Albertosaurus libratus, Lambe, 1914

Carr, T. D., 1999, Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria), Journal of Vertebrate Paleontology 19 (3), pp. 497-520 : 499-503

Ontogenetic Variation in Albertosaurus libratus

Premaxilla—The premaxilla of Stage 1 specimens of A. libratus is transversely narrow with a concave lateral margin, has a shallow alveolar process ( Fig. 5A ), narrow maxillary process ( Fig. 5C ), and the maxillary articular surface of the alveolar process is transversely narrow. The premaxillae of Stage 2 specimens are not distinctive. In Stage 3 specimens, the premaxillae are transversely broad in rostral view, which straightens the lateral margin of the bone.

Maxilla—In small Stage 1 specimens (e.g., CMN 12063 ) of A. libratus , the maxilla is transversely compressed. In large Stage 1 specimens (e.g., ROM 1247) in lateral view, the bone is thickened, and the slot for the maxillary process of the nasal is dorsolateral in position (1; Fig. 2E ). In small Stage 1 specimens, the margin of the antorbital fossa is sharply delimited, and its ventral margin may pass caudally in a concave arc, or is straight; in larger specimens, the rostroventral margin of the fossa grades into the textured bone surface (2; Figs. 2E, 5A ). In small specimens, the base of the interfenestral strut is flat, but is gently concave in large specimens (3; Figs. 2E , 5A ).

In Stage 1 specimens, the antorbital fenestra is longer than high ( Fig. 5A ). The lateral surface is textured and incised by shallow neurovascular sulci, the ventral margin of the antorbital fossa is bounded by a low ridge (4), and the alveolar process is shallow (5; Figs. 2E , 5A ). The vestibular bulla and passage of the subnarial foramen are laterally flat or transversely convex (6; Fig. 2E ).

Also in Stage 1 specimens, the rostroventral foramen of the premaxillary process is small and the rostrodorsal foramen is a cleft-like, ventrally opening slit ( Fig. 5A ). The caudal foramen of the ventral row of foramina bears a caudal elongate sulcus, that does not leave the ventrolateral margin of the jugal process (7; Figs. 2E , 5A ). The maxillary fenestra is positioned midway between the rostral margins of the antorbital fenestra and fossa, and is as long as deep or barely longer than high (8; Figs. 2E , 5A ). The lateral surface separating the antorbital fossa and nasal suture is a shallow tab (9; Figs. 2E , 5A ). Finally, the promaxillary fenestra is a slit-like foramen within the rostral margin of the antorbital fossa (10; Figs. 2E , 5A ) ( Russell, 1970).

In Stage 2 specimens (e.g., AMNH 5336), the maxilla is thickened laterally and bowed rostrally (11; Fig. 2F ). The rostrolateral surface is expanded rostrally and dorsoventrally ( Fig. 2F ) and its surface sculpturing is pronounced. The ventral margin of the antorbital fossa is either gently sigmoid or dorsally convex ( Fig. 2F ). The depression ventral to the antorbital fossa is shallow (12; Fig. 2F ). The maxillary fenestra is longer than high and approaches the rostral margin of the antorbital fenestra (13; Fig. 2F ). The promaxillary fenestra is recessed dorsally (14), and the lateral surface of the maxilla passes over the rostral margin of the antorbital fossa as a strut (15; Fig. 2F ). The rostroventral foramen of the premaxillary process is larger than the round rostrodorsal foramen. The ventral rim of the ventral jugal process is breached by the sulcus from the caudal foramen of the ventral row of foramina (16; Fig. 2F ).

In Stage 3 specimens (e.g., AMNH 5458), the rostrolateral surface of the bone is thickened, bowed, and expanded, displacing the articular surface for the nasal dorsally. The rostrolateral foramina are enlarged. The base of the interfenestral strut is concave, and the height of the antorbital fenestra approaches its length.

In medial view in A. libratus , shallow pneumatic excavations are present in the maxillary antrum in Stage 1 specimens (e.g., ROM 1247; Fig. 2I ). The caudoventral excavation may be shallow or pocket-like (17; Fig. 2I ). The floor of the promaxillary antrum is crossed by a low ridge above the third alveolus. The palatal process has a strong rostrodorsal sigmoid curvature (18); its caudal surface is flat such that its dorsal surface is visible (19) and its ventral margin extends beneath the level of the medial alveolar process (20; Figs. 2I, J ). The medial edge of the palatal process is cleaved by the articular surface for the palatine (21; Figs. 2I, J ). Also, the palatal process is transversely narrow; the tooth root bulges are low (22; Fig. 2J ), and the interdental depressions are shallow.

Nasal—In Stage 1 specimens (e.g., ROM 1247) of A. libratus , the nasal is lightly built, and moderately or strongly vaulted rostrally ( Figs. 2B, C , 5A, C ). The caudal plate expands slightly between the lacrimals (25; Figs. 2B, C , 5C ). The lacrimal articular facet overlaps the dorsolateral edge of the bone (26; Figs. 2B, C , 5C ). The dorsal surface of the nasal is irregular and bears rugosities, cusps, and bony papillae that interrupt the dorsal row of neurovascular foramina ( Fig. 5C ).

In Stage 2 specimens (e.g., ROM 4591 ), the nasal is thickened rostrally and the ventral surface is not strongly vaulted. The caudal plate expands between the lacrimals. The medial frontal process may be absent in Stage 2 specimens (e.g., AMNH 5336). In Stage 3 specimens, the lateral margins of the caudal plate expand or are parallel (e.g., CMN 2120) between the lacrimals.

Lacrimal—In small Stage 1 specimens (e.g., TMP 86.144.1 ) of A. libratus , the ventral process of the rostral ramus is absent ( Fig. 3A ); in larger specimens (e.g., ROM 1247), the ventral process is incipient (25; Fig. 3B , 5A ). In small Stage 1 specimens, the cornual process is a weak ridge with three apices and is shallower than the lacrimal pneumatic recess (26; Fig. 3A ). In large Stage 1 specimens, the cornual process projects rostrodorsally, bears two apices, and is as deep or deeper than the lacrimal pneumatic recess (27; Figs. 3B , 5A ). The lacrimal of Stage 1 specimens is T-shaped, with the supraorbital ramus projecting strongly behind the ventral ramus (28; Figs. 3A, B ; 5A ). In larger specimens, the rostrolateral margin of the cornual process is eave-like, and dished above the lacrimal pneumatic recess (29; Figs. 3B , 5A ). In large Stage 1 specimens the dorsolateral surface of the supraorbital ramus bears a strong shelflike ridge (30; Figs. 3B , 5A ).

In small Stage 1 specimens, the rostral ramus is divided into lateral and medial processes; the former is situated dorsal to the latter, so that the ramus is forked in lateral view (31; Fig. 3A ). In larger Stage 1 specimens, the processes overlap in lateral view, losing the forked appearance (32; Fig. 3B ). In small specimens, the lacrimal antorbital fossa of the dorsal ramus is fully exposed to view (33; Fig. 3A ). In larger specimens, the lateral external surface is extruded ventrally as a lamina, partly concealing the fossa in lateral view, creating a slot-like passage (31; Figs. 3B , 5A ).

In Stage 1 specimens, the textured dermal surface and the smooth fossa surface merge at their juncture (32; Fig. 2A ). In Stage 1 specimens, the rostral margin of the ventral ramus merges with the ventral lip of the lacrimal antorbital fossa (33; Fig. 3A, B , 5A ). Also, the rostral margin of the rostroventral lamina of the lacrimal is straight or concave in lateral view (34; Figs. 3A, B , 5A ). The jugal articular surface of the rostroventral lamina exceeds that of the ventral ramus. The caudal margin of the jugal articular surface of the ventral ramus is subvertical ( Fig. 5A ).

In Stage 2 specimens (e.g., AMNH 5336), the ventral process of the rostral ramus is developed, but is shorter than the dorsal process (35; Fig. 3C ). The cornual process may be bulbous and has one apex (36; Fig. 3C ). The ventral margin of the lateral lamina of the rostral ramus matches that of the lacrimal antorbital fossa (37; Fig. 3D ). The caudal margin of the jugal articular surface of the ventral ramus slopes caudodorsally (38; Fig. 3E ). This is also in D. torosus (39; Fig. 3H ) The rostral margin of the ventral ramus is embayed by the lacrimal antorbital fenestra (40; Fig. 3C ).

In Stage 3 specimens (e.g., CMN 2120), both the dorsal and ventral processes of the rostral ramus are elongate (41; Fig. 3E ). The cornual process bears a single erect apex, situated above the ventral ramus (42; Fig. 3E ). The lateral surface around the lacrimal pneumatic recess is not dished (43; Fig. 3E ). The lacrimal antorbital fossa is attenuated rostrally by the lateral lamina (44; Fig. 3E ). This condition is also seen in Stage 4 D. torosus (45; Fig. 3G ) and T. rex . In A. libratus , it is equivocal whether or not the lateral and medial laminae are fused ventrally; the specimen in which this is observed (CMN 2120) is mediolaterally crushed in this region ( Fig. 3E ).

In Stage 3 specimens (e.g., AMNH 5458), an edge separates the rostral margin of the ventral ramus from the lacrimal antorbital fossa. The rostral margin of the rostroventral lamina is convex in lateral view (46; Fig 3E ) and the extent of its contact with the jugal is matched by that of the ventral ramus (47; Fig. 3E ). This is also in Stage 4 D. torosus ( Fig. 3F ).

Jugal—In small Stage 1 specimens (e.g., TMP 86.144.1) of A. libratus , the postorbital ramus is caudodorsally declined. In Stage 1 specimens (e.g., ROM 1247), the orbital margin is low and elongate (48; Fig. 3I ) and the maxillary ramus of the jugal is straplike and shallow (49; Fig. 3I ). In larger Stage 1 specimens, the jugal pneumatic recess is a transversely narrow slit ( Fig. 5A, C ); its caudal edge may be united with that of the lacrimal (50; Russell, 1970) ( Figs. 3I , 5A ). Infrequently, the caudal margin of the recess may be resorbed, exposing the rostral extent of the secondary fossa to lateral view (51; Figs. 3I , 5A ).

The caudal margin of the lacrimal articular surface is subvertical in lateral view (52; Figs. 3I , 5A ). The articular surface for the postorbital is shallow and extends to the ventral orbital margin (53; Figs. 3I , 5A ). The medial articular surface for the lacrimal is overlapping. The caudal margin of the postorbital ramus is sinuous or concave (54; Figs. 3I , 5A ). The area ventral to the postorbital ramus is flat or convex in lateral view (55; Figs. 3I , 5A ). The transversely flat cornual process may be pronounced or its caudal margin may be low (56; Figs. 3I , 5A ). Finally, the quadratojugal articular surface passes rostrodorsally at or caudal to the midlength of the ventral process of the quadratojugal ramus, either horizontally or at a steep angle (57; Figs. 3I , 5A ).

In Stage 2 specimens (e.g., AMNH 5336), the postorbital articular surface ends above the ventral margin of the orbit (58; Fig. 3J ), the caudal margin of the postorbital ramus is strongly convex or straight at midheight (59; Fig 3J ), and the cornual process is pronounced (60; Fig. 3J ).

In Stage 3 specimens, the maxillary ramus is dorsoventrally deep (61; Fig. 3K ). The caudal margin of the jugal foramen is resorbed, exposing the rostral margin of the secondary fossa (62; Fig. 3K ). The lateral surface at the base of the postorbital ramus is shallowly concave (63; Fig. 3K ). The caudodorsal margin of the jugal pneumatic recess merges with the lateral surface of the jugal beneath the lacrimal contact (64; Fig. 3K ). Finally, the caudal margin of the lacrimal articular surface slopes caudodorsally along an elongate and shallow lateral overlap (65; Fig. 3K ).

Postorbital—In lateral view, the postorbital of small Stage 1 specimens of A. libratus is a slender and delicate bone ( Fig. 3M ). The frontal ramus approaches the length of the squamosal and jugal rami (66; Fig. 3M ). The squamosal ramus is slender and arched in lateral view (67; Fig. 3M ). The cornual process is a low, striated depression at the caudodorsal margin of the orbit (68; Fig. 3M ). The jugal ramus is elongate and slender (69; Fig. 3M ), and the jugal articular surface is shallow. The lateral surface of the jugal ramus bears dorsally arched sulci (70; Fig. 3M ). The rostral and caudal margins of the jugal process are parallel in lateral view (71; Fig. 3M ) and reach the ventral margin of the orbit. In dorsal view, the dorsotemporal fossa is shallow and is not bounded rostrally by a ridge.

In large Stage 1 specimens, the ventral margin of the squamosal process is sinuous (72; Fig. 3N ). In Stage 1 specimens, the dorsal margin of the bone tends to be vertically oriented ( Fig. 5C ). In large Stage 1 specimens, the incipient cornual process is a flattened, ear-like tab of bone with a horizontal ridge beneath its dorsal margin (75; Fig. 3N ). The dorsally-arched sulci reach its rostral and caudal margins (74; Fig. 2P ). The squamosal articular surface extends forward of the rostral margin of the laterotemporal fenestra. The frontal ramus is short in lateral view (76; Fig. 3N ), and is broad in dorsal view with a moderately deep dorsotemporal fossa bounded by a low ridge rostrally.

In Stage 2 specimens, the sulci of the jugal ramus are restricted rostral to its caudal margin (77; Fig. 3O ). The laterodorsal bone margin is everted medially (78; Fig. 3O ) and the bone ends above the orbit floor. The frontal ramus is stout and deep in lateral view (79; Fig. 3O ). The cornual process is prominent (80). In Stage 3 specimens, the cornual process of the postorbital may be enlarged, consisting of a thick ridge separated by a deep crease from an enlarged caudoventral boss. This is also true for D. torosus (81; Fig. 3P ).

Frontal—In large Stage 1 specimens (e.g., ROM 1247) of A. libratus , the orbital margin is within a vertical slot between the articular surfaces for the lacrimal and postorbital in lateral view ( Figs. 5A, C ). The lacrimal notch is long and narrow in dorsal view ( Fig. 5C ). The paired frontals are longer than wide ( Fig. 5C ). In Stage 1 specimens, the dorsotemporal fossa is shallow ( Fig. 5C ). In Stage 2 specimens (e.g., AMNH 5336), the fossa is deep. In Stage 1 specimens, the frontals are flattened to meet at the midline ( Fig. 5C ). In Stage 2 specimens, the frontals slope dorsomedially to their contact.

In Stage 1 specimens (e.g., ROM 1247) of A. libratus , the prefrontal articular surface is narrow in dorsal view.

Parietal—In Stage 1 specimens (e.g., ROM 1247) of A. libratus, the nuchal crest is low in caudal view, only as deep as the dorsal process of the supraoccipital. The rostrodorsal surface of the crest is rugose laterally and the dorsolateral margin of the crest flares caudolaterally ( Fig. 5C ). The sagittal crest is low ( Fig. 5A ).

In Stage 2 specimens (e.g., AMNH 5336), the rostrodorsal margin of the nuchal crest is rugose to the midline and the dorsolateral margin is rugose caudally. The dorsal margin of the sagittal crest is concave in lateral view. In Stage 3 specimens (e.g., AMNH 5458), the nuchal crest is tall, twice as deep as the dorsal process of the supraoccipital.

Basioccipital—In Stage 1 specimens (e.g., ROM 1247) of A. libratus , the occipital condyle is caudoventrally flattened and the lateral margins converge ventrally. The ventral surface of the basituberal web is flat and arches dorsally in caudal view. The surface beneath the occipital condyle is convex in frontal section and flares ventrally between the ascending scars (sensu Bakker et al., 1988). Finally, the basal tuber is poorly developed.

In Stage 2 specimens (e.g., AMNH 5336), the occipital condyle is spherical. The basal tuber forms a rugose block. In Stage 3 specimens (e.g., AMNH 5458), the caudal surface of the basioccipital is concave ventral to the occipital condyle.

Basisphenoid—In Stage 1 specimens (e.g., ROM 1247) of A. libratus , the basipterygoid process is flattened rostrolaterally, the basisphenoid pneumatic foramina are small and set above the ventral margin of the basipterygoid web. In lateral view, the ventral margin of the bone slopes at a low rostroventral angle such that the dorsal margin of the basipterygoid process does not project below the level of the caudoventral comer of the bone. The oval scar (sensu Bakker et al., 1988) is smooth, ventromedially oriented, and narrow.

In Stage 2 specimens (e.g., AMNH 5336), the oval scar is broad. In Stage 3 specimens (e.g., AMNH 5458), the ventral margin of the basisphenoid descends steeply rostroventrally in lateral view such that the dorsal margin of the basipterygoid process extends ventral to the level of the caudoventral corner of the bone. The oval scar is laterally expanded, ventrally oriented, and dished.

Vomer—In lateral view, the vomer of Stage 1 specimens (e.g., ROM 1247) of A. libratus has a horizontal ventral margin which curves gently caudoventrally, caudal to midlength (83; Fig. 4A ). A slender neck is present between the transverselyexpanded maxillary process and the dorsally deep body of the bone (82; Fig. 4A ). In Stage 2 specimens (e.g., AMNH 5336), the ventral margin curves strongly caudoventrally behind the midlength of the bone.

Palatine—In small Stage 1 specimens (e.g., TMP 86.144.1 ) of A. libratus , the caudalmost of the two pneumatic recesses in the lateral surface of the bone is rostrocaudally elongate and vertical struts are present on the medial wall of the rostral recess. The septum between the pneumatic recesses is narrow. Also, the palatine body is transversely compressed. In larger Stage 1 specimens (e.g., USNM 12814 ), the palatine is transversely inflated. In small Stage 1 specimens, the vomerine ramus is dorsoventrally shallow in lateral view; it is deep in larger Stage 1 specimens.

Surangular—In small Stage 1 specimens (e.g., TMP 86.144.1 ) of A. libratus , there is no ridge lateroventral to the glenoid in lateral view and the caudal margin of the retroarticular process is concave. In small and larger Stage 1 specimens (e.g., ROM 1247), the bone is shallow, such that the rostroventral margin is convex and slopes at a low angle caudoventrally (84; Figs. 4E , 5B ). The intramandibular process is stout and deep, its ventral margin meets the rostroventral margin of the bone along a shallow curve, or is confluent (85; Figs. 4E , 5B ). The rostral plate is externally flat (86; Figs. 4E , 5B ). The surangular shelf slants rostroventrally (87; Figs. 4E , 5B ) or horizontally; its lateral margin projects laterodorsally. The dorsomedial flange may be low and blade-like (88) or high ( TMP 86.144.1 ) with a narrow shelf separating it from the surangular shelf, in which the insertion scar of M. adductor mandibulae extemus ( Molnar, 1991) is indistinct and confined medial to the surangular shelf ( Figs. 4E , 5B ).

In large Stage 1 specimens (e.g., ROM 1247), the surangular foramen is not deeply recessed caudodorsally (89; Figs. 4E , 5B ). The fossa ventrolateral to the glenoid is shallow and smooth (90; Figs. 4E , 5B ). The caudal margin of the retroarticular process is straight to convex, such that the flange for the articular is weakly developed ( Figs. 5B ).

In Stage 2 specimens (e.g., AMNH 5336), the bone is deep such that the rostroventral margin slopes at a steep angle caudoventrally. The ventral margin of the intramandibular process and rostroventral margin of the surangular meet at an angle.

The surangular shelf passes horizontally, rostroventrally, or rostrodorsally, and its lateral margin projects horizontally. The surangular foramen is large and recessed. The sulcus lateroventral to the glenoid is dorsoventrally deep and rugose.

In Stage 3 specimens (e.g., CMN 2120), the intramandibular process is elongate and meets the rostroventral margin at an angle (91; Fig. 4F ). The surangular shelf passes rostrodorsally (92; Fig. 4F ) and the dorsomedial process is tall. The dorsolateral muscle scar is delimited rostrally by a sharply inset facet, which extends to the lateral surface of the surangular shelf (93; Fig. 4F ). The caudal margin of the retroarticular process is concave.

Prearticular—In small Stage 1 specimens (e.g., TMP 86.144.1 ) of A. libratus , the ventral portion of the articular surface of the angular is reduced and the lateral surface of the contact is aliform. In Stage 1 specimens (e.g., ROM 1247), the dorsal margin of the caudal ramus is restricted caudally (94; Fig. 4J ). The caudal ramus is shallow with parallel dorsal and ventral margins (95; Fig. 4J ). The rostral lamina is strap-like (96) and pointed distally (97); its caudodorsal margin is smooth ( Fig. 4J ). In large Stage 1 specimens, the angular facet is flat and not aliform.

In Stage 3 specimens (e.g., CMN 2120), the dorsal margin of the caudal ramus, with the adductor attachment surface, is shifted rostrally toward the mid-shaft (98; Fig. 4K ). The caudal ramus is deep such that the dorsal and ventral margins are gently convex and converge rostrally (99; Fig. 4K ). The caudodorsal margin of the rostral lamina bears a rugose surface for muscle attachment.

Dentary—In small Stage 1 specimens ( TMP 94.12.155 ) of A. libratus , the shallow dentary is as wide as it is deep. In larger Stage 1 specimens (e.g., ROM 1247), the dentary is deeper than wide. The angular process is dorsoventrally shallow ( Fig. 5B ). The symphyseal facet is flat and textured by stout caudodorsal bony papillae. The splenial articular surface is indicated by light, arcuate rostroventral ridges. The ventral bar beneath the Meckelian foramen is lightly rugose and rostroventrally excavated by a broad sulcus. Usually in tyrannosaurids the angular process is bifurcated by the external mandibular fenestra in lateral view; in ROM 1247 ( Fig. 5B ) this emargination is absent on both sides. In Stage 2 specimens (e.g., AMNH 5336), the angular process is deep. In Stage 3 specimens (e.g., AMNH 5458), the symphyseal surface may be rugose.

Figures

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.