Albertosaurus lancensis
Creators: name: Carpenter, K.
Publisher: name: Zenodo
Year: 1992
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Dinosauria
Tyrannosauridae
Albertosaurus
Albertosaurus lancensis

description: <i> <i> ? Albertosaurus lancensis <b>(Gilmore) 1946</b></i> </i> Fig. 2F <i>= Gorgosaurus lancensis Gilmore 1946</i> <b>Holotype:</b> CMNH 7 541 skull and lower jaws from the Hell Creek Formation of Montana, U.S.A. <b>Diagnosis:</b> (Cranial) skull long and low;maxillary fenestra small and separated from antorbital fenestra by a wide bar of the maxillary; antorbital fenestra longer than tall;nasals do not form, a wedge between the frontals as in <i>A. libratus</i> and <i>A. sarcophagus</i>, in dorsal view, muzzle well differentiated from cranium so that there was stereoscopic vision. (Postcranial) fourth trochanter more proximal than in <i>A. li ­ bratus</i>; proximal articular face of tibia almost triangular, rather than quadrangular as in most tyrannosaurids (except for <i>Maleevosaurus</i>) proximal expansion of fibula is symmetrical relative to long axis of the shaft. <b>Stratigraphic and Palaeobiogeographic Distribution:</b> Hell Creek Formation of Montana, U.S.A. <b>Discussion:</b> I agree with Russell (1970) that <i>A. arctunguis</i> is a junior synonym of <i>A. sarcophagus</i> despite the supposed differences between the tarsus of <i>A. sarcophagus</i> and <i>A. arctunguis</i> as noted by Welles and Long (1974). I do not consider these differences significant because Parks (1928) stated that part of the astragulus of <i>A. arctunguis</i> was damaged, and that both the astragulus and calcaneum were displaced. New skeletons of <i>A. sarcophagus</i> presently under study by Phil Currie are expected to verify these observations. Russell (1970) synonymized <i>Gorgosaurus</i> with Albertosaurus claiming that the holotype and paratype skulls closely resemble those referable to <i>Gorgosaurus libratus.</i> However,both the holotype and paratype skulls of <i>A. sarcophagus</i> are incomplete and crushed making such comparisons suspect. AMNH 5222, on the other <i>hand</i>,is a partial, disarticulated skull. Nevertheless,it shares with the holotype and paratype skulls a number of features, such as the short, broad premaxillaries, a pro-maxillary fenestra that is visible in lateral profile and widely separated from the maxillary fenestra, and in having a narrow bar of the maxillary separating the maxillary fenestra from the antorbital fenestra. AMNH 5222 has been rearticulated and gives us an undistorted view of the skull of <i>A. sarcophagus.</i> This shows that the skull is short and deep, as in <i>Tyrannosaurus</i>, rather than long and low as in <i>A. libratus</i> (compare Fig. 2 D with Figs. 2 E). <i>A. sarcophagus</i> differs from <i>A. libratus</i> in a number of other points as well. These include a pro-maxillary fenestra clearly visible in lateral profile, a broad bar of the maxillary separating the pro-maxillary from the maxillary fenestra, and a narrow bar of the maxillary separating the maxillary fenestra from the antorbital fenestra. There is also some indication that the postorbital extended into the orbit in <i>A. sarcophagus</i> in a manner similar to that seen in adult <i>Tyrannosaurus.</i> Postcranially, <i>A. sarcophagus</i> differs from <i>A. libratus</i> in having more gracile limbs, and in having a short, <i>Tyrannosaurus-</i> like trunk (compare Figs. 5 D and E). The differences between <i>A. sacrophagus</i> and A. <i>libratus</i> seem extreme enough to suggest that two distinct genera are present. However, pending a complete review of <i>A. sarcophagus</i> by P. Currie based on several new skeletons,Russell’s synonymy of <i>Gorgosaurus</i> with <i>Albertosaurus</i> is tentatively accepted. Albertosaurus <i>libratus</i> is the best known tyrannosaurid in North America, being represented by nine skulls and partial skeletons. Among this wealth of material includes juveniles permitting Russell (1970) to document ontogenetic changes. Not surprisingly, many of these changes paralleled those seen in <i>Tyrannosaurus bataar</i> (Rozhdestvensky,1965). <i>A. lancensis</i> is a problematical species despite it being represented by a complete skull. In profile, the skull most closely resembles that of <i>A. libratus</i>, especially in the development of the muzzle (compare Figs. 2 E and F). <i>Thus</i>,it is understandable why Gilmore assigned it to <i>Gorgosaurus.</i> However, when the skull is viewed dorsally, the muzzle is well differentiated from the cranium, a feature only known in <i>Tyrannosaurus.</i> In <i>addition</i>,as shown with <i>T. bataar</i>, juvenile <i>Tyrannosaurus</i> skulls are <i>Atbertosaurus-</i> like. in profile. Therefore,is <i>A.</i> lancensis a juvenile <i>T. rex</i> ? Gilmore (1946) stated that many sutures are coalescent, indicating an adult individual. This condition is exaggerated, as the only completely coalescent bones are the parietals; both the frontals and nasals are separate anteriorly. The coalescence of cranial bones is known to be variable in dinosaurs (Brown and Schlaikjer, 1940), making its usability to ‘age’ individuals suspect. Besides the Albertosaurus- like profile, two other features of the? <i>A. lancensis</i> skull that seem to differ from <i>T. rex</i> are the oval-shaped orbit and the shape of the antorbital fenestra. The shape of the orbit changes ontogenetically in <i>T. bataar</i> as may be seen in Figure 4. In the smallest skull the orbits are almost oval in shape, whereas in the larger skulls the orbit becomes progressively taller and is subdivided by the orbital process of the postorbital. Considering that the skull of <i>A. lancensis</i> is smaller than the smallest <i>T. bataar</i> skull, and by extending backwards ontogenetically the changes that occur in <i>T. bataar</i> skulls, the oval shape of the orbit in <i>A. lancensis</i> might be a juvenile character. The shape of the antorbital fenestra in <i>A. lancensis,</i> however, seems to exceed that expected ontogenetically based on <i>T. bataar.</i> As may be seen in Figure 4,the anterior edge of the fenestra remains rounded throughtout life in <i>T. bataar.</i> In <i>T. rex</i> this area is variable, being rounded, as in <i>T. bataar,</i> to squared (Carpenter,in press). In <i>? A. lancensis</i>, this region is more acute (Fig. 2 F). But because the <i>A. lancensis</i> skull is considerably smaller than the smallest known <i>T. bataar</i> skull, it is possible that area undergoes early ontogenetic change (this hypothesis is partially inferred from the early ontogenetic changes in the hadrosaur <i>Maiasaura</i>;Horner in preparation). A single fragmentary skeleton of <i>A. lancensis</i> is <i>known</i>,and identification is tentative (Molnar, 1980). Molnar does describe a partial lacrymal,which,if it belongs to? <i>A. lancensis</i> would indicate the absence of a horn. In the holotype skull this region is either damaged or restored,but enough of the horizontal ramus exists on the left side to indicate that at a small horn was present. Because Molnar did not figure the <i>element</i>,it is not possible to verify his identification. Molnar’s identification of the material to <i>A. lacensis</i> is due to its small size and more gracile appearance. However, as discussed above,a juvenile <i>A. libratus</i> (AMNH 5664) is more gracile than the adult (AMNH 5458) and the small size is might also indicate a young animal. One feature Molnar (1980) noted about the fragmentary skeleton that might differentiate it from <i>T. rex</i> is the bisymmetrical proximal expansion of the fibula. However, this is moe apparent than real and is based upon the orientation of the specimen. At this time there is nothing about the fragmentary skeleton to prevent its being a juvenile <i>T. rex.</i>
description: Published as part of <i>Carpenter, K., 1992, Tyrannosaurids (Dinosauria) of Asia and North America, pp. 250-268 in Aspects of Nonmarine Cretaceous Geology, Beijing :China Ocean Press</i> on pages 258-260, DOI: 10.5281/zenodo.1038212
description: {"references": ["------ 1946, A new carnivorous dinosaur from the Lance Formation of Montana: Smithsonian Miscellaneous Collections, v. 106, p. 1 - 19.", "Russell, D., 1970, Tyrannosaurs from the Late Cretaceous of Western Canada: National Museums of Canada Publication in Palaeontology, v. l, p. 1 - 34.", "Welles, S. and Long, R., 1974, The tarsus of theropod dinosaurs: Annals of the South African Museum, v. 64, p. 191 - 218.", "Parks, W., 1928, Albertosaurus arctunguis, a new species of theropodous dinosaur from the Edmonton Formation of Alberta: University of Toronto Studies, Geological Series, v. 25, p. 1 - 42.", "Rozhdestvensky, A., 1965, Growth changes in Asian dinosaurs and some problems of their taxonomy. paleontologicheskii Zhurnal, p. 95 - 109.", "Brown, B. and Schlaikjer, E., 1940, The structure and relationships of Protoceratops: New York Academy of Sciences Annals, v. 40, p. 133 - 266.", "Carpenter, K., in press, Examples of variation in Tyrannosaurus rex: in Currie, P. and Carpenter, K., eds., Dinosaur Systematics: Cambridge, Cambridge University Press", "------, 1980, An albertosaur from the Hell Creek Formation of Montana, Journal of Paleontology, v. 54, p. 102 - 108."]}