ABELISAURIDS & CARCHARODONTOSAURIDS

[darren.naish@port.ac.uk]

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 
these.

"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"

DARREN NAISH
darren.naish@port.ac.uk