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Re: More pterosaur gossip
Forwarded to the list for Kevin Padian, who is not currently subscribed to the
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 !
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
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