Albertosaurus Osborn 1905

Russell, Dale A., 1970, Tyrannosaurs from the Late Cretaceous of western Canada, Ottawa: National Museum of Natural Sciences, Publications in Palaeontology, No. 1 : 4-5

Genus Albertosaurus Osborn 1905 , p. 265

Gorgosaurus Lambe 1914 , p. 13

Type species: Albertosaurus sarcophagus Diagnosis

Premaxilla contacts nasal below external naris. Maxillary teeth relatively smaller than in Daspletosaurus , posteriorly become more recurved and smaller still. Anteriormost antorbital fenestra within maxilla visible in lateral aspect, first antorbital fenestra longer than high. Nasals expand between lacrimals, invaded posteromedially on either side by short broad tongue of frontal. Frontal broadly exposed on skull roof, posteriorly recurved vertical cleft in bone above orbit. Prefrontal either unexposed dorsally, or with small process situated between lacrimal and frontal. Lacrimal ' horn' rectangular in lateral aspect, with apex centred well in advance of antorbital ramus of bone; exceeds postorbital ' horn' in development. Anteroexternal edge of antorbital ramus of lacrimal curves anteroventrally across jugal, bounding pocket in jugal laterally. Ventral process of ectopterygoid uninflated, with small ventrally opening sinus; jugular process of bone similarly uninflated. Angular terminates posteriorly behind surangular foramen.

Ventral openings of main basisphenoid sinus situated on either side of midline of skull, dorsomedial to each basipterygoid process. Optic fenestra not bridged medially by presphenoid. Cranial nerves III and IV exit separately on anterior surface of laterosphenoid, although pit for III much larger than that of IV. Exit for cranial nerve Vj situated close to anterolateral edge of laterosphenoid.

Neural spines of fourth and fifth cervicals subequal, broadly flattened laterally. Spine of sixth cervical narrower, but does not come to point dorsally. Presacral vertebrae tend to be longer and lower than in Daspletosaurus ; basal caudal vertebrae diminish more rapidly in size posteriorly than in this genus.

The length of the humerus in adults is equal to about 30 per cent of that of femur, forelimb lightly constructed. Anterodistal condyle of metacarpal I large, causing first and second digits to be approximately parallel. First phalanx of digit II slender in adults. Dorsal edge of ungual of digit I passes through arc of about 120 degrees.

Anterior blade of ilium does not cover diapophysis of twelfth thoracic vertebra. The circumference of the femur in adults is equal to 34 to 37 per cent of the femur's length. In comparison with Daspletosaurus specimens of similar femur length, the presacral vertebral column is longer, thoracic ribs are slightly shorter, forelimb is shorter, ilium and sacrum are shorter, and metatarsus is slightly longer.

Comments

The generotypic species of Albertosaurus is based on two crania found at diff"erent stratigraphie levels within the Edmonton Formation. These specimens so closely resemble skulls referable to Gorgosaurus libratus , from the Oldman Formation, that there can be little question of their generic identity. Gorgosaurus is therefore considered as a junior synonym of Albertosaurus , and A. libratus is in all probability the immediate ancestor of the Edmonton form. It has been suggested that interdental plates medial to the marginal dentition ( Osborn 1905: 265; Lambe 1914: 14) and an intercoronoid ( Lambe 1914: 14) are absent in this genus, although this is not the case. Lambe (1914: 14; 1917: 16) has emphasized the similarity of the first maxillary tooth in the type of A. libratus to those in the premaxilla, considering this as characteristic of the genus. The carinae of the anterior maxillary teeth of A. libratus (and of Daspletosaurus also) are incipiently arranged as in the premaxillary teeth, and the number of small anterior maxillary teeth is variable, ranging from none in FMNH PR 308 to two in AMNH 5664 . Although it is partly crushed, the first maxillary tooth of the type of A. libratus is probably not so premaxilliform as is indicated in Lambe's figure ( 1917, fig. 10B).

The dental formula of Albertosaurus is as follows: premaxillary teeth 4, maxillary teeth 13 -15, dentary teeth 14-16. There are 10-14 (in extreme cases 17) serrations per 5 mm on the carinae of the marginal teeth, the serrations being coarsest near the centre of the jaws and finest near the posterior end of the jaws of immature individuals.