Tyrannosaurus, OSBORN, 1905
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publication ID |
https://doi.org/ 10.1111/j.1096-3642.2004.00130.x |
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persistent identifier |
https://treatment.plazi.org/id/03C72A48-FFB2-FFF1-FCCE-FE50DC5FFBA3 |
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Diego |
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Tyrannosaurus |
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GENUS TYRANNOSAURUS OSBORN, 1905 TYRANNOSAURUS REX OSBORN, 1905
Aublysodon molnaris ( Leidy, 1868) - Paul, 1988b: p. 325.
Aublysodon cf. A. mirandus ( Leidy, 1868) - Molnar & Carpenter, 1989: figs 1-4, p. 447.
Stygivenator molnari ( Paul, 1988b) - Olshevsky & Ford, 1995: fig. 36, p. 116.
Albertosaurus cf. A. lancensis Molnar, 1980 : figs 1-7, p. 102.
Albertosaurus megagracilis Paul, 1988b: p. 333 .
Nanotyrannus lancensis Bakker, Williams, & Currie, 1988 : pls. 1-4, figs 1-7, 11, 12, p. 2.
Dinotyrannus megagracilis Olshevsky & Ford, 1995 : fig. 37, p. 117.
Type species: Tyrannosaurus rex Osborn, 1905 .
Discussion and description
The first skeletal material referred to cf. Aublysodon was an isolated frontal ( TMP 80.16.485) from the Dinosaur Park Formation of Alberta ( Currie, 1987). Currie tentatively referred the frontal to Aublysodon because, in contrast to Albertosaurus libratus , the frontal is relatively thin and narrow for a given length. The presence of Aublysodon in the Dinosaur Park Formation was supported by the presence of isolated nondenticulate premaxillary teeth and ‘gracile’ denticulate maxillary and dentary teeth, thought to be referable to the taxon ( Currie, 1987: 55). Currie suggested that the Jordan Theropod might be referable to Aublysodon .
Lehman and Carpenter (1990: 1026; figs 1–6) referred a partial skeleton to Aublysodon that is likely referable to a new genus of basal tyrannosauroid. Molnar & Carpenter (1989) described aspects of the specimen, LACM 28471, pertinent to a revision of Aublysodon . Their referral of the specimen to Aublysodon cf. A. mirandus was based on the presence of an associated nondenticulate premaxillary tooth. The specimen provided the opportunity to ascribe skeletal characters to the taxon. In their abstract, Molnar & Carpenter (1989: 445) listed one skeletal and one dental character to characterize Aublysodon : a ‘peculiar first dentary tooth’ and a V-shaped frontoparietal suture ( Fig. 2A, B, L View Figure 2 ). Their revised diagnosis of A. mirandus included the following additional characters: slender dentaries, an emargination in the dentary caudal to the third alveolus, and an acute angle
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Left maxilla
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LACM 28471 CMNH 7541 CMNH 7541 LACM 28471 LACM 28471 CMNH 7541 CMNH 7541 Apex
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3 Right (reversed) Right (reversed) LACM 28471 LACM 28471 CMNH 7541 apex
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CMNH 7541 LACM 28471 CMNH 7541
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LACM 28471 LACM 29471 LACM 28471
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between the dorsal and rostroventral margins of the dentary ( Fig. 3D, E View Figure 3 ). They also listed additional dental characters for A. mirandus : the mesial carina of the first dentary tooth is displaced lingually ( Fig. 3E View Figure 3 ), all dentary teeth but the first ( Fig. 3D, E View Figure 3 ) are laterally compressed as well as those of the maxilla ( Figs 3D, E View Figure 3 , 4 View Figure 4 ), denticles extend midway down the mesial carina and to the base on the distal carina and the mesial denticles are smaller than the distal ones.
Comparison of the Jordan theropod ( LACM 28471) with tyrannosaurid material weakens the validity of A. mirandus . All tyrannosaurids, except adult Daspletosaurus and Tyrannosaurus , have V-shaped frontoparietal sutures. Slender dentaries are typical of juvenile tyrannosaurids ( Carr, 1999). Regarding the emargination in the dentary, in tyrannosaurids the alveolar margin is convex adjacent to the rostral four or five teeth and is then concave through the midlength of the bone ( Fig. 5 View Figure 5 ). The ‘step’ is the transition point between these margins, and in LACM 23871 this is exaggerated by the fragmentary nature of both bones ( Figs 3D, E View Figure 3 , 5 View Figure 5 ). Molnar & Carpenter (1989: 447) cite the ‘rostroventral border of dentaries with an acute angle relative to alveolar border’ as unique. However, this condition is typical of small tyrannosaurid dentaries ( Fig. 5 View Figure 5 ).
Molnar & Carpenter (1989) considered the extent and coarseness of denticles to be diagnostic for A. mirandus . They stated that in the dentary and maxillary teeth of LACM 28471, the mesial denticles only extend halfway down their carinae and the entire extent of the distal carinae is denticulate. We find that this character is typical of most tyrannosaurids, except Daspletosaurus torosus in which the mesial carina reaches the crown base, and is therefore not diagnostic of A. mirandus . Also, the condition of mesial denticles being finer than the distal denticles is typical of tyrannosaurids ( Carr & Williamson, 2000) and is not a diagnostic character of any species.
The partial skull of LACM 28471 displays many characters typical of Tyrannosauridae . The frontal is diagnostic among Late Cretaceous theropods ( Currie, 1987). The bone displays most of the diagnostic tyrannosaurid features, including the following: separate joint surfaces of the lateral and medial frontal processes of the nasal; short orbital rim; expanded frontopostorbital suture with a buttress (although undeveloped in LACM 28471) and caudolateral suture; frontals separated on the midline by the parietals; frontals flat between the orbits; dorsotemporal fossa covering much of the dorsal surface of the frontals ( Fig. 2A- E, G View Figure 2 ).
LACM 28471 displays additional tyrannosaurid characters, including a first maxillary tooth that is small and incisiform ( Figs 1D, E View Figure 1 , 4 View Figure 4 ) and a subconical first dentary tooth that is the smallest member of the tooth row ( Fig. 3D, E View Figure 3 ). In lateral view, a large neurovascular foramen pierces the maxilla rostral to the antorbital fossa ( Fig. 1D View Figure 1 ), the alveolar margin of the maxilla is convex ( Fig. 1D, E View Figure 1 ) and that of the dentary is concave caudally and convex rostrally ( Fig. 3D, E View Figure 3 ); the rostral-most neurovascular foramen that pierces the lateral surface of the dentary is the largest of its row and is situated ventral to the first alveolus ( Fig. 3E View Figure 3 ). This foramen is also in dromaeosaurids ( Paul, 1988a). In dorsal view, the nasals are constricted at midlength between the maxillae ( Fig. 1A– C View Figure 1 ); the parietals are wedge-shaped and bear a prominent sagittal crest ( Fig. 2I–M View Figure 2 ). Finally, in ventral view, the parietals possess a plug-like process on the rostral midline for apposition with the frontals ( Fig. 2M View Figure 2 ). Thus, there are no characters that contradict assignment of LACM 28471 to Tyrannosauridae .
Taxonomic identity
As in Daspletosaurus torosus and Tyrannosaurus rex , the lingual surface of each of the third left and right maxillary crowns in LACM 28471 bear a prominent apicobasal ridge that creates a crease adjacent to the mesial carina ( Fig. 4 View Figure 4 ). In other tyrannosaurids (e.g. Albertosaurus ), this ridge and sulcus are absent. As in juvenile (e.g. CMNH 7541) and adult (e.g. AMNH 5027) T. rex , the rugose lateral surface of the maxilla stops far short of the proximal end of the ascending process of the maxilla ( Figs 1F View Figure 1 , 6 View Figure 6 ), indicating that the antorbital fossa reached the nasal suture. Also, the sagittal crest is present on the frontals, a state absent in tyrannosaurids other than Daspletosaurus and Tyrannosaurus .
Several characters of LACM 28471 are shared with T. rex . As in juvenile T. rex (e.g. CMNH 7541), the dorsal and lateral surfaces of the rostral third of the nasals are set at abrupt angles to one another, such that the bones are angular in cross-section ( Fig. 1A View Figure 1 ). This is also found in dromaeosaurids ( Paul, 1988a). The nasals do not separate the distal ends of the nasal processes of the premaxillae by a pair of median ridges, indicating that the processes were appressed through their entire length, as in T. rex (e.g. AMNH 5027, CMNH 7541; Fig. 7 View Figure 7 ). On the basis of these shared derived characters, we consider LACM 28471 to be referable to Tyrannosaurus rex .
Ontogenetic stage
In addition to its small size (estimated skull length: ~450.0 mm), LACM 28471 shares many features that typify juvenile tyrannosaurids (sensu Carr, 1999). In general, the teeth are labiolingually narrow (‘bladelike’) and the cranial remains are lightly built, especially those of the dorsal skull roof such as the nasals, frontals, and parietals ( Figs 1–3 View Figure 1 View Figure 2 View Figure 3 ). LACM 28471 represents a tyrannosaurid growth stage that precedes that of ‘small Stage 1’ described by Carr (1999) for Albertosaurus libratus .
Molnar (1978: 74) observed that the joint surface for the maxilla on the nasal is smooth and favourably compared this with dromaeosaurids in contrast to the ‘dentate appearance’ of this region in tyrannosaurids. A serrate nasomaxillary suture is characteristic of large adult tyrannosaurids and that of subadult and juvenile specimens is a smooth tongue-in-groove contact ( Carr, 1999). Thus, the smooth state in LACM 28471 is a juvenile character ( Fig. 1D–G View Figure 1 ).
The frontal process of the nasal is not constricted between the lacrimals in LACM 28471 ( Fig. 1B, C View Figure 1 ). The same is also true for juvenile Daspletosaurus (e.g. TMP 94.143.1) in contrast to the constricted state in adults (e.g. CMN 8506, TMP 85.62.1). An unconstricted nasal is present in a juvenile individual of T. rex (e.g. CMNH 7541) where only the unmodified right lateral frontal process is preserved ( Gilmore, 1946; Bakker et al., 1988; Carr, 1999). In adult T. rex , the process is compressed to a pair of rods between the lacrimals ( Osborn, 1912; Fig. 8 View Figure 8 ).
As in juvenile T. rex (e.g. CMNH 7541), the nasals of LACM 28471 are not rugose, their condition in large adult specimens ( Fig. 1A–E View Figure 1 ). This smooth condition differs from a small juvenile A. libratus (e.g. TMP 86.144.1), in which the nasals are rugose. As in juvenile and adult tyrannosaurids, the internasal suture is open along the rostral and caudal extremities of the bone, but the remainder is closed in external view ( Fig. 1A–C View Figure 1 ). Thus, growth of this bone was not inhibited by closure of this suture and indicates the functional importance of a stable snout for small juvenile tyrannosaurids.
The maxilla, as in juvenile tyrannosaurids ( Carr, 1999), is dorsoventrally shallow and transversely narrow ( Fig. 1A, D–G View Figure 1 ), indicating a long, low snout of delicate construction. The triangular profile of the maxilla noted by Molnar (1978) typifies the proportions of this bone in juveniles and all small theropods and is not taxonomically diagnostic ( Currie & Dong, 2001b; Fig. 6 View Figure 6 ). Molnar observed that the angle of the ventral and rostrodorsal margins converge at a low angle (50∞) in LACM 28471. In contrast to larger tyrannosaurids, this difference is growth related, juvenile tyrannosaurids having shallower snouts than adults ( Carr, 1999). This is also indicated by the low angle of the margins of the maxilla, which Molnar (1978: 73) also observed in A. libratus . The low angle in LACM 28471, in contrast to the higher angle in A. libratus (70∞), is undoubtedly related to the fact that LACM 28471 was the smallest specimen in Molnar’s sample.
The term ‘maxillary flange’, used herein, refers to the convex ala that extends dorsally from the dorsolateral margin of the maxilla, which overlaps the ventrolateral surface of the nasal. The flange represents the lateral half of a deep longitudinal slot in the maxilla that receives a blade-like process from the ventral surface of the nasal. As in juvenile T. rex (e.g. CMNH 7541), the maxillary flange of LACM 28471 is present but is not as prominent as that of larger specimens ( Figs 1D–G View Figure 1 , 6 View Figure 6 ).
Comparable to juvenile Tyrannosaurus (e.g. CMNH 7541, PIN 552–2) and Daspletosaurus (e.g. TMP 94.143.1), the rostral margin of the maxillary fenestra in LACM 28471 does not approach the rostral margin of the external antorbital fenestra as it does in adults ( Figs 1D, E View Figure 1 , 6 View Figure 6 ). In adult T. rex , the maxillary fenestra extends medial to the rostral margin of the external antorbital fenestra ( Fig. 6 View Figure 6 ), while in Daspletosaurus adults, a T. bataar juvenile (GIN 100/177), and the holotype of Shanshanosaurus huoyanshanensis ( Currie & Dong, 2001a) , the maxillary fenestra is separated from the margin of the external antorbital fenestra by a narrow rim of bone ( Fig. 6 View Figure 6 ). Also as in juvenile tyrannosaurids, in LACM 28471 a low ridge encircles the rostroventral margin of the antorbital fossa ( Fig. 1D View Figure 1 ), while the ventral foramen within the fossa of the subnarial foramen opens rostroventrally ( Carr, 1999; Fig. 1D View Figure 1 ); in subadults and adults, this foramen is large and opens rostrally ( Carr, 1999).
The dorsotemporal fossa of the frontal is present in LACM 28471 and, as in juvenile tyrannosaurids ( Carr, 1999), the rostral margin of the depression is indistinct (e.g. CMNH 7541; Fig. 2A, B View Figure 2 ) whereas the rostral margin of the dorsotemporal fossa is distinct in large and mature specimens ( Fig. 8 View Figure 8 ). As in other juvenile tyrannosaurids (e.g. CMNH 7541), most of the rostral margin of the fossa is not delimited by a distinct ridge and sulcus, except in LACM 28471 where these features are present at the lateral extent of the fossa in both bones ( Fig. 2A, B View Figure 2 ). Also, the sutural surface of the postorbital is not modified into a dorsoventrally deep abutting and peg-in-socket contact as in larger specimens, but is composed of deep grooves and ridges ( Fig. 2E, G, H View Figure 2 ).
In dorsal view, the joint surface of the lacrimal is rostrocaudally elongate and transversely narrow, as in juvenile tyrannosaurids ( Carr, 1999; Fig. 2A, B View Figure 2 ). This is unlike adults of Daspletosaurus and Tyrannosaurus in which the frontal, caudal to the lacrimal, is transversely wide ( Fig. 8 View Figure 8 ). The same is also true for the prefrontal- frontal suture in T. rex ( Fig. 8 View Figure 8 ). In LACM 28471, the joint surfaces of the medial and lateral frontal process of the nasal are separate in dorsal view ( Fig. 2A, B View Figure 2 ), as in other tyrannosaurids. In adult T. rex (e.g. AMNH 5027, BHI 3033, BHI 4100, FMNH PR2081), the medial frontal process of the nasal is displaced ventrally out of view and the lateral frontal process itself is compressed toward the midline beneath the lacrimal ( Fig. 8 View Figure 8 ). Thus in adults, the joint surface of the medial frontal process is pinched out below and between the compressed nasal processes of the frontal.
As in other juvenile tyrannosaurids ( Carr, 1999), the muscle attachment area on the lateral surface of the surangular, ventral to the glenoid fossa, is a shallow groove ( Fig. 3A View Figure 3 ). In larger and more mature individuals this is a dorsoventrally deep and rugose scar. The shallow dentary of LACM 28471 also indicates the juvenile status of the specimen ( Figs 3D, E View Figure 3 , 5 View Figure 5 ). In tyrannosaurid growth, dentary depth exhibits positive allometry; in the smallest juveniles, the width of the bone approaches its height.
Molnar (1978) claimed that the dentary of LACM 28471 is unique with regard to the shallow angle of the symphysis and the slope of the alveolar margin in lateral view. In our view, the low angle of the ventral margin of the symphyseal region relative to the ventral margin of the bone is typical of juvenile tyrannosaurids ( Figs 3D, E View Figure 3 , 5 View Figure 5 ), as noted by Molnar (1978) for A. libratus and Currie & Dong (2001a) for Shanshanosaurus . A steep rostroventral margin of the dentary typifies larger and deeper-jawed specimens. In tyrannosaurids, the alveolar margin of the dentary, in lateral view, is generally concave and becomes convex lateral to the rostral four alveoli ( Fig. 5 View Figure 5 ). This is also the case in LACM 28471 ( Figs 3D, E View Figure 3 , 5 View Figure 5 ). Therefore, on the basis of these features, LACM 28471 exemplifies a juvenile T. rex .
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.
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Tyrannosaurus
| Carr, Thomas D. & Williamson, Thomas E. 2004 |
Aublysodon molnaris ( Leidy, 1868 )
| Paul GS 1988: 325 |
Albertosaurus megagracilis
| Paul GS 1988: 333 |