Dinotyrannus megagracilis Olshevsky, 1995, n. comb.
Creators: name: Olshevsky, G.
Publisher: name: Zenodo
Year: 1995
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Dinosauria
Tyrannosauridae
Dinotyrannus
Dinotyrannus megagracilis

description: 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. <b>Genus</b> <i>Dinotyrannus</i> nov. Type species <i>Dinotyrannus megagracilis</i> (Paul, 1988) n. comb. Synonyms of type species: <i>Albertosaurus megagracilis</i> Paul, 1988 Etymology: <i>Dino-,</i> Latinized combining form of <i>deinos,</i> Greek for “terrible,” and <i>-tyrannus,</i> Latinized combining form of <i>tyrannos,</i> 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 <i>Nanotyrannus</i> and <i>Tyrannosaurus.</i> <b>Holotype</b> <i>specimen</i> of <b>type species</b>: 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 (<i>portions</i> 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 <i>fibula</i>, 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. <b>Diagnosis</b>: The genus <i>Dinotyrannus,</i> with an adult body length estimated at 11-12 meters, differs from all albertosaurinids (members of the paratribe Albertosaurini), such as <i>Albertosaurus,</i> 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 <i>Gorgosaurus,</i> and some specimens of <i>Tarbosaurus efremovi).</i> Nasals less constricted at junction with frontals than in either <i>Nanotyrannus</i> or <i>Tyrannosaurus,</i> precluding reference to either of those genera. Contact between frontal and prefrontal less angular in dorsal aspect than in <i>Tyrannosaurus.</i> Broadened posterior region of frontals suggests the occiput was expanded and the eyes somewhat forwardly directed, though not to the extent seen in <i>Nanotyrannus</i> and <i>Tyrannosaurus.</i> As noted by Molnar (1980), the frontal has a distinctly different shape from that of <i>Tyrannosaurus.</i> 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 <i>Tyrannosaurus).</i> 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 <i>Tyrannosaurus,</i> 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 <i>Maleevosaurus novojilovi,</i> which otherwise does not seem closely related to this form. The slenderness and length of the hind limb suggest that <i>Dinotyrannus</i> had a lighter weight and more gracile body proportions for its size than albertosaurinids or <i>Tyrannosaurus,</i> hence the trivial name <i>megagracilis</i> given to the type species by Paul. <b>Referred</b> specimens: None, <b>Referred</b> species: None. <b>Discussion</b>: Though clearly different from the other two Lance tyrannosaurids, <i>Dinotyrannus</i> 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 <i>Tyrannosaurus rex</i> should be reexamined and compared with the holotype limb elements to see whether they might be better referred to <i>Dinotyrannus.</i> It is also difficult to classify <i>Dinotyrannus</i> 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 <i>Dinotyrannus is</i> 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 <i>Dinotyrannus</i> type material, lies in isolated teeth and indeterminate skeletal elements, such material should more properly be referred to the genus <i>Dinotyrannus</i> as <i>Dinotyrannus</i> sp. or <i>Dinotyrannus</i> cf, <i>D. megagracilis</i> instead of to <i>Albertosaurus,</i> As far as is known, no member of paratribe Albertosaurini survived <i>in</i> 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 <i>Dinotyrannus</i> skull elements for this review, and his contribution is most gratefully acknowledged.
description: Published as part of <i>Olshevsky, G., 1995, The origin and evolution of the Tyrannosaurids, Part 1, pp. 92-119 in Dino-Frontline 9</i> on pages 3-5, DOI: 10.5281/zenodo.1038228