some bones, as
illustrated by xray of a specimen of *Dorygnathus* in Wellnhofer's Pterosaur
book, will lay within the mass. This is true also of the holotype *Microraptor*
mainpart (which was combined with parts of *Yanornis* to make a "Archaeoraptor"
-- or in Czerckas slang, *Archaeovolans*, now a synonym of *Yanornis*). This is
caused by bones being preserved in a non-horizontal plane relative to the
horizontal bedding plane; fractures of the slabs reveal peices, not a whole.
However, another example occurs called "pedestalling" in which the bones sit
atop a base of rock, and the counterpart is this feature in relief. This occurs
in the majority of pterosaurs from Solnhofen quarries, as well as the
aforementioned *Archaeopteryx*.

<3. Of what use that long string tail is â?? or what an aerodynamic burden it
must have been â?? are beyond my ability to answer.>

  First, prove it exists. And yes, I see on the mainslab what Dave is looking
at. And no, if there is no hint of a wing membrane but preservation of
"impressions" of a skull and dewlap and a tail, I think this is a false signal
brought about by examining dark regions compared to light regions.

  A large series of curved irregular lines to the right of the skeleton in the
photo have the appearance (to this eye) of small "plateaus" of rock, formed
from the pedestalling effect of slab splitting. One can look at virtually any
slab of rock and see the same effect. In fact, one can do this with granite
pebbles you get from quarries, where fracturing of flakes and splitting from
other rocks causes interesting patterns of plates, smooth surfaces, and
irregular lumps all over that define the classic "shapeless" rock.

<1. There is no question that this specimen had a head and cervical series once
before.>

  I'm sorry, but I am questioning this right now. So there IS a doubt. The
presence of a contiguous series of dorsals, a few disarticulated cervicals, and
no skull are rather suggestive of a rather neatly removed head. It's a just so
story, but it explains the data without entering in line tracings.

  There actually appears to be a non-tracing inclusion above the knuckle of
mdIV-2 and mdIV-3. Dave doesn't illustrate this.

<2. The ephemeral head and neck matches in all aspects the embryo described in
Nature and it resembles the skulls of adult Pterodaustro. This type of soft
crest is seen in a skull-only specimen and in other ctenochasmatids. The gular
sac is to be expected.>

  The embryo described in _Nature_ is not illustrated save for a region of the
ribs, humerus, and metacarpal IV. This also illustrates a large problem with
the photo-interpretation: if the shell region to the right of fig. 1c in the
Chiappe _Nature_ paper were not listed as "sh", would Dave identify it as
shell? Would be be aware of the microcomposition of the material and be aware
if it were an impression in the rock, a bone, or membraneous residue? Or shell?
How would he tell?
 
<5. Wing unguals and manual digit V are also seen, but rarely reported, in most
other pterosaurs. Typically they do not preserve as well.>

  Only from Dave. No one else examining the specimens for the last 200 years
has seen it. And I doubt it is due to them "not knowing what to look for". The
region in question is a short, arcuate region evenly segmented by four lines
into five segments distal to a round bone at the proximal end of the fourth
metacarpal, which may be a carpal itself. If this round actual bone is a
carpal, then it's very possible this curved element, if a bone and if real, is
just a feature of the concordant metacarpals. Or a pisiform element. Or maybe
it';s just a trick of light. at the base of this curved feature Dave
illustrates a dark mass on the proximal end of thefourth metacarpal itself,
which to me at any rate appears to be a break in the bone, but, it is possible
this is a discolored region due to bacterial activity. The prominence of a
light region of bone which Dave sees as the mcIV directly adjacant to this and
the abrupt margins imply this may be so, but then ... this is a photo ... and
interpretations are probably as terrible as Hoagland's.

<In any case, here is a great demonstration (albeit at reduced resolution) of
the way one can pinpoint details in a photograph with a precision tracing and
make it available worldwide. Now, whether what I see is valid or not, at least
I hope there's no question that this technique can be useful for sharing data
quickly and precisely.>

  The use of camera lucida and drawn-by-eye sketching or tracing has been in
use for over a century for fossils. There is nothing new with this technique
other than finding new artifacts and new interpretations. Provative data is
still a far way off. Thus no new information appears to exist. Sorry.

  Cheers,

Jaime A. Headden

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


                
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