Gorgosaurus libratus Lambe 1914

In the following sections, elements of Gorgosaurus (Figs. 1–4) will be described first. The rationale is that this is the most common (and possibly least derived) Alberta tyrannosaurid. The same elements of Albertosaurus are compared (Figs. 6–16), followed by those of Daspletosaurus (Figs. 18–36). The figures of Gorgosaurus, Albertosaurus, and Daspletosaurus are arranged taxonomically, rather than in the order they are referred to in the text. This is to improve the reader’s chances of finding figures when leafing through the paper. TMP 91.36.500 (Fig. 1) is a virtually complete skeleton of Gorgosaurus libratus that is 5.1 m in length from the premaxilla to the tip of the tail is. It was presumably a young adult. Although the neural arches are fused to the centra, the sutures are still visible. Albertosaurus sarcophagus is represented (Figs. 6–16) by TMP 81.10.1, which was about 8 m long at the time of death, and TMP 86.64.1, an almost complete, 6.5 m long articulated skeleton found near the Tyrrell Museum. Most of the Daspletosaurus specimens are large individuals. However, the best−preserved skull is that of TMP 94.143.1 (Figs. 18–36), an animal which was approximately 5.8 m long. The skull (Fig. 2) of TMP 91.36.500 (G. libratus) is 640 mm long when measured between the premaxilla and the occipital condyle, and 670mm between the premaxilla and the posteroventral margin of the quadrate. The only smaller, reasonably complete skull of G. libratus (TMP 86.144.1) is disarticulated, but would have been 50cm long. AMNH 5664 (the type specimen of Gorgosaurus sternbergi) has a slightly larger skull with a lateral length of 678 mm. Cranial fragments and a pair of nearly complete lower jaws (TMP 94.12.155) are from a smaller G. libratus individual whose skull would have been 364 mm long (based on a logarithmic comparison [y = 1.1068x – 0.0317, r 2 = 0.95] of 22 tyrannosaur skulls where both skull and jaw lengths are known). The small skull of Daspletosaurus sp. (TMP 94.143.1) is about 620mm long between the premaxilla and quadrate. Small, juvenile skulls are known for two other tyranno−saurids—that of Nanotyrannus lancensis (CM 7541) is 575 mm long (premaxilla to occipital condyle) and that of Shanshanosaurus huoyanshanensis (IVPP V4878) is an estimated 288 mm long (Currie and Dong 2001). The skulls of smaller tyrannosaurids are relatively long and low compared to the adults. The maximum cranial width of TMP 91.36.500 is 160 mm across the postorbitals, which is half that of an adult G. libratus (UA 10), suggesting that the skull was relatively narrower at the back in juveniles. The equivalent width measurements in Daspletosaurus sp. (TMP 94.143.1) and Nanotyrannus, which have shorter skulls, are respectively 154 and 210mm. These genera are clearly broader across the postorbital region of the skull at any equivalent age. The antorbital fenestra of the small G. libratus (TMP 91.36.500) makes up 37% of the antorbital skull length and 62% of the preorbital height. These proportions are almost the same (38%, 63%) in an adult G. libratus (UA 10), which shows that there is little ontogenetic change in the length of the antorbital fenestra during growth. Unlike allosaurids, sinraptorids and most other theropods, the tyrannosaurid antorbital fossa does not extend onto the nasal. At the level of the back of the maxillary tooth row, the ventral edge of the antorbital fossa extends more than 10 mm below the ventral border of the antorbital fenestra in Gorgosaurus, whereas the two margins almost coincide in Daspletosaurus (Fig. 16). A. sarcophagus (NMC 5601, TMP 81.10.1, 85.98.1) seems to have a wider, more horizontal separation between the two margins than G. libratus. The promaxillary slit is relatively smaller in Daspletosaurus (Fig. 18) and Tyrannosaurus (Brochu 2002) than it is in Albertosaurus (Russell 1970) and Gorgosaurus. The maxillary fenestra is midway between the anterior margins of the antorbital fossa and antorbital fenestra (Figs. 2A, B, 3A) in Gorgosaurus of any age. Small tyrannosaurine specimens are similar to Gorgosaurus in that the maxillary fenestra is relatively small and central (Fig. 3E, F). However, mature tyrannosaurines have greatly enlarged maxillary fenestrae with anterior margins that are coincident with the anterior margins of the antorbital fossae (Fig. 3D, G, H). The orbits of juvenile tyrannosaurids are only slightly higher than long. Contrary to Carr (1999), this is not so much of a juvenile characteristic as it is of small size, and all theropod adults smaller than tyrannosaurid juveniles also have large, round orbits. Premaxilla.—The pitted external surface of the premaxilla of Gorgosaurus is higher than long. The supranarial process only diverges slightly from the long subnarial process. The gently curving contact with the maxilla is interrupted by a slit−like subnarial foramen. The distal end of the subnarial process is separated from the maxilla by the nasal. In dromaeosaurids and ornithomimimids, the subnarial process of the premaxilla is wedged between the nasal and maxilla. The subnarial process of the Gorgosaurus premaxilla meets and overrides the nasal beneath the external naris as in Daspletosaurus (Fig. 18), Nanotyrannus, Tarbosaurus (Maleev 1974), and Tyrannosaurus. This may not be the case in some specimens of Tyrannosaurus (Brochu 2002). Russell (1970) reported that the premaxilla did not meet the nasal below the external naris in Daspletosaurus torosus, and used this characteristic to distinguish Daspletosaurus from Albertosaurus. However, the sutures for the premaxilla and nasal on the maxillae (NMC 8506, TMP 89.17.53, 94.172.115) demonstrate that the subnarial processes of Daspletosaurus met below the external naris in all specimens. The nasal process in immature and mature specimens of Gorgosaurus, Albertosaurus, juvenile Daspletosaurus, and juvenile Tarbosaurus bataar (GIN 100/777) are distally forked. The medial process of the fork sits in a groove on the dorsal surface of the nasal, and the distal ends of the paired premaxillae are separated from each other by the nasals. In Nanotyrannus, Tyrannosaurus (Holtz 2001, Brochu 2002), and mature specimens of Daspletosaurus and Tarbosaurus, the nasal processes are closely appressed and taper posteriorly to separate the anterior tips of the nasals. This character is gradational with the posterior tips of the nasal processes always being separated in albertosaurines and juvenile specimens of Daspletosaurus and Tarbosaurus. At least some mature Daspletosaurus torosus (NMC 8506) and T. rex specimens (BHI 3033) show a remnant of the groove in the nasal for the posterior tip of the premaxilla. The nasal processes of the premaxillae are separate distally in at least one specimen of T. rex (TMP 81.6.1). In ventral view, the teeth of the paired premaxillae of G. libratus (TMP 91.36.500) form a semicircle that is wider than anteroposteriorly long. The premaxilla forms the anterior wall of the alveolus for the first maxillary tooth, an unusual arrangement amongst theropods. Maxilla.—Almost half the maxillary length of G. libratus (TMP 91.36.500) is in front of the antorbital fenestra. One of the foramina on the external surface is relatively large and anteriorly oriented, and is connected to the subnarial foramen by a depression (Fig. 2A). As in Albertosaurus (Fig. 6A), Alioramus (Kurzanov 1976), Daspletosaurus (Fig. 18A), and Tyrannosaurus (Brochu 2002), there is a second major row of foramina that arches above the alveolar margin. The posterior end of the maxilla bifurcates into two processes (Fig. 2A), the lower of which underlies the jugal. The medial surface of this process extends higher than the lateral surface, and the jugal sits in a shallow trough between the two. The upper process is a vertical plate that separates two processes at the front of the jugal, the outer one of which covers the lateral surface of the maxilla. The medial edge of the distal end of the maxilla reaches the ectopterygoid (Fig. 2B), although there are no conspicuous contact surfaces on either bone. In all tyrannosaurids, the antorbital fossa forms a smoothwalled depression in the ventral margin of the posterodorsal process. Dorsally there is a laterally rugose ridge that separates the antorbital fossa and the nasal other than for a short distance posteriorly. Distally, the posterodorsal process bifurcates to embrace the margins of the anterodorsal process of the lacrimal. The lower of the two prongs contacts the lower surface of this process, whereas the shorter upper prong overlaps the lateral surface. In TMP 83.36.100 (G. libratus), the lacrimal extends anteriorly more than 5 cm along the medial surface of the posterodorsal process of the maxilla. The maxilla−lacrimal contacts of A. sarcophagus (TMP 86.64.1) and Daspletosaurus (Fig. 18A) are similar, although the anterior tip of the lacrimal is not separated from the maxilla by the nasal in mature specimens of Daspletosaurus sp. (TMP 85.62.1, 98.48.1), Tarbosaurus (Hurum and Sabath 2003), and some specimens of Tyrannosaurus. The anterodorsal process on the medial side of the maxilla in tyrannosaurids protrudes only a short distance beyond the end of the bone to contact the inner surface of the premaxilla. The intermaxillary suture of G. libratus (TMP 85.11.3) has three prominent ridges and grooves above the first four maxillary teeth along the internal surface of the palatal shelf of the maxilla. The vomer overlapped the ventral surface at the back of this process. From a point above the fourth maxillary tooth to the level of the anterior margin of the antorbital fenestra above the eighth maxillary tooth, the internal margin of the palatal shelf is smooth and rounded to form part of the boundary of the internal naris. Behind this point there is a well−defined suture for the palatine. The lower surface of the palatal shelf in Gorgosaurus has pronounced depressions that coincide with the positions of dentary t

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