Tyrannosaurus rex (Thomas D. Carr & Thomas E. Williamson 2004)
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Ordo: Dinosauria
Familia: Tyrannosauridae
Genus: Tyrannosaurus
Name
Tyrannosaurus rex Osborn, 1905 – Wikispecies link – Pensoft Profile
- Tyrannosaurus rex Thomas D. Carr, 2004, Zoological Journal of the Linnean Society : 481-511.
Discussion
LACM 23845: DISCUSSION Taxonomic identity Molnar (1980) described the partial skull and skeleton (LACM 23845) of a medium-sized tyrannosaurid recovered from a quarry adjacent to one containing an adult T. rex (LACM 23844) on the L. D. Engdahl Ranch in Garfield County, Montana. He referred the specimen to Albertosaurus cf. A. (= Nanotyrannus) lancensis. This specimen was later made the holotype of a new species, Albertosaurus megagracilis by Paul (1988b). Olshevsky & Ford (1995) placed this species in a new genus, Dinotyrannus. We suggest that LACM 23845 represents a subadult T. rex. LACM 23845 displays several characters that are shared by Daspletosaurus and Tyrannosaurus. In dorsal view, the external surface of the frontal process of the nasals is constricted between the lacrimals (Figs 8, 9B, D). Also, between the constriction and the rostral extent of the joint surface for the lacrimal, the dorsolateral margin of the nasal is notched to receive a stout process from the dorsomedial edge of the lacrimal (Figs 8, 9B). In dorsal view, the frontals are transversely wide such that the frontolacrimal and frontoprefrontal sutures are also widened (Fig. 8). As in subadult and adult specimens of Tyrannosaurus and Daspletosaurus, the sagittal crest is divided on the frontals such that a deep midline cleft separates the paired crests (Fig. 8). In other tyrannosaurids and in juvenile T. rex (e.g. CMNH 7541, LACM 28471), a large foramen pierces the sagittal crest on the midline, and the crest diminishes rostrolateral or rostroventral to the opening. The dorsotemporal fossa is deep, and the dorsal surface rostral to the fossa is rostrocaudally short, only 2 cm longer than in the juvenile LACM 28471 (Fig. 8). Also, the frontoparietal suture is transversely orientated (Figs 8, 15C, H), except on the midline, as in Daspletosaurus and Tyrannosaurus (Fig. 8). Finally, the lingual ridge and groove are present in an associated crown that is probably a mesial maxillary tooth. LACM 23845 displays several Tyrannosaurus characters. As in most specimens of T. rex (except on the left side in LACM 23844), the caudolateral process of the nasal is long (Figs 9, 10). This process is short or absent in Daspletosaurus (e.g. CMN 8506, TMP 85.65.1) and T. bataar (e.g. PIN 553–1), and long in Albertosaurus (Fig. 10). The lacrimal of LACM 23845 does not have a cornual process; instead, the dorsal surface is wide and flat, and the accessory pneumatic fossa is located distal to the lacrimal recess (Figs 10, 11C). In Daspletosaurus the cornual process is a low mound; in Albertosaurus it is sharply defined, with the dorsal surface of the bone sloping dorsolaterally to the cornual process and the accessory pneumatic fossa proximal in position (Fig. 10). As in subadult and adult specimens of Tyrannosaurus, the sagittal crest extends rostrally on the frontals such that a midline cleft separates the paired crests (Fig. 8). In other tyrannosaurids and in juvenile T. rex (e.g. CMNH 7541, LACM 28471), a large foramen pierces the sagittal crest on the midline and the crest diminishes rostrolateral or rostroventral to the opening. As in Tyrannosaurus and Albertosaurus, the antorbital fossa depresses the lateral surface of the rostral ramus ahead of the lacrimal pneumatic recess (Fig. 10). In Daspletosaurus the fossa faces ventrally in large specimens (Fig. 10). The gentle twist in the rostrolateral margin of the squamosal process of the quadratojugal (Figs 11G, I, 12) is less well developed than the notch in the same region of large adult specimens (e.g. AMNH 5027, BHI 3033, FMNH PR 2081, SDSM 12047). This character is diagnostic for T. rex. In dorsal view, the joint surfaces of the nasal processes of the premaxillae are not separated proximally (Fig. 9B). Although the midline struts that separate the premaxillae on the midline are present, they are set below the external surface of the bone, indicating that the processes were probably appressed throughout their entire length, as in juvenile (e.g. CMNH 7541) and adult (e.g. AMNH 5027, FMNH PR 2081) T. rex (Fig. 7). As in other specimens of T. rex (e.g. FMNH PR 2081, LACM 23844), the caudal surangular foramen is smaller than it is in other tyrannosaurids (Fig. 13). This is also present in some Albertosaurus libratus (e.g. ROM 1247; Fig. 13).
Finally, two postcranial characters are consistent with other T. rex specimens. In the scapula in ventral view, the caudal margin of the joint surface of the humerus is narrower than the cranial margin (see e.g. FMNH PR 2081; Fig. 14C), while in lateral view, the acromion is elongate (e.g. FMNH PR 2081; Fig. 14A, B), characters first noted by Carpenter & Smith (2001). Thus, we consider
to be referable to T. rex.
Ontogenetic stage Molnar (1980) provided four reasons why LACM 23845 is not an immature T. rex, each of which is contestable: presence of a tall nuchal crest, smooth prefrontofrontal and quadratojugoquadrate sutures, and the form of the proximal fibula. The height of the nuchal crest is thought to be ontogenetically variable in tyrannosaurids (Russell, 1970). This idea is based on the subadult A. libratus specimen AMNH5664 in which the nuchal crest is ‘only about one-fourth as large as in adults’ (Russell, 1970: 7). However, the nuchal crest in AMNH 5664 is completely broken off and missing. Prominent nuchal crests are present in much smaller juvenile tyrannosaurid specimens, including T. rex(e.g. CMNH 7541; Carr, 1999) and so were presumably originally tall in AMNH 5664. Therefore, the presence of a tall nuchal crest in LACM 23845 does not indicate adulthood. In rostral and lateral views, we observed that the central region of the joint surface of the prefrontal in the frontal of LACM 23845 is coarsened by low papillae and rostrodorsally trending ridges, indicating that the prefrontal- frontal union is not smooth. Thus in T. rex a socket for the prefrontal in the frontal is present late in maturity (cf. Currie, 1987). The shallow and smooth joint surfaces of the quadrate on the quadratojugal (Molnar, 1980; Fig. 11H, L) are interpreted here as size-dependent features in LACM 23845. In large adult T. rex the joint surfaces are coarse and deeply excavated (e.g. LACM 23844). Molnar suggested that the form of the proximal fibula was different from that of Tyrannosaurus. In LACM 23845, in lateral view, the proximal end of the fibula is more symmetrical than in Tyrannosaurus, the bone is more slender, and the dorsal margin is flatter (Molnar, 1980). Although described as symmetrical, the proximal portion of the fibula in lateral view has a nearly vertical rostral margin and the caudal margin dilates proximally. This condition is present in adult T. rex specimens (e.g. FMNH PR 2081) and in subadult Albertosaurus(e.g. , ) and so does not appear to reflect relative maturity or size. The slenderness of the bone probably reflects the smaller size of the animal. Molnar (1980: 107) also stated that the ‘projection of the proximal face of metatarsal III’ indicated the specimen’s maturity (Fig. 20 H, I, K), but the meaning of this character is unclear. In our view, LACM 23845 is probably a subadult T. rex. In addition to a low estimated skull length (~ 80 cm), several features indicate that it was a subadult or small adult at death. Molnar observed that the nasals are not as rugose as in T. rex. However, his comparative material for the latter taxon (e.g. AMNH 5027, LACM 23844) consists of larger and more mature specimens; thus the more rugose condition might reflect greater maturity. As noted by Molnar (1980), the nasal contact of the frontal is W-shaped, but this is due to the subadult status of the specimen. The condition in LACM 23845 is therefore transitional between the unconstricted condition in juveniles (e.g. CMNH 7541, LACM 28471) and the constricted condition of adults (e.g. AMNH 5027, BHI 3033, FMNH PR 2081; Fig. 8). The dorsal surface of the lacrimal is situated relatively close to the dorsal margin of the lacrimal recess, indicating that the bone is not as inflated as in large adult specimens (e.g. AMNH 5027, BHI 3033, FMNH PR 2081,
- Carr, 1999; Figs 10, 11C). The medial joint surface for the nasal on the frontal is visible in dorsal view (Fig. 15C, D). In large adult specimens (e.g. AMNH 5027, BHI 3033, FMNH PR 2081), the joint surface for the medial frontal process of the nasal is displaced ventrally out of view between the compressed rods of the frontal process.
Unlike large adult specimens of T. rex, the parasagittal crest of the frontal in LACM 23845 does not extend far rostrally, but diminishes after a short distance (Figs 8, 15C, D). This state is also present in Albertosaurus of any size. In dorsal view, the suture for the prefrontal on the frontal is angular on the left and arcuate on the right in LACM 23845 (Fig. 15C, D). The asymmetrically developed dorsal surface of the nuchal crest is not developed into a rugose platform as in large adult T. rex (e.g. AMNH 5027), and so might indicate immaturity (Fig. 15 J- M). In LACM 23845, only the right half of the crest is thickened, but with a smooth dorsal surface (Fig. 15 J- M). The tubercle for attachment of the tendon of M. depressor mandibulae of the otoccipital (Fig. 16A, B) is low, and not prominent as in large adults (e.g. AMNH 5027, FMNH PR 2081). The margin of the external mandibular foramen of the angular is developed into a rugose ridge (Fig. 17C). A low irregular scar is present in juvenile tyrannosaurids (Carr, 1999). The prearticular of LACM 23845 is typical of subadult tyrannosaurids: the caudal ramus is dorsoventrally deep, the dorsal and ventral margins converge toward each other rostrally, and the bone is dorsoventrally flattened and transversely wide at midshaft (Fig. 17D – G). Unlike adult specimen LACM 23844, the medial surface of the bone is not excavated by the joint surface for the splenial, the bone is shallower, the rostroventral surface of the joint surface for the angular is flat and not concave, and the bone is narrower at midshaft. As in mature tyrannosaurids, the dentary is much deeper than wide (Fig. 18A, B). Thus, LACM 23845 displays features that are intermediate in development between juveniles (e.g. CMNH 7541) and adults and therefore may be a subadult or small adult T. rex.
Description
Revised description Our opinion of some details of the osteology of LACM 23845 differs from that of Molnar (1980). We summarize the differences in interpretation in Table 2, but several of his observations require comment here. We found that the dorsal ramus of the lacrimal is identical to that of T. rex and does not, as reported by Molnar (1980) resemble that of Albertosaurus (Fig. 10). We did not find the supraoccipital, but the median portion of the nuchal crest of the parietal is preserved (Fig. 15 J- M). We regard the asymmetry in thickness between each ala of the nuchal crest as individual variation, in contrast to Molnar (1980) who regarded this as a shared similarity with Albertosaurus libratus. Likewise, we regard the absence of a medial shelf along the ventral margin of the surangular process of the dentary as individual variation. We found that the olecranon process of the ulna is completely broken off and missing (Fig. 14E, F), accounting for there being ‘no sign of a pronounced olecranal process as in Albertosaurus and Daspletosaurus ’ (Molnar, 1980: 105). Molnar listed similarities between LACM 23845 and Albertosaurus. Molnar stated that in LACM 23845, ‘ A. lancensis ’, and A. libratus, the sagittal crest reaches the dorsal margin of the nuchal crest, but in Tyrannosaurus the crest does not reach the dorsal margin of the nuchal crest. However, among most tyrannosaurids (Table 3), the extent of the sagittal crest is variable in most taxa and in LACM 23845 the sagittal crest actually stops short of the dorsal margin of the crest (Fig. 15M). Therefore, this character is individually variable and does not have diagnostic value except for A. libratus, in which the crest reaches the dorsal margin of the nuchal crest (Table 3). The form of the distal end of the fibula does not differ in any significant way from that of other tyrannosaurids. Molnar’s comparison of the metatarsals with Albertosaurus – in contrast to the stout bones in Tyrannosaurus – may simply reflect the smaller size of LACM 23845. The supposed manual ungual is actually from digit I of the pes, based on the robust nature of the ungual and the low flexor tubercle. The angular form of the joint surface of the ungual is, in part, an artefact of breakage (cf. Molnar, 1980). PREVIOUS TAXONOMIC WORK ON LACM23845 Paul (1988b: 324). referred LACM 23845 to the new species Albertosaurus megagracilis on the basis of its ‘extremely atrophied forelimbs, down-bent nasals, very long snout... long hind limbs’ and its ‘overall large size and gracile build’. With regard to the long snout, this is based on Molnar’s inference of a longer snout than in CMNH 7541 based on the greater (25%) length of the nasals in contrast to the slightly greater (10%) length of the frontals and parietals in LACM 23845. Without an independent estimate of body size, it is difficult to determine what allometric trends these differences describe, and Paul’s (1988b) use of them as evidence of a taxonomic difference is undermined. Because LACM 23845 was shown above to be a subadult T. rex, A. megagracilis is a junior subjective synonym of T. rex. Olshevsky & Ford (1995) state that LACM 23845 is not complete enough to be diagnosable but proceed to refer the specimen to a new taxon that they diagnose. Their diagnosis repeats characters from Molnar (1980) and they add the wide caudal region of the frontals, which indicates the orbits faced rostrally, but not as much in Tyrannosaurus. However, the caudal region of the frontal lies within the dorsotemporal fossa and is independent of orbit orientation (Figs 8, 15C, D). Olshevsky and Ford claim that there is a wide gap between the lacrimal and postorbital joint surfaces. However, the joint surface of the postorbital is missing on both sides, while a wide gap does not occur in subadult and adult tyrannosaurids (e.g. , AMNH 5664, , ROM 1247). Unlike Olshevsky & Ford (1995), we suggest that LACM 23845 is diagnosable and can be referred to T. rex. Thus, D. megagracilis is a junior subjective synonym of T. rex.
Taxon Treatment
- Thomas D. Carr; Thomas E. Williamson; 2004: Diversity of late Maastrichtian Tyrannosauridae (Dinosauria: Theropoda) from western North America, Zoological Journal of the Linnean Society ': 481-511. doi
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