Albertosaurus libratus (Lambe, 1914)
|
publication ID |
https://doi.org/ 10.1080/02724634.1999.10011161 |
|
DOI |
https://doi.org/10.5281/zenodo.4456652 |
|
persistent identifier |
https://treatment.plazi.org/id/0F0287A7-FFAA-FFDC-737B-FBE6FC2AF7A8 |
|
treatment provided by |
Jeremy |
|
scientific name |
Albertosaurus libratus |
| status |
|
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 View FIGURE 5 ), narrow maxillary process ( Fig. 5C View FIGURE 5 ), 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 View FIGURE 2 ). 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, View FIGURE 2 5A View FIGURE 5 ). In small specimens, the base of the interfenestral strut is flat, but is gently concave in large specimens (3; Figs. 2E View FIGURE 2 , 5A View FIGURE 5 ).
In Stage 1 specimens, the antorbital fenestra is longer than high ( Fig. 5A View FIGURE 5 ). 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 View FIGURE 2 , 5A View FIGURE 5 ). The vestibular bulla and passage of the subnarial foramen are laterally flat or transversely convex (6; Fig. 2E View FIGURE 2 ).
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 View FIGURE 5 ). 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 View FIGURE 2 , 5A View FIGURE 5 ). 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 View FIGURE 2 , 5A View FIGURE 5 ). The lateral surface separating the antorbital fossa and nasal suture is a shallow tab (9; Figs. 2E View FIGURE 2 , 5A View FIGURE 5 ). Finally, the promaxillary fenestra is a slit-like foramen within the rostral margin of the antorbital fossa (10; Figs. 2E View FIGURE 2 , 5A View FIGURE 5 ) ( Russell, 1970).
In Stage 2 specimens (e.g., AMNH 5336), the maxilla is thickened laterally and bowed rostrally (11; Fig. 2F View FIGURE 2 ). The rostrolateral surface is expanded rostrally and dorsoventrally ( Fig. 2F View FIGURE 2 ) and its surface sculpturing is pronounced. The ventral margin of the antorbital fossa is either gently sigmoid or dorsally convex ( Fig. 2F View FIGURE 2 ). The depression ventral to the antorbital fossa is shallow (12; Fig. 2F View FIGURE 2 ). The maxillary fenestra is longer than high and approaches the rostral margin of the antorbital fenestra (13; Fig. 2F View FIGURE 2 ). 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 View FIGURE 2 ). 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 View FIGURE 2 ).
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 View FIGURE 2 ). The caudoventral excavation may be shallow or pocket-like (17; Fig. 2I View FIGURE 2 ). 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 View FIGURE 2 ). The medial edge of the palatal process is cleaved by the articular surface for the palatine (21; Figs. 2I, J View FIGURE 2 ). Also, the palatal process is transversely narrow; the tooth root bulges are low (22; Fig. 2J View FIGURE 2 ), 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 View FIGURE 2 , 5A, C View FIGURE 5 ). The caudal plate expands slightly between the lacrimals (25; Figs. 2B, C View FIGURE 2 , 5C View FIGURE 5 ). The lacrimal articular facet overlaps the dorsolateral edge of the bone (26; Figs. 2B, C View FIGURE 2 , 5C View FIGURE 5 ). 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 View FIGURE 5 ).
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 View FIGURE 3 ); in larger specimens (e.g., ROM 1247), the ventral process is incipient (25; Fig. 3B View FIGURE 3 , 5A View FIGURE 5 ). 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 View FIGURE 3 ). 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 View FIGURE 3 , 5A View FIGURE 5 ). The lacrimal of Stage 1 specimens is T-shaped, with the supraorbital ramus projecting strongly behind the ventral ramus (28; Figs. 3A, B View FIGURE 3 ; 5A View FIGURE 5 ). In larger specimens, the rostrolateral margin of the cornual process is eave-like, and dished above the lacrimal pneumatic recess (29; Figs. 3B View FIGURE 3 , 5A View FIGURE 5 ). In large Stage 1 specimens the dorsolateral surface of the supraorbital ramus bears a strong shelflike ridge (30; Figs. 3B View FIGURE 3 , 5A View FIGURE 5 ).
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 View FIGURE 3 ). In larger Stage 1 specimens, the processes overlap in lateral view, losing the forked appearance (32; Fig. 3B View FIGURE 3 ). In small specimens, the lacrimal antorbital fossa of the dorsal ramus is fully exposed to view (33; Fig. 3A View FIGURE 3 ). 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 View FIGURE 3 , 5A View FIGURE 5 ).
In Stage 1 specimens, the textured dermal surface and the smooth fossa surface merge at their juncture (32; Fig. 2A View FIGURE 2 ). 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 View FIGURE 3 , 5A View FIGURE 5 ). Also, the rostral margin of the rostroventral lamina of the lacrimal is straight or concave in lateral view (34; Figs. 3A, B View FIGURE 3 , 5A View FIGURE 5 ). 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 View FIGURE 5 ).
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 View FIGURE 3 ). The cornual process may be bulbous and has one apex (36; Fig. 3C View FIGURE 3 ). The ventral margin of the lateral lamina of the rostral ramus matches that of the lacrimal antorbital fossa (37; Fig. 3D View FIGURE 3 ). The caudal margin of the jugal articular surface of the ventral ramus slopes caudodorsally (38; Fig. 3E View FIGURE 3 ). This is also in D. torosus (39; Fig. 3H View FIGURE 3 ) The rostral margin of the ventral ramus is embayed by the lacrimal antorbital fenestra (40; Fig. 3C View FIGURE 3 ).
In Stage 3 specimens (e.g., CMN 2120), both the dorsal and ventral processes of the rostral ramus are elongate (41; Fig. 3E View FIGURE 3 ). The cornual process bears a single erect apex, situated above the ventral ramus (42; Fig. 3E View FIGURE 3 ). The lateral surface around the lacrimal pneumatic recess is not dished (43; Fig. 3E View FIGURE 3 ). The lacrimal antorbital fossa is attenuated rostrally by the lateral lamina (44; Fig. 3E View FIGURE 3 ). This condition is also seen in Stage 4 D. torosus (45; Fig. 3G View FIGURE 3 ) 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 View FIGURE 3 ).
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 View FIGURE 3 ) and the extent of its contact with the jugal is matched by that of the ventral ramus (47; Fig. 3E View FIGURE 3 ). This is also in Stage 4 D. torosus ( Fig. 3F View FIGURE 3 ).
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 View FIGURE 3 ) and the maxillary ramus of the jugal is straplike and shallow (49; Fig. 3I View FIGURE 3 ). In larger Stage 1 specimens, the jugal pneumatic recess is a transversely narrow slit ( Fig. 5A, C View FIGURE 5 ); its caudal edge may be united with that of the lacrimal (50; Russell, 1970) ( Figs. 3I View FIGURE 3 , 5A View FIGURE 5 ). Infrequently, the caudal margin of the recess may be resorbed, exposing the rostral extent of the secondary fossa to lateral view (51; Figs. 3I View FIGURE 3 , 5A View FIGURE 5 ).
The caudal margin of the lacrimal articular surface is subvertical in lateral view (52; Figs. 3I View FIGURE 3 , 5A View FIGURE 5 ). The articular surface for the postorbital is shallow and extends to the ventral orbital margin (53; Figs. 3I View FIGURE 3 , 5A View FIGURE 5 ). The medial articular surface for the lacrimal is overlapping. The caudal margin of the postorbital ramus is sinuous or concave (54; Figs. 3I View FIGURE 3 , 5A View FIGURE 5 ). The area ventral to the postorbital ramus is flat or convex in lateral view (55; Figs. 3I View FIGURE 3 , 5A View FIGURE 5 ). The transversely flat cornual process may be pronounced or its caudal margin may be low (56; Figs. 3I View FIGURE 3 , 5A View FIGURE 5 ). 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 View FIGURE 3 , 5A View FIGURE 5 ).
In Stage 2 specimens (e.g., AMNH 5336), the postorbital articular surface ends above the ventral margin of the orbit (58; Fig. 3J View FIGURE 3 ), the caudal margin of the postorbital ramus is strongly convex or straight at midheight (59; Fig 3J View FIGURE 3 ), and the cornual process is pronounced (60; Fig. 3J View FIGURE 3 ).
In Stage 3 specimens, the maxillary ramus is dorsoventrally deep (61; Fig. 3K View FIGURE 3 ). The caudal margin of the jugal foramen is resorbed, exposing the rostral margin of the secondary fossa (62; Fig. 3K View FIGURE 3 ). The lateral surface at the base of the postorbital ramus is shallowly concave (63; Fig. 3K View FIGURE 3 ). The caudodorsal margin of the jugal pneumatic recess merges with the lateral surface of the jugal beneath the lacrimal contact (64; Fig. 3K View FIGURE 3 ). Finally, the caudal margin of the lacrimal articular surface slopes caudodorsally along an elongate and shallow lateral overlap (65; Fig. 3K View FIGURE 3 ).
Postorbital—In lateral view, the postorbital of small Stage 1 specimens of A. libratus is a slender and delicate bone ( Fig. 3M View FIGURE 3 ). The frontal ramus approaches the length of the squamosal and jugal rami (66; Fig. 3M View FIGURE 3 ). The squamosal ramus is slender and arched in lateral view (67; Fig. 3M View FIGURE 3 ). The cornual process is a low, striated depression at the caudodorsal margin of the orbit (68; Fig. 3M View FIGURE 3 ). The jugal ramus is elongate and slender (69; Fig. 3M View FIGURE 3 ), and the jugal articular surface is shallow. The lateral surface of the jugal ramus bears dorsally arched sulci (70; Fig. 3M View FIGURE 3 ). The rostral and caudal margins of the jugal process are parallel in lateral view (71; Fig. 3M View FIGURE 3 ) 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 View FIGURE 3 ). In Stage 1 specimens, the dorsal margin of the bone tends to be vertically oriented ( Fig. 5C View FIGURE 5 ). 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 View FIGURE 3 ). The dorsally-arched sulci reach its rostral and caudal margins (74; Fig. 2P View FIGURE 2 ). 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 View FIGURE 3 ), 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 View FIGURE 3 ). The laterodorsal bone margin is everted medially (78; Fig. 3O View FIGURE 3 ) and the bone ends above the orbit floor. The frontal ramus is stout and deep in lateral view (79; Fig. 3O View FIGURE 3 ). 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 View FIGURE 3 ).
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 View FIGURE 5 ). The lacrimal notch is long and narrow in dorsal view ( Fig. 5C View FIGURE 5 ). The paired frontals are longer than wide ( Fig. 5C View FIGURE 5 ). In Stage 1 specimens, the dorsotemporal fossa is shallow ( Fig. 5C View FIGURE 5 ). 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 View FIGURE 5 ). 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 View FIGURE 5 ). The sagittal crest is low ( Fig. 5A View FIGURE 5 ).
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 View FIGURE 4 ). A slender neck is present between the transverselyexpanded maxillary process and the dorsally deep body of the bone (82; Fig. 4A View FIGURE 4 ). 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 View Materials ), 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 View FIGURE 4 , 5B View FIGURE 5 ). 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 View FIGURE 4 , 5B View FIGURE 5 ). The rostral plate is externally flat (86; Figs. 4E View FIGURE 4 , 5B View FIGURE 5 ). The surangular shelf slants rostroventrally (87; Figs. 4E View FIGURE 4 , 5B View FIGURE 5 ) 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 View FIGURE 4 , 5B View FIGURE 5 ).
In large Stage 1 specimens (e.g., ROM 1247), the surangular foramen is not deeply recessed caudodorsally (89; Figs. 4E View FIGURE 4 , 5B View FIGURE 5 ). The fossa ventrolateral to the glenoid is shallow and smooth (90; Figs. 4E View FIGURE 4 , 5B View FIGURE 5 ). The caudal margin of the retroarticular process is straight to convex, such that the flange for the articular is weakly developed ( Figs. 5B View FIGURE 5 ).
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 View FIGURE 4 ). The surangular shelf passes rostrodorsally (92; Fig. 4F View FIGURE 4 ) 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 View FIGURE 4 ). 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 View FIGURE 4 ). The caudal ramus is shallow with parallel dorsal and ventral margins (95; Fig. 4J View FIGURE 4 ). The rostral lamina is strap-like (96) and pointed distally (97); its caudodorsal margin is smooth ( Fig. 4J View FIGURE 4 ). 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 View FIGURE 4 ). The caudal ramus is deep such that the dorsal and ventral margins are gently convex and converge rostrally (99; Fig. 4K View FIGURE 4 ). 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 View FIGURE 5 ). 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 View FIGURE 5 ) 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.
No known copyright restrictions apply. See Agosti, D., Egloff, W., 2009. Taxonomic information exchange and copyright: the Plazi approach. BMC Research Notes 2009, 2:53 for further explanation.