Re: Dwarf Tyrannosaurs?

[Dinogeorge@aol.com]

In a message dated 7/13/99 0:45:11 AM EST, qilongia@yahoo.com writes:

<< 2. *Dinotyrannus megagracilis* Paul, 1988 (=
 Gorgosaurus megagracilis). Very small specimen also
 found to be a possible juvenile of *T. rex.* _Not_
 another name for *Nanotyrannus.* >>

This was originally Albertosaurus megagracilis Paul, 1988, not Gorgosaurus 
megagracilis, and it's not "very small" at all, but about 80% of the size of 
an adult Tyrannosaurus rex. When Ralph Molnar described it in 1980, he 
pointed out numerous differences between the type specimen and Tyrannosaurus 
rex and concluded it was not referable to that species. He thought it might 
very well represent a fully grown adult Nanotyrannus lancensis (then known as 
Albertosaurus lancensis). Greg Paul noted differences between the type 
specimen and A. lancensis and created the new species Albertosaurus 
megagracilis for it, and I coined the generic name Dinotyrannus for this 
species in Dino-Frontline #9, 1995. Here is the slightly edited and 
unillustrated text of a preliminary description of the genus Dinotyrannus I 
prepared for Dino-Frontline #9 and 10, which carried my article on 
tyrannosaurids. Unfortunately, there was no room in the issues to actually 
publish the description, and I will include it in the forthcoming Mesozoic 
Meanderings #3


Description of Dinotyrannus
Linnaean hierarchy:
Class Archosauria
 Subclass Dinosauria
  Infraclass Aves
   Parasuperorder Theropodomorpha
    Paraorder Theropoda
     Suborder Tyrannosauria
      Family Tyrannosauridae
       Subfamily Tyrannosaurinae
        Tribe Tyrannosaurini
         Genus Dinotyrannus Olshevsky, 1995
          Type species Dinotyrannus megagracilis (Paul, 1988) Olshevsky, 1995

Synonyms of type species:
        Albertosaurus megagracilis Paul, 1988
        
        Etymology: Dino-, Latinized combining form of deinos, Greek for 
"terrible," and -tyrannus, Latinized combining form of tyrannos, Greek for 
"tyrant," "master," or "despot"; thus, "terrible tyrant," referring to its 
obvious carnivorous nature as well as to its close phyletic relationship to 
the genera Nanotyrannus and Tyrannosaurus.

        Holotype specimen of type species: LACM 23845, a partial skeleton 
including scattered skull and jaw elements (portions of nasals, maxillae, 
right lacrimal, prefrontals, frontals, partial parietal, supraoccipital, 
pterygoids, partial quadratojugals, partial right dentary, incomplete 
angulars and prearticulars, nearly complete right surangular, partial left 
surangular) and limb bones (left ulna, metacarpal II, manual ungual, partial 
left femur and tibia, complete right fibula, right astragalus, right 
metatarsals II and III, and 11 pedal phalanges), found in association and 
presumably belonging to the same individual. Presently at the Los Angeles 
County Museum of Natural History. The looseness of the articulations of the 
skull bones strongly suggest the holotype individual was subadult.

        Type locality and horizon: Hell Creek Formation, on the Lester D. 
Engdahl ranch near Jordan, Garfield County, Montana, USA. Discovered in 1967 
by a Los Angeles County Museum field party under the direction of J. Reed 
MacDonald.

        Diagnosis: The genus Dinotyrannus, with an adult body length 
estimated at 10 meters, differs from all albertosaurinids (members of the 
paratribe Albertosaurini), such as Albertosaurus, in lacking a lacrimal 
"horn"; that is, the lacrimal dorsal margin is more or less level with the 
dorsal surface of the skull. This likely 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 the 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 those genera. Contact 
between frontal and prefrontal less angular in dorsal aspect 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 specimen. Medial shelf on ventral 
margin of posteroventral process of dentary absent (unlike in Tyrannosaurus).

        Ulna straight and untapered, without strong olecranon process; it 
differs in these characters from those of other North American 
tyrannosaurids. Manual ungual phalanx without proximal tendon tubercle and 
with angular rather than arcuate articular surface, differing from all other 
tyrannosaurids in this 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 
any 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 not seem 
closely related to this form. The slenderness and length of the hind limb 
suggest that Dinotyrannus had a lighter weight and more gracile body 
proportions for its size than albertosaurinids or Tyrannosaurus, hence the 
trivial name megagracilis given to the type species by Paul. These may, 
however, be juvenile features, and fully adult specimens of Dinotyrannus 
could well be the same size as those of Tyrannosaurus (and thus confused with 
this genus in collections).

Referred specimens: None.

Referred species: None.

        Discussion: Though clearly different from the other two Lance 
tyrannosaurids, Dinotyrannus remains difficult to diagnose because of the 
scantiness of the holotype material and the lack of any referred specimens. 
The limb bones and other material at the Smithsonian Institution (USNM 2110, 
6183, and 8064: see Gilmore [1920]) from the Lance Formation presently 
referred to Tyrannosaurus rex should be reexamined and compared with the 
holotype limb elements to see whether they might be better referred to 
Dinotyrannus. It is also difficult to classify Dinotyrannus at the level of 
tribe; it could be either a derived tarbosaurinid or a primitive 
tyrannosaurinid. It is more plausible to find a tyrannosaurinid in the Lance 
of western North America than a tarbosaurinid, so Dinotyrannus is considered 
a primitive member of the tribe Tyrannosaurini. Inasmuch as the only other 
evidence for an Albertosaurus-size tyrannosaurid in the latest Maastrichtian 
of North America, besides the Dinotyrannus type material, lies in isolated 
teeth and indeterminate skeletal elements, such material should more properly 
be referred to the genus Dinotyrannus as Dinotyrannus sp. or Dinotyrannus cf. 
D. megagracilis instead of to Albertosaurus. As far as is known, no member of 
paratribe Albertosaurini survived in that epoch in North America.

        Most of the preceding diagnosis was distilled from the published 
descriptions of the holotype specimen by Ralph E. Molnar (1980) and Paul 
(1988). Molnar also provided unpublished drawings of some Dinotyrannus skull 
elements for this review [the article in Dino-Frontline], and his 
contribution is most gratefully acknowledged.