Daspletosaurus torosus (Russell, 1970)

Carr, T. D., 1999, Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria), Journal of Vertebrate Paleontology 19 (3), pp. 497-520 : 503-508

Taxonomic Variation

Premaxilla—In rostral view, the premaxillae of the holotype (Stage 4) of D. torosus ( CMN 8605 ) are fused by struts of bone at the base of the nasal process; however, it is possible that the trabeculae are pathological exostoses. In addition, “skirts” of bone extrude from the external margin of the alveoli, and the alveolar process is deep (see Appendix 1 for a synoptic comparison of taxonomic variation).

In caudal view ( Fig. 2A ), the medial process of the premaxilla of D. torosus has a pronounced dorsal flange (100), and there is a crest-like ridge that delimits the subnarial foramen ventrally (101; Fig. 2A ). In D. torosus and Stage 4 T. rex specimens, the maxillary process is broad, elongate, and flattened ( Fig. 8C, I ). In T. rex the maxillary process broadly overlaps the medial edge of the maxillary process of the nasal.

Maxilla----In lateral view in Stage 4 specimens of D. torosus (e.g., CMN 8506 ), the maxilla is thickened transversely, eliminating the depression ventral to the antorbital fossa, and the ventral floor of the fossa is ledge-like (102; Fig. 2G ). The lateral surface sculpture is pronounced, with deep neurovascular sulci (103; Fig. 2G ). The nasal articular surface is displaced medially (104; Fig. 2G ). The vestibular bulla is swollen and convex in transverse section (105; Fig. 2G ) and its foramina are separated by the rugose lateral surface. The lateral surface separating the antorbital fossa from the nasal suture is as deep as the fossa beneath it (106; Fig. 2H ). The articular surface for the nasal may be transversely broad, forming a deep peg-and-socket articulation (107; Fig. 2G, K ).

The alveolar process of the maxilla in D. torosus is deep and expanded; the dorsal and ventral margins gradually converge caudally (108; Fig. 2G ). Alveolar skirts are pronounced (109; Fig. 2G ). The first tooth is subincisiform and the succeeding teeth have subconical crowns. In contrast, the first tooth in A. libratus is incisiform (110; Fig. 2E ), and the crowns of successive teeth are labiolingually compressed. The ventral margin of the antorbital fossa passes caudally along a dorsally convex arc (102; Fig. 2G ). The antorbital fenestra is as tall as long (see Russell 1970:fig. 6). The maxillary fenestra is rostrally elongate and dorsoventrally deep (110), forming a strut medial to the rostral fossa margin ( Fig. 2G ). The promaxillary fenestra lies between these laminae and is dorsally recessed (111; Fig. 2G ).

In medial view in D. torosus (e.g., CMN 8506 ), the dental impressions are pit-like (112; Fig. 2K ). The dorsal margin of the maxillary antrum passes rostroventrally (113; Fig. 2K ), whereas in A. libratus the margin is horizontal, beneath which the maxillary fenestra is situated up to half of its height below (114; Fig. 2I ). In D. torosus , the dorsal margin of the maxillary fenestra approaches (115; Fig. 2K ) or extends to the dorsal margin of the antrum. The caudoventral excavation of the maxillary antrum is enlarged and deep (116; Fig. 2K ). The floor of the promaxillary recess is crossed by a strut above the third alveolus (117; Fig. 2K ).

The palatal process is wide and bounded by a distinct medial ridge; its ventromedial margin passes above the ventral margin of the alveolar process (118; Fig. 2K ). The palatine articular surface is a dorsoventrally deep and flat facet (119; Fig. 2K ). The tooth root bulges are not visible caudally on the palatal process (120; Fig 2K ). The interdental plates are enlarged but unfused (121; Fig. 2K ); in some specimens (e.g., AMNH 3456) only the triangular apices of the rostral plates are visible.

Nasal—In Stage 4 D. torosus (e.g., CMN 8506) and one T. rex specimen (MOR 555), the caudolateral processes are extremely short ( Russell, 1970: fig. 6 ). In D. torosus , the nasals are thickened dorsoventrally throughout their length. The caudal plate is over- and underlapped marginally by the lacrimal, and is constricted between the paired bones (122; Fig. 2D ). Caudal plate constriction is also seen in T. bataar (e.g., PIN 551-2; Maleev, 1974:fig. 3). In D. torosus , the lateral margins of the caudal plate are dorsally everted such that the tract rostral to it is dished (123; Fig. 2D ). The medial frontal process is elongate (124; Fig. 2D ).

In Stage 4 specimens of T. rex (e.g., AMNH 5027), the caudal plate is reduced to an elongate rod by the medially expanded lacrimals ( Fig. 8C ) and the articular surface for the maxilla is a peg-and-socket contact ( Fig. 8E ).

Lacrimal—In D. torosus (e.g., CMN 8506 ), the cornual process is inflated dorsally and transversely (125; Fig. 3F ), broadening its caudal frontal contact, and eliminating the shelf above the ventral ramus (126; Fig. 3F ). This inflated condition is also seen in the homologous region of Stage 4 T. rex specimens ( Fig. 8C, E, I ). In D. torosus , the cornual process is twice as deep as the lacrimal pneumatic recess (127; Fig. 3F, G ). The Tshape of the bone is obscured by the inflated rostral and supraorbital rami (128; Fig. 3F ). The rostral margin of the ventral ramus is embayed by the lacrimal recess (129; Fig. 3F ). The lacrimal antorbital fossa and the lateral surface of the ventral ramus are sharply separated by the leading edge of the ventral ramus (129; Figs. 3F, G ).

The lateral lamina of the rostral ramus appears to be fused with the medial lamina ventrally, rostral to the lacrimal pneumatic recess (45; Fig. 3F, G ). The dorsal process of the rostral ramus is elongate in contrast to the short ventral process (130; Fig. 3G ). The ventral ramus is rostrocaudally broad beneath the dorsal ramus ( Fig. 3F ). The nasal articular surface is deep and over- and underlaps the bone ( CMN 8506 ). The rostral margin of the rostroventral lamina is straight or convex (131; Fig. 3F ). This condition is also in Stage 4 specimens of T. rex ( Fig. 8E ).

Albertosaurus libratus , A. sarcophagus , and D. torosus all possess lacrimal cornual processes, in which the cornual process projects rostrodorsally, the apex set rostral or dorsal to the ventral ramus. In D. torosus , the rostral ramus is inflated such that the horn is not markedly offset in lateral view. The apex of A. sarcophagus is reduced, offset rostrally by a gentle emargination.

In both species of Tyrannosaurus , there is no cornual process ( Fig. 8C, E, I ). Also, the dorsal ramus of Stage 4 specimens is inflated, such that the bone is not dished above the small lacrimal pneumatic recess ( Fig. 8E ). In Stage 4 T. rex specimens, it appears that the medial and lateral laminae of the rostral ramus are fused, and the surface of the latter is excavated laterally by the antorbital fossa and is pierced by two pneumatic foramina ( Fig. 8E ). In Albertosaurus and D. torosus , the ventral and dorsal rami meet at a right angle ( Figs. 2G-K , 3G-I , 5A ). In Tyrannosaurus , the ventral ramus meets the dorsal at an acute angle ( Molnar, 1991) ( Fig. 8E ).

Jugal—In D. torosus (e.g., CMN 8506 ), the cleft between the processes of the maxillary ramus curves caudodorsally in lateral view. In A. libratus , the cleft is horizontal in lateral view (132; Fig. 3I ). The antorbital and secondary fossae of D. torosus are separated by a pronounced, rounded septum (133; Fig. 3L ) (except TMP 85.62.1 ). The caudal margin of the jugal pneumatic recess is rebsorbed, fully exposing the secondary fossa in lateral view (134; Fig. 3L ). This condition also pertains to T. rex ( Fig. 8E ). The postorbital articular surface is deep and braced ventrally by a bony shelf well dorsal to the ventral margin of the orbit (135; Fig. 3L ). The bone is strut-like beneath the postorbital articular surface, producing a deep lateral concavity (136; Fig. 3L ); a similar but less pronounced strut and depression are present in Tyrannosaurus (e.g., AMNH 5027). The cornual process is prominent and transversely swollen (137; Fig. 3L ). The quadratojugal articular surface passes rostrodorsally ahead of the midlength of the ventral process (138; Fig. 3L ). The medial articular surface for the lacrimal is braced ventrally by a ridge. Finally, the maxillary ramus is deepened at the level of the jugal pneumatic recess (139; Fig. 3L ) and the rostral extremity of the ramus is stout (except TMP 85.62.1 ).

As in Albertosaurus and D. torosus, Stage 4 Tyrannosaurus specimens bear a low cornual process ( Fig. 8C, E, I ). In T. rex , the contribution of the jugal to the antorbital fenestra is restricted between the maxilla and lacrimal ( Fig. 8C, E ), unlike the extensive exposure in Albertosaurus ( Fig. 5A ) and D. torosus ( Russell, 1970: fig. 6 ).

Postorbital—In D. torosus (e.g., CMN 8506 ), the cornual process may be enlarged to reach or project beyond the dorsal margin of the bone (81; Fig. 3P ). The jugal ramus tapers to a point (140). In A. libratus , the distal margin of the jugal process is angular in lateral view (69; Fig. 3M ). In D. torosus , the bone terminates far above the ventral margin of the orbit. Also, a small suborbital prong is present (141; Fig. 3Q ). In medial view, the articular surface for the jugal is bipartite and deeply slot-like. The dorsal articular surface for the squamosal terminates caudal to the rostral margin of the laterotemporal fenestra.

In T. rex (e.g., AMNH 5027), a dorsal and ventral differentiation of the patch-like cornual process is evident. A rugosity is sometimes developed from its rostral half that proceeds caudally, and may bear a skirt-like ventral ridge ( Molnar, 1991). In Tyrannosaurus , the suborbital prong is pronounced ( Fig. 8C, E, I ).

Frontal—In Stage 4 D. torosus (e.g., CMN 8506 ), the paired frontals are as wide as long. In Stage 4 D. torosus and T. rex , the lacrimal notch is short rostrocaudally and transversely broad caudally ( Fig. 8C ; Russell, 1970: fig. 6 ). The frontal slopes ventromedially to meet its counterpart at the midline. The rostral margin of the dorsotemporal fossa extends rostrolaterally and is straightened ( Russell, 1970: fig. 7 ).

In Stage 4 specimens of T. rex , the nasal ramus is foreshortened and the dorsotemporal fossa is deep, producing a distinct transverse bar at its rostral margin ( Fig. 8C ). In some specimens (e.g., TMP 81.6.1 ) the rostral fossa margin passes caudolaterally in dorsal view. Also in T. rex , the paired frontals are wider than each is long ( Fig. 8C ). The sagittal crest is concave in lateral view and the rostral extent of the crest is cleft sagittally in dorsal view ( Fig. 8C ).

Prefrontal—In Stage 1 specimens (e.g., ROM 1247) of A. libratus , the prefrontal is stout, slightly longer than wide in dorsal view, and does not extend far rostral to the nasal process of the frontal in dorsal view ( Fig. 5C ).

In small Stage 1 and Stage 4 specimens (e.g., CMN 8506 ) of D. torosus , the prefrontal is several times longer than wide, reaching or extending beyond the nasal process of the frontal rostrally ( Russell, 1970: fig. 7 ). In Stage 4 specimens, the proximal half of the bone is flat and transversely expanded ( Russell, 1970: fig. 7 ).

In Stage 4 T. rex specimens, the prefrontal is exposed dorsally as a sliver of bone between the frontal, nasal, and lacrimal ( Fig. 8C ).

Parietal—In Stage 4 D. torosus specimens (e.g., CMN 8506 ), the nuchal crest is tall, rostrally-everted in lateral view, and dorsolaterally expanded ( Russell, 1970: figs. 6 , 7 ). The lateral margins are deeply concave in caudal view. In lateral view, the dorsal margin of the keel-like sagittal crest is deeply concave rostrally. The lateral contact with the frontal is strengthened by a strong transverse ridge ( Russell, 1970: fig. 7 ).

In Stage 4 T. rex specimens, the nuchal crest may be rostrocaudally thick and bears a flat and rugose dorsal platform ( Fig. 8C, E, G, I ). The sagittal crest is transversely thick ( Fig. 7C ).

Supraoccipital—In Stage 1 specimens (e.g., ROM 1247) of A. libratus , the suture with the otoccipital is deeply interdigitating. In Stage 4 D. torosus (e.g., CMN 8506 ) and T. rex (e.g., TMP 81.6.1), the suture with the otoccipital may be coalesced. In Stage 4 T. rex , the dorsal ramus of the supraoccipital is divided into two processes separated by deep muscle insertions ( Fig. 8G ), possibly for M. splenius capitis (pers. obs. of Dromaius novaehollandiae).

Basioccipital—In Stage 4 D. torosus (e.g., CMN 8506 ), the basioccipital is broadly exposed on the floor of the foramen magnum between the exoccipitals. The ventral surface of the basituberal web is blade-like, as in T. rex . The basituberal web may be horizontal in caudal view. The basioccipital is excavated by a deep pit ventral to the neck of the occipital condyle. The basal tuber may be elaborated into a rugose block.

In Stage 4 T. rex (e.g., AMNH 5027), the subcondylar recess is shallow ( Fig. 8G ), induced by the transversely broad and inflated paroccipital region. The recess is occupied by the small, oval basioccipital pneumatic foramen, that pierces the dorsolateral comer of the bone next to the basioccipital-otoccipital suture ( Fig. 8G ).

Unlike the arcuate suture of other tyrannosaurids, the otoccipital-basioccipital suture of the occipital condyle of the topotype and a referred specimen (ROM 12790) of A. sarcophagus is angular in caudal view. The basioccipital separates the otoccipitals from the midline of the condyle by a triangular process. However, the holotype of A. sarcophagus displays the condition typical of other tyrannosaurids.

Basisphenoid—In Stage 4 D. torosus (e.g., CMN 8506 ), the rostral margin of the cultriform process curls medially to enclose the space between itself and the rostral surface of the bone. The ventral margin of the bone descends steeply rostroventrally. The basipterygoid facet faces rostrally. The basisphenoid foramina are dorsoventrally elongate and taper dorsally; they are positioned high within the basisphenoid recess. Contra Russell (1970), the presence of a single median pneumatic foramen in the holotype ( CMN 8506 ) is doubtful; the left wall of the recess is crushed medially, obscuring the region of the left foramen completely. In FMNH PR308 , the lateral margins of both foramina and a fragment of the median septum between them are visible, the recess in this specimen is otherwise filled with matrix. Further preparation of both specimens is required to clarify structural details.

In Stage 4 D. torosus , the oval scar may bear a strongly rugose surface that is convex in transverse section and passes uninterrupted onto the basal tuber. In FMNH PR308 the oval scar is dished and rugose.

In Stage 4 T. rex specimens (e.g., AMNH 5027), the rostral plate of the bone is rotated caudally to contact the rostral surface of the basioccipital, eliminating the basisphenoid recess ( Fig. 8I ). Also, the basisphenoid pneumatic foramina are small and their dorsal margins are at the level of the basioccipital ( Fig. 8A, G ). The oval scar is concave with a rough surface ( Fig. 8 A, G ).

In the A. sarcophagus specimens CMN 5600 and TMP 86.64.1, each basisphenoid pneumatic foramen lies within a distinct fossa.

Laterosphenoid—In Stage 1 specimens (e.g., ROM 1247) of A. libratus , the caudolateral surface is broadly convex in transverse section and gently excavated ventral to the dorsotemporal fossa. The rostrolateral margin is poorly developed, such that the profundus branch of the trigeminal nerve (N. trigeminus) (V) exits rostrally ( Russell, 1970).

In Stage 4 D. torosus specimens (e.g., CMN 8506 ), the caudolateral surface is interrupted by a strong, sharp-edged ledge that separates the dorsotemporal fossa from the rostrolateral surface of the bone. The strut-like rostrolateral margin of the bone displaces the exit for the profundus branch of N. trigeminus (V) caudolaterally ( Russell, 1970).

Vomer—In Stage 4 D. torosus specimens (e.g., CMN 8506 ), the ventral margin is deep, eliminating the neck-like region in lateral view (142; Fig. 4B ). In D. torosus and A. libratus , the maxillary process is expanded and parallel-sided unlike that of Tyrannosaurus , which is diamond-shaped ( Fig. 8A ; Osborn, 1912)

Ectopterygoid—In Stage 1 specimens (e.g., ROM 1247) of A. libratus , the ectopterygoid is dorsoventrally compressed with small, slit-like pneumatic foramina (143; Fig. 4C ). The muscle scar on the jugal ramus is caudodorsal in position (144; Fig. 4C ).

In Stage 4 D. torosus and T. rex , the ectopterygoid is inflated (145; Fig. 4D , 8A ), with enlarged, rounded pneumatic foramina (146; Figs. 4D , 8A ). The muscle scar on the jugal ramus is broad and swollen, and faces ventrally to flank the lateral pneumatic foramen (147; Fig. 4D ). In T. rex (e.g., MOR 555), the bone is transversely elongate.

Palatine—In Stage 4 D. torosus (e.g., CMN 8506), the caudal pneumatic recess is round and there is a broad separation between the recesses. These are also present in the skull of the holotype of A. sarcophagus , CMN 5600. In D. torosus , the palatine is transversely inflated, a condition seen in the A. sarcophagus specimens CMN 5600 and CMN 5601. The palatine is inflated in Stage 4 T. rex specimens (e.g., AMNH 5027).

Surangular—In Stage 4 D. torosus , the surangular may be deep (148; Fig. 4G ). The intramandibular process is deep and stout and meets the rostroventral margin of the surangular at a very low angle or is confluent with the rostroventral margin (149; Fig. 4G ). The rostral plate is laterally convex (150; Fig. 4G ). The dorsomedial flange may be low or dorsally expanded (151; Fig. 4G ). The dorsolateral muscle scar may be rugose rostrally. The surangular shelf is depressed over the surangular foramen (152; Fig. 4G ). This condition is also in T. rex . The surangular foramen is large and deeply recessed (153; Fig. 4G ). The fossa ventrolateral to the glenoid may be dorsoventrally deep or is a rugose pocket (154; Fig. 4G ). The caudal margin of the retroarticular process is concave with a caudoventrally projecting heel (155; Fig. 4G ).

The surangular of Stage 4 T. rex specimens is deep with a subvertical rostral margin and transversely convex rostral plate ( Fig. 8E ). The dorsomedial flange is low, and the muscle scar lateral to it is rugose. A deep rugose pocket is present lateroventral to the glenoid ( Fig. 8E ).

Angular—In Stage 1 specimens of A. libratus , the bone extends caudal to the surangular foramen ( Fig. 5B ). The dorsolateral scar passes medial to the caudal plate as a rugose sulcus (156; Fig. 4H ). The ventral margin of the rostral process and caudal plate forms a relatively continuous, convex ventral margin (157; Fig. 4H ).

In Stage 4 specimens of D. torosus , the caudal plate of the bone is dorsoventrally deep (158; Fig. 4I ) and may be restricted rostral to the surangular foramen when in articulation ( Russell, 1970: fig. 5 ). It extends caudally in FMNH PR308 ( Fig 1 ; contra Russell, 1970). The dorsolateral scar that passes medial to the caudal flange is strongly pronounced and rugose (159; Fig. 4I ). Finally, the rostral process is flexed dorsally relative to the caudal plate (160; Fig. 4I ).

In Stage 4 T. rex specimens, the angular is deep, the rostral process is flexed relative to the caudal plate ( Fig. 8E ), extends caudal to the surangular foramen ( Fig. 8E ; Russell, 1970), and the dorsomedial scar is a pronounced ridge.

Prearticular—In Stage 4 D. torosus , the dorsal margin of the deepened caudal ramus is rostral in position and may be developed into a strong keel (161; Fig. 4L ); its dorsal and ventral margins may be straightened as they taper rostrally (161; Fig. 4L ). The distal margin of the paddle-like rostral lamina (162) may be convex (163; Fig. 4M ). These features are also present in Stage 4 T. rex (e.g., AMNH 5027).

Splenial—In Stage 1 specimens (e.g., ROM 1247) of A. libratus , the rostral extent of the articular surface for the dentary is flat and forms a peg-and-socket contact at its rostral extent. In Stage 4 D. torosus (e.g., CMN 8506 ), the lateral articular surface for the dentary is reinforced by arcuate, interleaving ridges that fit into corresponding slots in the dentary.

Dentary—In Stage 4 D. torosus (e.g., CMN 8506 ), the angular process is dorsoventrally deep. In medial view, the symphysis may be contained rostrally between the rostromedially extending lateral surface and caudally by a pronounced bony ridge. The symphyseal surface is lightly rugose. The articular surface for the splenial extends rostrally, indicated by strong rostroventral ridges and slots. Two to four foramina are present (instead of one or two in A. libratus ) at the rostral end of the Meckelian canal. The ventral bar beneath the Meckelian foramina is extremely rugose and transversely convex, obliterating the ventrally passing sulcus. In addition, a low eminence is present at the rostral end of the ventral bar as in T. rex .

Dentition

In Stage 1 and Stage 3 specimens of A. libratus , the first maxillary tooth is incisiform. In Stage 4 D. torosus the first tooth is subincisiform, modified by labiolingual thickening of the tooth. In Stage 1 A. libratus , the maxillary teeth are labiolingually compressed and blade-like. The fourth maxillary tooth has a crown width/length ratio of 0.52 (ROM 1247); in Stage 4 D. torosus , the teeth are thicker—the crown width/length ratio of the fourth tooth is 0.77 ( CMN 8506 ). In D. torosus , the thick maxillary dentition likely increases the depth and width of the alveolar region.

In Stage 1 A. libratus , all of the dentary teeth are labiolingually narrow, except for the first. The width/length ratio for the crown of the fourth dentary tooth is 0.5 (ROM 1247); the width/length ratio increases caudally, reaching 0.71 at the 14th tooth. In Stage 4 D. torosus , the mesial dentary teeth are labiolingually thick. The crown width/length ratio of the fourth tooth ranges from 0.7 ( CMN 8506 ) to 0.9 (CMN 11594); the distal teeth are also wide, the ratio is 0.73 (CMN 8506).

Figures

APPENDIX 1. A genus-level comparison of tyrannosaurids: the growth changes of Albertosaurus libratus compared with the homologous characters in Daspletosaurus torosus and Tyrannosaurus rex. Abbreviations: aofen, antorbital fenestra; aofo, antorbital fossa; artic, articular; basis pn for, basisphenoid pneumatic foramen; cdrsl, caudodorsal; cnflt, confluent; cnvx, convex; corn, cornual; dtf, dorsotemporal fossa; fen, fenestra; hrz, horizontal; intramnd, intramandibular; ltfen, laterotemporal fenestra; M. add mand ext, Musculus adductor mandibular pars externus; max, maxilla; occ con, occipital condyle; pn, pneumatic; po, postorbital; pr, process; qj, quadratojugal; rd, rostrodorsal; rv, rostroventral; sbn, subnarial; shllw, shallow; smth, smooth; sq, squamosal; surf, surface.

FIGURE 1. Relative completeness of the skull and jaws of FMNH PR308 (Daspletosaurus torosus) in left lateral (A) and right lateral (B) views. Skull length is 1,050 mm.

FIGURE 2. Growth series of Albertosaurus libratus craniofacial bones: nasals in dorsal view (TMP 86.144.1, B; ROM 1247, C); maxillae in lateral (ROM 1247, E; AMNH 5336, F) and medial (CMN 12063, J; ROM 1247, I) views. Craniofacial bones of Daspletosaurus torosus'. premaxilla (CMN 8506, A); nasals (CMN 8506, D) in dorsal view; maxilla in lateral (CMN 8506, G; AMNH 5346, H) and medial (CMN 8506, K) views. Arrows indicate features discussed in text. Bones have been reversed to face right when required. Scale bar equals 50 mm; F is not to scale.

FIGURE 3. Growth series of Albertosaurus libratus craniofacial bones: lacrimals in lateral view (TMP 86.144.1, A; ROM 1247 B; AMNH 5336, C, D; CMN 2120, E); jugals in lateral view (ROM 1247, I; AMNH 5336, J; CMN 2120, K); and postorbitals in lateral view (TMP 86.144.1, M; AMNH 5664, N; AMNH 5336, O). Craniofacial bones of Daspletosaurus torosus'. lacrimals in lateral view (CMN 8506, F; TMP 85.62.1, G; CMN 11594, H); jugal in lateral view (FMNH PR308, L); and postorbitals in lateral view (CMN 11594, P; FMNH PR308, Q). Arrows indicate features discussed in text. Bones have been reversed to face right when required. Scale bar equals 50 mm; J is not to scale.

FIGURE 4. Comparison of palatal and mandibular bones: Albertosaurus libratus vomer (A) in lateral view, ectopterygoid (ROM 1247, C) in rostroventral view, growth series of surangulars (ROM 1247, E; CMN 2120, F) in lateral view, angular (ROM 1247, H) in lateral view, and growth series of prearticulars (ROM 1247, J; CMN 2120, K) in medial view. Daspletosaurus torosus vomer (CMN 8506, B) in lateral view, ectopterygoid (CMN 8506, D) in rostroventral view, surangular (CMN 8506, G) in lateral view, angular (CMN 8506, I) in lateral view, and prearticular (CMN 8506, L, M) in medial view. Arrows indicate features discussed in text. Bones have been reversed to face right when required. Scale bar equals 50 mm.

FIGURE 5. Composite (AMNH 5664, CMN 11315, ROM 1247, TMP 91.36.500, USNM 12815) early ontogeny (large Stage 1) skull and jaw of Albertosaurus libratus in lateral (A, B) and dorsal (C) views. Skull length approximately 750 mm.

FIGURE 6. Restored skull with occluded jaws of a Stage 1 Tyrannosaurus rex (CMNH 7541) in left lateral (A), dorsal (B), occipital (C), and palatal (D) views. Skull length is 572 mm. Rostral and transverse crushing of the suspensorium in A and B has not been corrected. Basisphenoid pneumatic foramina are restored in D; right squamosal is restored in B; occipital condyle is restored in C. Abbreviations: ang, os angulare; ar, os articulare; bo, os basioccipitale; bs, os basisphenoidale; bs rec cl, basisphenoid recess ceiling; co, os coronoideum; cr sag, crista sagittalis; dn, os dentale; ect, os ectopterygoideum; imf, fenestra intramandibulare; ipv, vacuita interpterygoideum; la, os lacrimale; mx, os maxillare; na, FIGURE 6. Continued os nasale; ot, os otoccipitale; pa, os parietale; pl, os palatinum; pmx, os premaxillare; pra, os prearticulare; prf, os prefrontale; pt, os pterygoideum; qj, os quadratojugale; qu, os quadratum; qu for, foramen quadratum; rec pnu bs, recessus pneumaticum basisphenoidale; rec pn pl, recessus pneumaticum palatinum; sq, os squamosum; spl, os spleniale; spl fen, fenestra spleniale; suborb fen, fenestra suborbitalis; sur, os surangulare; tp, tooth puncture; vo, vomer.

FIGURE 8. Comparison of Stage 1 (CMNH 7541) and Stage 4 (AMNH 5027, MOR 555) skulls of Tyrannosaurus rex in palatal (A, B), dorsal (C, D), lateral (E, F), caudal (G, H), and rostral (I, J) views. Numbered labels indicate T. rex autapomorphies: (1) nasal processes of the premaxillae tightly appressed throughout their entire length; (2) restricted exposure of the jugal within the antorbital fenestra; (3) antorbital fossa reaches the nasal suture caudodorsally; (4) transversely broad jugal pneumatic recess; (5) elongate frontal sagittal crest; (6) strongly divergent and short basal tubers; (7) rostroventrally-oriented caudal occipital plate; (8) shallow subcondylar recess; (9) rostroventrally deep basisphenoid plate and rostrocaudally-restricted basisphenoid recess; (10) inflated ectopterygoid; (11) strongly convex rostral plate of the surangular; (12) transversely narrow snout and broad temporal region relative to other tyrannosaurids; and (13) deep mandible relative to other tyrannosaurids.

Russell, 1970: Figure 7 Daspletosawus torosus, reconstruction of the skull in dorsal aspect, based on NMC 8506.