Re: More pterosaur gossip

[MKIRKALDY@aol.com]

Forwarded to the list for Kevin Padian, who is not currently subscribed to the 
dinosaur list.

Mary
______

I agree with Dave Unwin about the problems with the "prolacertiform" 
relationships of pterosaurs, and the difficulties are, as in most phylogenetic 
analyses, mainly with the choice and coding of characters.  

As to the problem of pterosaur footprints, perhaps I should summarize what I 
actually said.  Dave presents it like this:


> The problem is that the Crayssac tracks look just
> like all the other pteraichnid tracks (now reported
> from 30 or so Jurassic and Cretaceous sites from 
> all round the world), and share a suite of unique
> features in common with them - even down to the
> phalangeal formulae which are clearly visible in 
> some of the prints we examined at Crayssac as well
> as at other sites in Spain and North America.

Actually, that's not true of the published literature.  I pointed out in my 
talk that not a single complete phalangeal formula has ever been derived from a 
so-called pterosaur track or pterosaur track (including those from Crayssac) in 
any publication.  In some cases it is not even possible to determine how many 
digits are present, and workers do not agree on which digits they are.  I 
listed a set of criteria that characterize poorly preserved footprints, on 
which I expect all ichnologists would agree, and showed that pteraichnid tracks 
unexceptionally meet all of these criteria.  (As Don Baird noted, such 
footprints are not static records of anatomy and are unlikely to preserve some 
zoologically significant information.)  I listed several features that have 
been predicted for pterosaur tracks and showed that pteraichnid tracks did not 
have them.  And I also showed that every features listed the diagnosis of 
Pteraichnidae was present in the tracks of small crocodiles, such as !
!
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Olsen and I described.  Thus, wh
oever the trackmakers of Pteraichnidae were, they could have been crocodiles.  
Part of the ambiguity is because they are so poorly preserved in anatomical 
respects.  And there is a limit to what you can infer from such tracks, as Don 
Baird taught (see Bull. MCZ 1954, 1957).  

I reconstructed phalangeal formulae in the tracks of several examples of 
pteraichnid footprints, using standard methods, and based on the illustrations 
of those tracks by others.  Each time I was able to fit a crocodile foot 
skeleton into the track, but not a pterosaur foot.  Maybe others will have 
better luck.  In these tracks, the metatarsals are too short, the penultimate 
phalanges are too short, and the MP breadth is too great to match pterosaur 
feet.  Up to now people have merely asserted that pterosaur foot skeletons fit 
these tracks; they have not demonstrated it in any paper I've seen.  Don't take 
my word for it; read the literature and if you find a counterexample, please 
let me know.  

Does this mean that pterosaurs didn't make any of these tracks?  Of course not. 
 But first, I make a distinction between the Crayssac tracks, which we know are 
pterosaurian because they fit some criteria of pterosaur tracks (wide 
intermanual distances, long metatarsals, foot four times as long as broad) and 
pteraichnid tracks from elsewhere, which lack these features.  And yes, some 
poorly preserved Crayssac tracks have features that intergrade into some 
features of pteraichnid tracks from elsewhere.  (We might expect this; it's 
also difficult to tell apart ornithopod and theropod three-toed tracks, if 
they're poorly enough preserved.)  The point to be made from all this is that 
the characterizations, descriptions, and diagnoses of pteraichnid tracks, plus 
discussions about their potential trackmakers, have lacked the kind of 
methodological rigor that is needed to make sure that footprints of crocodiles 
and pterosaurs are not confused, especially when poorly preserved. It is d!
!
!
ifficult to conclude that _Purbe
ckopus_ is pterosaurian and not crocodylian, once you actually get down to 
fitting the feet to the tracks; yet it was assigned a pterosaurian trackmaker 
with full confidence.  So what I am saying is that a lot of this stuff needs a 
critical re-examination and cannot simply be assumed.  It is not to say that no 
pteraichnid tracks could have been made by pterosaurs.  

As soon as I saw the publication on the Crayssac tracks by Mazin et al. in 1995 
I accepted that they were pterosaurian.  This means that pterosaurs could walk 
quadrupedally, which I did not expect, and I still think that the hindlimbs 
were fully capable of supporting terrestrial locomotion (a different issue).  I 
worked at the Crayssac site with Jean-Michel and his crew several years ago, 
and was extremely impressed by the things that these tracks told us.  It was 
also clear that Mazin recognized that these issues were not simple, and did not 
answer many questions easily.  But he's been pursuing them; at the meeting 
Mazin and his co-workers showed a beautifully detailed computer animation of a 
pterodactylus-like animal walking (sometimes in suspended phase) in those exact 
trackways, and this will help us to test models of limb articulation and 
mobility.  Still, there are lots of questions left, as always with pterosaurs.  
This meeting showed us, I think, just how "ambiguous" p!
!
!
terosaurs are (as one participan
t put it), and how varied in their construction and function.  I think the 
participants did a beautiful job of showing this and of advancing a number of 
issues.

There are, for example, two ways that the pterodactylid could have walked in 
order to make these Crayssac tracks.  One basically fits the model that Chris 
Bennett described in JVP in 1997 (although he didn't show a top view of the 
trackmaking, it works kinematically).  This is basically a LM-RP-RM-LP series 
used by most quadrupeds, and it fits the computer animation by Mazin et al.  
The other is the way that Dave Unwin and Don Henderson modeled in their 
computer animation.  They had the left pes and manus moving more or less in 
overlapping rhythm, then the right pes and manus.  This would work too, as long 
as the manus got up and out of the way before the pes overstepped it.  But that 
shouldn't have been a problem especially if pterosaurs were mostly powered by 
their hindlimbs, which I think is right.  And that also seems to fit with Don's 
estimate of relative weight-bearing by the fore and hind limbs, though I 
wouldn't want to speak for him in this application.  

Much more to chew over, but I hope this gives an idea of both the progress and 
some of the remaining ambiguities.  I thought it was a terrific meeting.  -- kp