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Re: Questions of Various Kinds



In a message dated 96-10-06 18:39:46 EDT, Mickey Rowe writes:

>  I'm also a bit unhappy that George is doing nothing
>  more than giving an opinion here (which walks along the line of
>  violating another moderation rule).  Why should we care what your
>  opinion is if you don't tell us why you hold it (and that question
>  isn't directed only at George)? -- MR

For those who don't have a copy of _Dinosaur Folios_ #1, herewith a diagnosis
of _Albertosaurus_, differentiated from _Gorgosaurus_ and _Daspletosaurus_:

        Diagnosis (mainly from Russell [1970] and Carpenter [1992]): Large
tyrannosaurid with broad, relatively short muzzle, and comparatively short
forelimbs and long, slender hind limbs
                Cranium and mandible: Maxillary and dentary teeth
relatively smaller than in Gorgosaurus and Daspletosaurus; 15 dentary
teeth, of which the first is small and truncated; premaxillae extend
primarily transversely across the muzzle with little caudal extent, so
that the premaxilla is nearly hidden in lateral view by the front of
the maxilla; premaxillae contact nasals below the external naris, as
in Gorgosaurus but not as in Daspletosaurus; pro-maxillary fenestra
distant from maxillary fenestra and visible in lateral view; maxillary
fenestra taller than long and separated from antorbital fenestra by a
thin maxillary bar; moderately developed rectangular lacrimal horns;
postorbital bar present in most if not all adult individuals; small,
round surangular foramen
                Vertebral column: Dorsal vertebral series relatively
shorter than in Gorgosaurus but longer than in Daspletosaurus;
thoracic ribs relatively shorter than in Daspletosaurus
                Pectoral girdle: Scapula with forwardly hooked distal end
                Forelimb: Relatively shorter than in Daspletosaurus
and Gorgosaurus
                Pelvic girdle: Indistinguishable from that of
Gorgosaurus; less massively built than that of Daspletosaurus
                Hind limb: Slenderer than those of Gorgosaurus and
Daspletosaurus; tibia more gracile than that of Gorgosaurus

There's not a lot of difference among the three genera _Albertosaurus_,
_Daspletosaurus_, and _Gorgosaurus_, and arguments could be raised to keep
them all in one genus. But cladistically one would then have to show that
they're more closely related to one another than they are to any other
tyrannosaurids--that is, have a more recent common ancestor than, say, their
common ancestor with _Tyrannosaurus_ and _Nanotyrannus_. Some workers see a
morphocline that runs from _Albertosaurus_ through to _Tyrannosaurus_, with
_Gorgosaurus_ and _Daspletosaurus_ (and the unnamed Two Medicine
tyrannosaurid) as intermediate stem-forms. This mandates distinct genera for
each form unless you accept paraphyletic genera in your taxonomy.