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My Thoughts on Limusaurus' hand
I finally had a chance to check out Xu et al.'s claims regarding Limusaurus and
tetanurine hands being II-III-IV-V instead of I-II-III-IV, and my feelings
mirror those of Andrea Cau on his Theropoda blog.
Xu et al. (2009) hypothesized Limusaurus may indicate metacarpals I-II-III-IV
of tetanurines are homologous to metacarpals II-III-IV-V in other amniotes,
based on several characters.
Digit I in Limusaurus and Aucasaurus are highly reduced, with no phalanges. Yet
Ceratosaurus shows an articular surface for phalanx I-1, showing the condition
in Limusaurus may be derived within ceratosaurs as opposed to a basal
ceratosauroid (ceratosaur+tetanurine) state.
Metacarpal II is medially twisted in Limusaurus, Dilophosaurus and some
Coelophysis specimens, similar to metacarpal I in other saurischians.
Unfortunately, this is not easily determinable from most figures, so the
condition in basal tetanurines is unknown.
While Xu et al. note metacarpal III lies ventral to metacarpal II in
tetanurines, as metacarpal IV does to III in Limusaurus and coelophysoids, this
is also true of metacarpal IV in tetanurines (e.g. Guanlong, as seen in its
supplementary information; Tanycolagreus).
Similarly, while metacarpal I does not overlap II in non-tetanurines, this is
seemingly also true in Xuanhanosaurus and Acrocanthosaurus (overlap is present
in "Szechuanoraptor", Megaraptor, Torvosaurus, Allosaurus and Guanlong however).
There is a dorsolateral flange on metacarpal II of Dilophosaurus and Limusaurus
which is similar to one on metacarpal I of some tetanurines (e.g.
"Szechuanoraptor", Allosaurus, Guanlong). But Xuanhanosaurus and Megaraptor
also have such a flange on metacarpal II, but not I, like basal theropods. Both
flanges seem to exist in Acrocanthosaurus, while none exist in Aucasaurus and
Xu et al. state metacarpal II is more robust than I in non-tetanurine
theropods, homologizing it to the robust metacarpal I in tetanurines, but the
situation is more complex. It's clearly the size of the base which is
important, since even Coelophysis and Dilophosaurus have metacarpal I shafts
more robust than those of II. Yet basal tetanurines (e.g. Xuanhanasaurus,
"Szechuanoraptor", Torvosaurus, Megaraptor, Acrocanthosaurus, Allosaurus) have
metacarpal II more robust than I, while Aucasaurus and Herrerasaurus have the
opposite condition. This is true in Xu et al.'s tetanurine example of Guanlong
too, while even their example of Deinonychus has more proximal area and depth
on metacarpal II, just less width.
Phalanx I-1 in tetanurines is said to be longer than phalanx I-1 in
Herrerasaurus, Dilophosaurus and ceratosaurs, but phalanx I-1 is not preserved
in any ceratosaur except for two questionably identified elements in
Masiakasaurus. Furthermore, it is not as if any phalanx on digit II in
Dilophosaurus or Herrerasaurus is notably more elongate than their phalanx I-1,
and some basal tetanurines like Torvosaurus actually have an extremely short
Metacarpal II is longest in tetanurines, while III is longest in more basal
theropods. Yet Limusaurus and Ceratosaurus resemble tetanurines in this (contra
Xu et al.'s statements and measurements about the former), and Dilophosaurus
and Megapnosaurus are polymorphic (e.g. II longer in the paratype of
Dilophosaurus, III longer in the holotype).
There is a proximal dorsolateral process on metacarpal III in coelophysoids and
Limusaurus, similar to one on metacarpal II in some basal tetanurines like
Guanlong. Yet Guanlong also has a process on metacarpal III, which partly
covers metacarpal IV, even though the latter is not illustrated in Xu et al.'s
paper. Acrocanthosaurus and Allosaurus also have a processes on metacarpal III,
while "Szechuanoraptor" lacks processes on metacarpals II or III. This process
on metacarpal III of tetanurines could be homologous to the process on III in
basal theropods as easily as it could the process on II.
Finally, Xu et al. state metacarpal III in tetanurines is short, slender and
proximally triangular like metacarpal IV in basal theropods. Of course,
metacarpal IV in tetanurines is also short and slender (even moreso than III),
with those of Xuanhanosaurus and "Szechuanoraptor" resembling metacarpal IV in
basal theropods more than their metacarpal III does. This is a case where the
more reduced metacarpal III in derived tetanurines like Guanlong and
Deinonychus (illustrated by Xu et al.) resembles basal theropod metacarpal IV
more than metacarpal III in basal tetanurines (e.g. Xuanhanosaurus,
"Szechuanoraptor", Torvosaurus, carnosaurs) do, with the thicker shaft and
robust distal articulation in the latter taxa. As for their triangular proximal
outline, metacarpal IV in Dilophosaurus and Limusaurus are more round than
triangular, but "Szechuanoraptor" shows basal tetanurines have triangular
metacarpal IV too in any case.
Xu et al. ran a phylogenetic analysis which determined that when characters
states are ordered, the resulting tree assuming tetanurines have digits
II-III-IV-V is six steps longer than if they are assumed to have digits
I-II-III-IV. The length of the unordered trees is equal, but leaving characters
unordered potentially leads to ridiculous "intermediate synapomorphies" like
coelophysoids and Herrerasaurus being diagnosed by having a single phalanx on
digit IV (not more or less) or taxon being diagnosed by having an intermediate
ratio, as opposed to relatives with low and high ratios. Furthermore, neither
Xuanhanosaurus, "Szechuanoraptor" or Megaraptor were included in their matrix,
though these taxa show high homoplasy if II-III-IV-V is assumed, as noted
above. I conclude that there is little evidence tetanurine hands lacked digit I.
The Theropod Database- http://home.comcast.net/~eoraptor/Home.html
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