Albertosaurus libratus ( Lambe 1914 ), Russell, 1970

Russell, Dale A., 1970, Tyrannosaurs from the Late Cretaceous of western Canada, Ottawa: National Museum of Natural Sciences, Publications in Palaeontology, No. 1 : 5-10

Albertosaurus libratus ( Lambe 1914)

Gorgosaurus libratus Lambe 1914 , p. 13

Gorgosaurus sternbergi Matthew and Brown 1923 , p. 7

Distribution

Oldman Formation, near Steveville, Alberta.

Referred specimens

* NMC 8782 incomplete manus and pes, fragments from hind limbs (3 miles south of Steveville, upper half of beds).

* AMNH 5423 jaw fragments, four caudal vertebrae, fragments of pelvis, left hind limb (Little Sandhill Creek basin).

NMC 2270 maxilla.

NMC 12063 maxilla.

ROM 4591 nasals (east side of coulee, at Denhart, Alberta).

AMNH 5664 (type of G. sternbergi) nearly complete skeleton (quarry 54 of Sternberg 1950).

USNM 12814 nearly complete skeleton, lacking tail (Little Sandhill Creek basin).

* NMC 2250 ilium.

ROM 1422 fragmentary skull.

ROM 1237 skeleton, lacking presacral vertebrae and right hind limb (quarry 21 of Sternberg 1950).

UA 10 skull, several presacral vertebrae and ribs, right humerus, left metatarsus (quarry 48 of Sternberg 1950).

AMNH 5432 left maxilla, left tibia-metatarsus and some phalanges of pes (quarry 59 of Sternberg 1950).

NMC 11593 pelvis, hind limbs, base of tail (quarry 50 of Sternberg 1950).

ROM 683 maxillae.

ROM 436 left premaxilla and maxilla.

ROM 1247 palatal elements.

AMNH 5336 skull (quarry 57 of Sternberg 1950).

AMNH 5458 skull and nearly complete skeleton (Little Sandhill Creek basin).

NMC 2120 (type of G. libratus ) nearly complete skeleton (quarry 36 of Sternberg 1950).

NMC 2193 surangular.

FMNH PR 308 skull and skeleton, lacking hind limbs and tail (Little Sandhill Creek basin).

NMC 11814 braincase.

Diagnosis

Length of dentary tooth row 71 per cent of length of fourth metatarsal ( USNM 12814 ?, NMC 2120 ). In adult animals ( NMC 2120 ) combined length of scapula-coracoid greater than that of femur. In adult animals ( AMNH 5458 , NMC 2120 ), combined length of tibia and astragalus 95 per cent of that of femur.

Comments

Gorgosaurus sternbergi , with the most complete skeleton known of a North American tyrannosaur as its type, was characterized by Matthew and Brown (1923: 7) as follows: "It is of smaller size and more slender proportions that in G. libratus . The jaws are much less massive and the muzzle is more slender, the maxilla more elongate and shallow, the orbital fenestra more circular. The tibia is considerably longer than the femur." Gilmore (1946: 3) suggested that the apparent slenderness of the muzzle in the type was caused by crushing, and Rozhdestvensky (1965) has demonstrated that the tibia /femur ratio decreases during growth in tyrannosaurs { Tarbosaurus ).

Matthew and Brown (1923: 7) were aware of the possibility that these characters might be due to the juvenility of the type specimen, noting that the pelvic elements were not co-ossified. The sutures between the bones of the skull roof are widely open, and there is nothing in the morphology of the specimen to suggest that it is not an A. libratus approximately two-thirds grown. Interestmgly, the supraoccipital alae of the parietals are only about one-fourth as large as in adults, indicating that these crests become more powerfully developed with maturity.

Rozhdestvensky's (1965) important studies of growth changes in the limb proportions of Tarbosaurus require that North American material be examined for similar growth effects. Although articulated remains of juvenile tyrannosaurs are far from being the commonest of fossils, a barely adequate sample of halfgrown to adult specimens of /I. libratus is available, from which some impression may be gained of allometric growth in the species.

Six specimens of A. libratus from the badlands of the Oldman Formation near Steveville, Alberta, have been examined for changes in bodily proportions correlating with the size of the animal. The smallest individual of the series AMNH 5423 was considered as possibly referable to A. libratus because the lengths of the tibia and metatarsus continue the same trends seen in larger specimens of the species, because Albertosaurus is three times as abundant as Daspletosaurus in this area of outcrop of the Oldman Formation, and because only adult specimens of the latter genus have so far been found here. The other five specimens ( AMNH 5664 , USNM 12814 , NMC 11593 , AMNH 5458 , NMC 2120 ) have been assigned to A. libratus on the basis of their skeletal anatomy {see diagnoses). The precise horizon from which each of these specimens was collected is known in only three cases, although dinosaurian fossils are restricted to an approximately 200- foot vertical interval in the area ( Sternberg 1950). In view of the great resemblance oï A. sarcophagus from the younger Edmonton Formation to A. libratus , it is improbable that allometric changes would be obscured by evolutionary changes over a stratigraphie interval of this magnitude.

The length of the femur was arbitrarily chosen as a standard to which the lengths of other skeletal structures in a specimen were compared {see Table 1). The allometric changes are apparent when the lengths of the skeletal structures are expressed as a percentage of the femur length and plotted against femur length on a graph. These data, although based on a minimal number of measurements, seem to indicate that as the animal increases in size from half-grown to fully mature, the presacral vertebral column, ribs, scapula-coracoid, pubis, and ischium increase more rapidly in length than does the femur; the skull, sacrum, humerus, and radius-ulna grow at approximately the same rate; and the caudal vertebral column, manus, tibia, metatarsus, and pes grow at a slower rate. These relative rates of growth appear to be rather constant, although in the case of the combined length of the tibia-astragalus the rate of increase is slow, relative to that of the femur, until the animal attains the size of USNM 12814 , when the growth rate abruptly changes to equal that of the femur.

'

The comparative changes in the proportions of skeletal structures may have been of the same general magnitude during the earlier, more rapid stages of growth. Assuming that this was true, it would be possible to estimate the relative bodily proportions of a very young (hatchling) Albertosawus libratus . For a femur length of 100 mm, the lengths of the following structures were estimated graphically as follows: skull 88 mm, presacral vertebral column 210 mm, sacrum 70 mm. anterior twenty-four caudal vertebrae 390 mm, scapula-coracoid 63 mm, humerus 26 mm, ulna 20 mm, manus 48 mm, second metacarpal 7 mm, ilium 100 mm, pubis 68 mm, ischium 54 mm, circumference of femur 28 mm, tibia-astragalus 140 mm, metatarsal three 85 mm, third digit of pes 56 mm.

Using these figures as a guide, an attempt has been made to reconstruct the skeleton of a hatchling tyrannosaur. The adult morphology of A. libratus has been modified by reducing the development of muscle scars and tuberosities, and the number of teeth in the jaws and by increasing the size of the orbit. The ungual phalanges of the manus and pes are shown, as in a very immature specimen (NMC 8782), referred to A. libratus because the first ungual of the manus, unlike in Daspletosaurus , is strongly curved. The lengths of the ribs are entirely conjectural, as the increase in rib length is apparently too rapid in more mature individuals to have been sustained throughout the ontogeny of the animal. It is hoped that this reconstruction ( Figure 3 ) will serve both as an estimation of the actual appearance of a very young specimen of A. libratus , and as a means of focusing attention on misinterpretations unknowingly incorporated into it.

Figures

Figure 3 Albertosaurus libratus, restoration of a hypothetical hatchling. The length of the femur is 100 mm.