Albertosaurus sarcophagus 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 : 10-13

Albertosaurus sarcophagus Osborn 1905

Laelaps incrassatus . Cope 1892, p. 240

Dryptosaurus incrassatus , Lambe 1904, p. 6

Albertosaurus sarcophagus Osborn 1905 , p. 265

Albertosaurus arctunguis Parks 1928 , p. 7

Distribution

Lower Edmonton Formation, Red Deer River, Alberta.

Referred Specimens

NMC 5601 (paratype of A. sarcophagus ) skull and lower jaws, fragments of sacral vertebrae and ilium, distal end of tibia with astragalus, fourth metatarsal, three ungual phalanges of pes (sec.?, tp. 33, rge. 22, W. 4th mer., east bank of Red Deer River, Member B of Edmonton Formation).

AMNH 5222 scattered skull (sec. 12?, tp. 33, rge. 22, W. 4th mer., 150 feet above east bank of Red Deer River, Member B of Edmonton Formation).

ROM 807 (type of ^4. arctunguis) sacrum and adjacent vertebrae, left scapulacoracoid and forelimb, left half of pelvic girdle and associated hind limb (sec. 27 or 34, tp. 30, rge. 21, W. 4th mer., 100 feet above west bank of Red Deer River, Member A of Edmonton Formation).

NMC 5600 (type of A. sarcophagus ) palatal region of skull, braincase and lower jaws (sec. 11?, tp. 29, rge. 21, W. 4th mer., approximately 100 feet above Red Deer River, along Kneehills Creek, Member A of Edmonton Formation).

? NMC 2196 scapula-coracoid, abdominal ribs (sec. 35?, tp. 28, rge. 20, W. 4th mer., west bank of Red Deer River, Member A of Edmonton Formation).

Diagnosis

Length of dentary tooth row 65 per cent of length of fourth metatarsal ( NMC 5601 ). In adult animals ( ROM 807 , if the distal end of the scapula has not been broken off) combined length of the scapula-coracoid about 80 per cent of that of femur. In adult animals ( ROM 807 ) combined length of tibia and astragalus equal to (or slightly greater than) that of femur.

Comments

In defining Albertosaurus arctunguis . Parks (1928: 5-7) stressed the dissimilarities between the distal end of the fourth metatarsal of his specimen ( ROM 807 ) and that of the paratype of A. sarcophagus ( NMC 5601 ). The slightly pathologic condition of this portion of the latter element, as Parks himself noted ( 1928: 7), accounts for some of the differences, while the remainder are due to the fact that the posteroexternal edge of the distal articulating surface has been broken off, which Parks failed to recognize. The ungual phalanges associated with NMC 5601 , considered by Parks as belonging to the manus, actually belong to the pes. Altogether, it is not possible to separate the type of A. arctunguis from the type and paratype of ^. sarcophagus on morphological grounds, and the great resemblance of each of these specimens to the corresponding region of the skeleton of A. libratus argues convincingly for their inclusion within a single species.

In addition to the characters noted in the diagnosis, the forelimb of ROM 807 seems to be slightly shorter than in A. libratus (NMC 2120), and the pubis is slightly larger than in the latter specimen {see Parks 1928: 12, 20, 23). A. sarcophagus occurs in both member A and B of the lower Edmonton Formation ( Russell and Chamney 1967: 10, 12). It is unfortunate that the species is not known from more complete material.

In tyrannosaurs, the bone beneath the brain stem gives the appearance of being solid. However, this region of the skull is almost entirely filled with large sinuses, which are separated from one another only by thin walls. Fenestrae, through which these sinuses communicate with the surface of the bone, have occasionally been confused with openings for nerve or blood vessel conduits {see for example Lambe 1904: pl. 7, fig. 15; Gilmore 1946: 10). A reconstruct tion of the ventral part of the braincase in Albertosaurus is presented here ( Figure 4 ), based primarily on a partial dissection of the braincase of NMC 5600 ( A. sarcophagus ), with the parasphenoidal area restored from ROM 1237 ( A. libratus ). The large sinuses located in the parasphenoid rostrum and ventral part of the basisphenoid are seen in sagittal section. Dotted lines show the approximate extent of the latter sinus and two other sinus systems on either side of the cranial midline. The fenestrae linking each sinus system with the exterior are indicated by arrows; the arrow labelled " X " passed through the fenestra draining the sinus system in the paroccipital process. The more dorsal sinus systems are partly subdivided by thin webs of bone, which would probably exhibit a high degree of variability in diflFerent individuals.

In Tyrannosaurus (AMNH 5117), and also in Allosaurus ( Osborn 1912: figs. 9-10), there is a foramen in the basisphenoid near the anteroventral edge of the laterosphenoid, which Osborn identified as the ventroposterior opening of the carotid canal. Although the distal part of the canal was not observed in NMC 5600 , a small foramen in the pituitary fossa may mark its anterior termination. Behind this is another foramen, which may have contained the sixth nerve.

Neither the posterior course of the canal leading to the latter foramen nor the point of penetration of the sixth nerve into the floor of the medullary cavity was identified in this specimen. Not shown in Figure 4 are the conduits for nerves X to XII, which lie in the bone immediately ventrolateral to the medullary cavity and emerge on the occipital face of the skull in three foramina situated beside the neck of the occipital condyle. A small rod of bone, measuring 30 mm between its broken ends and 2.5 mm in diameter, was found in the matrix between the broken right quadrate and paroccipital process in NMC 5600. No doubt it is the stapes.

It should be noted that in tyrannosaurs the median elements of the dermal skull roof are solidly sutured to each other and to the braincase below. This rigidity, coupled with the structural weakness of the basipterygoid processes, absolutely precludes any possibility of normal kinetic movement in the skull. Perhaps the diarthroid joint between the quadrate and squamosal, loose overlapping of the basipterygoid processes by the pterygoids, and split nature of the posterior part of the palate would allow some displacement of the temporal muscle mass in a medial direction. In similar fashion, the loose dorsal and posterior contacts of the jugal and quadratojugal, in combination with a vertically flexible antorbital lacrimal bar, may have allowed some displacement of the temporal muscle mass in a lateral direction. It is probable, however, that these suites of structures operated in an entirely passive manner, in response to the struggles of a massive prey held between the jaws. Even movements of the postulated kind must have been very limited, serving only to prevent the breakage of bones within the skull.

Figures

Figure 4 Alberiosaunis, reconstruction of the lower half of the braincase seen in sagittal section. Abbreviations: CC carotid canal, MED medullary cavity, PIT pituitary fossa, ST sella turcica, VI hypothetical course of sixth cranial nerve. For further explanation see text.