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Re: Archaeopteryx Pes
Nick Longrich wrote-
Actually, Jacques Gauthier was the first to propose a
hyperextensible digit II in Archaeopteryx (in his 1986 theropod
phylogeny paper) and observation of the Eichstatt specimen (or casts
like the ones I've seen) makes it pretty clear he's right: the end of
the first phalanx in digit II has a distinctly enlarged articular
surface that would have allowed the toe to retract. It is much larger
than the other joints, and like the rest of the skeleton the toes are
very well-articulated, as far as I can tell.
Unfortunately, I don't have a copy of Elzanowski and Pasko's paper on hand.
If I remember correctly they determined phalanx II-1 of the Eichstatt
specimen was preserved upside down, so that what appears to be the dorsal
surface is actually the ventral. Thus, the distal articulation is expanded
ventrally and would not have been able to retract the rest of the digit.
Other phalanges are also preserved at different angles, like IV-2 which has
its ventral side up. I'll check the article tomorrow and correct my
statements if I got any details wrong.
It's hard to see, since on one side the phalanx is partially
obscured by another digit, but in *both feet* the condyle extends
dorsally, not ventrally. Arguing for a 180 degree rotation in both,
while leaving the rest of the foot largely undisturbed, really would
stretch it. Also, check out Fig 8. and particularly Fig. 9 in Chiappe
et al., as well as the right foot of the Changchenornis type. You'll
notice that the second digit is slightly extended at the joint
between the first and second phalanges, a common occurrence in
articulated feet of Confuciusornis.
A good reference to have on hand. Currie and Carpenter argue rather
strenuously that _Acro_ is an allosaurid and not a carcharodontosaurid,
though most of the character states they mention are also consistent, to my
mind, with a position for _Acro_ as a primitive carcharodontosaurid, more or
less intermediate in form between _Allo_ and the carcharodontosaurines
(_Giga_ and _Carcharo_). That is certainly what it looks like to me.
They do have a cladogram, but for an outgroup they use the highly derived
Abelisauridae rather than the more conservative _Ceratosaurus_ (or something
like _Torvosaurus_), which I would have preferred. I also disagreed with
some of their character polarities.
Yeah, some megalosaurids, spinosaurids, etc. might have been really
good to include. Good taxon sampling tends to be more important than
getting lots of characters in reconstructing phylogeny, and like you
say abelisaurs are pretty weird animals.
In general I think you are likely right about
Acrocanthosaurus, with one modification. I'd suggest that
Acrocanthosaurus may be sort of intermediate between
carcharodontosaurines and sinraptorids. The skull roof has a lot of
unusual similarities in these guys, what's more they all seem to have
low finbacks (I'm going by Paul's restoration of Giganotosaurus for
carcharodontosaurs). Finbacks are fairly common, being in spinosaurs
and Ceratosaurus after all, but all the same it's suggestive. So
where does Allosaurus fit in? Well, I had been figuring, "oh, no
prob, it'll fall out with those guys..." I have yet to get Allosaurus
falling out with a sinraptor-acrocanthosaur-carcharodontosaur clade
since I started entering these guys into my matrix and the more I
look at it, the less I'm sure it even should. Sinraptorids all have
an obturator foramen in the pubis (yep, apparently even Sinraptor...
see Gao's paper on "Yangchuanosaurus hepingensis") which Allosaurus
sure doesn't, and they have big caudal hyposphene-hypantrum
articulations, while Allosaurus apparently lacks them in the proximal
caudals (though I think the last sacral/first caudal articulate with
one) and though the putative fourth metacarpal of Sinraptor wasn't
found with the hand (which makes me want more evidence it is a fourth
metacarpal), that could still be a fourth metacarpal in which case
you'd have another primitive feature... and in both the obturator
process and the ascending process of the astragalus, Allosaurus looks
more coelurosaur-like than Sinraptor-like. Some of the putative
synapomorphies don't stack up either, "W-shaped premaxilla-nasal
articulation" is in *much* more primitive theropods. The expansion of
the antorbital fossa on the lateral part of the nasals could be a
synapomorphy, but then it's in Monolophosaurus which doesn't even
have an obturator process; on the other hand the prominent nasal
ridges in both Allosaurus and sinraptorids could be synapomorphic.
Then there's Fukuiraptor- it has a larger ascending process of the
astragalus than any putative allosaur, megalosaurid, ceratosaur, etc.
Which raises a point: all this arguing about taxonomy and
what to call this or that seems a little misplaced. Before we can
erect a taxonomy, we need a decent idea of how things are related,
and we don't have that yet, we've got a systematic quagmire we're
trying to use as the foundation for our taxonomy. _Monty Python And
the Holy Grail_ has this to say about building on a quagmire:
"Listen, lad. I built this kingdom up from nothing. When I started
here, all there was was swamp. Other kings said I was daft to build a
castle on a swamp,but I built it all the same, just to show 'em.
It sank into the swamp. So, I built a second one. That sank
into the swamp. So, I built a third one. That burned down, fell over,
then sank into the swamp, but the *fourth* one... stayed up! !"
Foundation, then structure. Otherwise, we get situations like
a Ceratosauria that uses Coelophysis as a reference taxon, so
"Ceratosauria" may exclude Ceratosaurus... it just makes stuff more
complicated. More than anything, we need taxonomy to be useful, but
if stuff like this happens and names proliferate because we are too
eager to rush in and name every concievable node and stem... well,
heck if I know a solution to it all.
"I'm not dead yet."