Dinotyrannus megagracilis (Paul, 1988) Olshevsky, 1995

Olshevsky, G., 1995, The origin and evolution of the Tyrannosaurids, Part 1, Dino-Frontline 9, pp. 92-119 : 3-5

Box 3: Description of Dinotyrannus

Linnaean hierarchy:

Paraclass Reptilia

Parasubclass Diapsida

Parainfraclass Archosauria

Parasuperorder Theropodomorpha

Paraorder Theropoda

Suborder Tyrannosauria nov.

Family Tyrannosauridae

Subfamily Tyrannosaurinae

Tribe Tyrannosaurini nov.

Genus Dinotyrannus nov.

Type species Dinotyrannus megagracilis (Paul, 1988) n. comb.

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 (figure above by Tracy Lee Ford, view of right side of reconstructed skull, reversed), 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 ÏI 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 11-12 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 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 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.

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 tyraimosaurinid. 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 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, and his contribution is most gratefully acknowledged.

Figures

Holotype specimen of type species: LACM 23845 (figure above by Tracy Lee Ford, view of right side of reconstructed skull, reversed), 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 ÏI 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