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Re: Comments on T?. megagracilis



For what it's worth, herewith a copy of my diagnosis of _Dinotyrannus_ as=

submitted to Gakken but not yet published. It is quite closely related to=

_Tyrannosaurus rex_ and at least belongs in the same tribe with that spec=
ies
and _Nanotyrannus lancensis_. The tribe is characterized by relatively
strongly forward-vectored orbits caused by lateral expansion of the occip=
ut,
loss of lacrimal horn and concomitant enlargement of the postorbital
rugosity, and downwardly angled occipital condyle. These characters all
relate to the overlap of the visual fields across the top of the muzzle t=
o
permit stereoscopy.


        Diagnosis: The genus _Dinotyrannus_, with an adult body length 
estimated=
 at
11=9612 meters, differs from all albertosaurinids (members of the paratri=
be
Albertosaurini), such as Albertosaurus, in lacking a lacrimal "horn"; tha=
t
is, the lacrimal dorsal margin is more or less level with the dorsal surf=
ace
of the skull. This character bars it from Albertosaurini and strongly
supports inclusion in the tribe Tyrannosaurini (or paratribe Tarbosaurini=
).
Nasals markedly downturned in front (giving a convex dorsal profile to th=
e
muzzle reminiscent of AMNH 5336 and USNM 12814, skulls perhaps wrongly
referred to _Gorgosaurus_, and some specimens of _Tarbosaurus efremovi_).=

Nasals less constricted at junction with frontals than in either
_Nanotyrannus_ or _Tyrannosaurus_, precluding reference to either of thos=
e
genera. Contact between frontal and prefrontal less angular in dorsal asp=
ect
than in _Tyrannosaurus_. Broadened posterior region of frontals suggests =
the
occiput was expanded and the eyes somewhat forwardly directed, though not=
 to
the extent seen in _Nanotyrannus_ and _Tyrannosaurus_. As noted by Molnar=

(1980), the frontal has a distinctly different shape from that of
_Tyrannosaurus_. Dorsal orbital margin with wide gap between lacrimal and=

postorbital for frontal, but this may be due to juvenility of the specime=
n.
Medial shelf on ventral margin of posteroventral process of dentary absen=
t
(unlike in _Tyrannosaurus_).


        Ulna straight and untapered, without strong olecranon process; it 
differ=
s in
these characters from those of other North American tyrannosaurids. Manua=
l
ungual phalanx without proximal tendon tubercle and with angular rather t=
han
arcuate articular surface, differing from all other tyrannosaurids in thi=
s
combination of characters. Forelimb quite short relative to hind limb,
perhaps due more to relative elongation of hind limb than to forelimb
reduction.

        General hind-limb proportions large but relatively slenderer than in an
albertosaurinid of comparable size. Hind limb relatively longer than in a=
ny
albertosaurinids or _Tyrannosaurus_. Tibia triangular in proximal view,
unlike that of albertosaurinids, which is quadrangular in proximal view.
Cnemial crest of tibia prominent and extends more laterally than in other=

tyrannosaurids except _Maleevosaurus novojilovi_, which otherwise does no=
t
seem closely related to this form. The slenderness and length of the hind=

limb suggest that _Dinotyrannus_ had a lighter weight and more gracile bo=
dy
proportions for its size than albertosaurinids or _Tyrannosaurus_, hence =
the
trivial name _megagracilis_ given to the type species by Paul.
-------------------------------------------------------------------------=
-----
------
Unfortunately, not enough of the dentary is known for a complete tooth co=
unt,
but I would not be surprised if it were one or even two greater than the
"standard" 13 of _Tyrannosaurus_.

Going strictly by anatomical characters rather than body size, I'd say
_Nanotyrannus_ is closer to _Tyrannosaurus_ than is _Dinotyrannus_. If
_Dinotyrannus_ is made a junior synonym of _Tyrannosaurus_, probably
_Nanotyrannus_ should be, too.

G.O.