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Re: Pterosaur morphological evolution



Tim,

Briefly: Buffetaut et al. (2002) rejected the idea that Hatz. was
flightless because the structure of its humerus, not the limb
proportions. Indeed, we don't have a single complete bone for Hatz. The
expanded humeral diaphysis and millimetre-thick bone walls are
consistent with being a flier, as is the oversize deltopectoral crest.
For what it's worth, I figure that the massive DPC would be one of the
first things to go if pterosaurs abandoned flight: the energy and
material put into developing and sustaining  their flight musculature
would be totally wasted in a grounded form, so I imagine all
osteological correlates of it would disappear quite readily.

On a related note, has anyone ever wondered why
Raeticodactylus/Caviramus has such a narrow humeral diaphysis? It's
seriously stick thin. It's still got a large DPC and flight finger,
though, so I don't know that it necessarily reflects flightless habits.
Interestingly, its femur is probably more robust than its humerus, a
feature only otherwise seen in Preondactylus. Looks to me like
Raeticodactylus/Caviramus is just begging for some a nice biomechanical
investigation of its limb strength: it would be particularly interesting
to know if it could quad launch.

Right, back to work.

Mark

--

Dr. Mark Witton
www.markwitton.com
Palaeobiology Research Group
School of Earth and Environmental Sciences
University of Portsmouth
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Portsmouth
PO1 3QL

Tel: (44)2392 842418
E-mail: Mark.Witton@port.ac.uk

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>>> Tim Williams <tijawi@gmail.com> 07/07/2011 07:05 >>>
Michael Habib <MHabib@chatham.edu> wrote:

> Looks to be a very interesting paper!  Pretty impressive for a
Masters Thesis project, too.  My only disappointment was the caption of
the press release labeling Quetzalcoatlus as "probably flightless" -
> considering that Mark Witton and I wrote an entire PLoS ONE
manuscript debunking this idea (more or less), it was a bit annoying. 
But so it goes.


Your PLoS ONE paper convinced me that even the biggest azhdarchids
could fly; just as the Witton & Naish PLoS ONE paper convinced me that
azhdarchids were terrestrial stalkers.


But what these papers also make me wonder is that IF an azhdarchid was
flightless, then we might not expect to see much change in its overall
limb proportions.  Unlike terrestrial flightless birds, in which the
forelimbs (wings) become truncated through lack of locomotory use, a
terrestrial flightless pterosaur would presumably retain elongate
forelimbs for terrestrial quadrupedal locomotion. It would shift from
a part-time terrestrial stalker to a full-time terrestrial stalker.


Apparently the original description of _Hatzegopteryx_ (which I
haven't seen) raises the possibility of this giant Hateg azhdarchid
being flightless.  (This idea is tied into the "island effect" -
herbivorous dinosaurs got smaller due to island dwarfism, whereas
_Hatzegopteyx_ is a possible example of island gigantism.)  However,
this flightless hypothesis for _Hatzegopteryx_ was rejected by this
same study due to the length of the forelimb elements, which retained
proportions consistent with flight.  But if the azhdarchids were
terrestrial stalkers (see Figs 8 and 9 of Witton & Naish, 2008) then a
secondary loss of flight might not have much effect on the postcranial
proportions - except that the hypertrophied fourth manual digit would
probably shrink to oblivion.  Most of the changes would be reflected
in the integument - such as in the disappearance of the patagial
flight membrane.


I'm not actually saying that any known pterosaur was flightless.  Just
that, hypothetically speaking, the effect of flightlessness in
pterosaurs would be very different to their avian cousins, owing to
the different (quadrupedal vs bipedal) terrestrial locomotor modules
in the two groups.




Cheers

Tim