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ABELISAURIDS & CARCHARODONTOSAURIDS
Can there ever be a novel thread concerning any aspect of theropod
systematics? Not today:)
This thing about carcharodontosaurids being close relatives of
abelisaurids is, in my opinion, poorly supported. It's Fernando
Novas' idea and pends on about three characters, all of them cranial
and all of them apparently prone to homoplasy in theropods as well as
in other dinosaur, and other reptile, taxa. Rugosity of the dorsal
surface of the nasals, invasion of the orbit by a rostral process of
the postorbital bar and I think rugosity on the dorsal surface of the
postorbital are some of the characters that have been advanced in
favour of this relationship: all are seen in both abelisaurids and
carcharodontosaurids. However, these characters are also seen in
other theropods (notably tyrannosaurids) and that they are -probably-
associated with social behaviour makes them somewhat suspicious as
ostensibly shared characters.
I do appreciate that, in order to understand character distribution,
we should ignore function and favour homology, but I suspect it is
logical to be cautious about features that seem prone to homoplasy
e.g. things like robustness of forearms, orbit size, presence of
horns, crests etc. Is there a body of cladistic literature on this
area? I am familiar with Oliver Rieppel's arguments about homology.
It may also be that we can already see the same character complex
convergently evolving within one particular group of theropods: the
tyrannosaurids. If diagrams of _Aublysodon_ and _Shanshanosaurus_
(whatever its current status) are to be trusted, aublysodonts
(=shanshanosaurines) lack rugose nasals and postorbitals.
Aublysodonts and tyrannosaurines like _Gorgosaurus libratus_ also
show that the tyrannosaurid orbit is not, ancestrally, invaded by
the postorbital process in tyrannosaurids, yet this condition seems
to have convergently evolved in _Tyrannosaurus_ and _'Albertosaurus'
arctunguis_ (though this might be arguable - please comment if so).
More important is that carcharodontosaurids share far more characters
with other allosauroids than they do with abelisaurids. See Holtz
(1994), Sereno et al. (1996), and Harris (1998) for listings of
"We are not hard-wired souls, etched into the circuitry of life; we
are not angelic rocks, fixed immovable into the fabric of time; we
are but fleeting shadows, flitting across the canvas of the globe"