Tyrannosauroidea

Holtz, TR jr., 2004, Tyrannosauroidea, The Dinosauria, University of California Press, pp. 111-136 : 1-7

Tyrannosauroidea

THOMAS R. HOLTZ JR.

Tyrannosauroids are among the most distinctive and best known of Mesozoic theropods ( table 5.1). They are characterized by large skulls with a specialized, heterodont dentition, a derived squamosal-quadratojugal flange, and a highly pneumatic basicranium; greatly reduced forelimbs (both in size and in digit count); and elongate hindlimbs bearing a pinched third metatarsal (arctometatarsus).

In recent years new discoveries and the use of modern phylogenetic analytical techniques have revealed the presence of taxa outside Tyrannosauridae sensu stricto but closer to this clade than to all other theropod groups. These include the newly described Early Cretaceous Eotyrannus lengi (Hutt et al. 2001; Naish et al. 2001), the Late Jurassic Stokesosaurus (Madsen 1974; Foster and Chure 2000; Rauhut 2000a), and the Late Cretaceous Dryptosaurus aquilunguis (Cope 1866; Carpenter et al. 1997; Carr et al., in press). The latter is historically significant, for it was the taxon that revealed that carnivorous dinosaurs were obligate bipeds rather than quadrupeds (Cope 1866). Together with Tyrannosauridae , these aforementioned taxa are members of Tyrannosauroidea, a clade of basal coelurosaurs. As recognized here there are at least six, and possibly nine, genera within Tyrannosauridae , as well as three genera for which a basal tyrannosauroid position is well supported. Based on more questionable evidence, basal tyrannosauroids may be additional taxa.

Whereas most other taxa of large-bodied theropods (abelisaurids, spinosauroids, carnosaurs) are represented by only a few, often incomplete skeletons, several species of tyrannosaurids are represented by multiple complete skulls and postcrania ( Tyrannosaurus rex , Tarbosaurus bataar , Daspletosaurus torosus , Albertosaurus sarcophagus , and Gorgosaurus libratus ) of various ontogenetic stages. Among multi-tonne theropods only Allosaurus fragilis is better represented in the fossil record. Furthermore, tyrannosaurids are characterized by copious synapomorphies relative to other well-known theropod clades; basal tyrannosauroids reveal some of the sequence of acquisition of these features.

Diagnostic skeletal material for Tyrannosauridae is at present limited to the last part of the Late Cretaceous (Campanian and Maastrichtian) of eastern and central Asia (China and Mongolia) and North America; the more inclusive Tyrannosauroidea includes taxa from the Late Jurassic of Europe and North America and the Early Cretaceous of Europe and Asia (and possibly South America). Abundant bones of tyrannosauroids (likely tyrannosaurids) are present throughout the Late Cretaceous of western North America and eastern and central Asia (Weishampel et al., this vol.); however, due to their incompleteness they cannot be definitively referred to Tyrannosauridae as that taxon is defined here. The oldest confirmed tyrannosauroid is Stokesosaurus from the Kimmeridgian of the western United States (the same taxon or a related form is present in deposits of the same age in Portugal [Rauhut 2000b]). Based on phylogenetic inference, however, if Eshanosaurus from the Hettangian of China (Xu et al. 2001a) proves to be a member of Therizinosauroidea (implying a split between tyrannosauroids and maniraptoriforms, including therizinosauroids, had already occurred by the Early Jurassic), then tyrannosauroids were present from the Early Jurassic. The latest Maastrichtian western North American Tyrannosaurus rex (and Nanotyrannus lancensis , if it is a distinct taxon) is the latest-known tyrannosaurid and among the latestknown nonavian theropods.

Tyrannosauroids make up the sole large-bodied carnivorous theropods known from the Campanian–Maastrichtian of central and eastern Asia and North America; presumably these taxa were the top predators of those communities. Before that, however, tyrannosauroids were of only moderate body size and lived in faunas in which ceratosaurs, spinosauroids, and/or carnosaurs represented the largest carnivorous theropods (Foster et al. 2001; Farlow and Holtz 2002).

Tyrannosaurids include some of the largest known theropod taxa (up to 13 m long and weighing six or more tonnes). The five most completely known species ( Tyrannosaurus rex , Tarbosaurus bataar , Daspletosaurus torosus , Albertosaurus sarcophagus , and Gorgosaurus libratus ) are all represented by individuals with femora more than 100 cm in length, reaching 138 cm in the largest Tyrannosaurus rex (Brochu 2002) . The basal tyrannosauroid Dryptosaurus does not reach this size, but with a femoral length of more than 77 cm it was still a large-bodied theropod (Carpenter et al. 1997). Skulls exceeding 1 m in length are known for Tyrannosaurus , Tarbosaurus , and Daspletosaurus torosus ; the premaxilla-occipital condyle length of the largest T. rex skull (153 cm) exceeds the same measurement of the skull of Giganotosaurus (148 cm). However, despite being among the largest known theropods, tyrannosaurids are also characterized by proportionately elongate and slender hindlimbs. Tyrannosaurids are markedly heterodont for theropods, and the D- or U-shaped premaxillary teeth of all tyrannosaurids and incrassate (thickened) “lateral” (i.e., maxillary and dentary) teeth of tyrannosaurids are easy to distinguish from those of nontyrannosaurid taxa.

TABLE 5.1 Tyrannosauroidea

Nomina dubia Material

Aublysodon amplus Marsh, 1892 Tooth

Aublysodon cristatus Marsh, 1892 Tooth

Aublysodon lateralis Cope, 1876a Teeth

Aublysodon mirandus Leidy, 1868 Premaxillary teeth

Deinodon horridus Leidy, 1856 Teeth

Laelaps macropus Cope, 1868 Fragmentary hindlimb elements

Tomodon horrificus Leidy, 1865 Tooth

(type of Diplotomodon Leidy, 1868a )

Tyrannosaurus lanpingensis Yeh, 1975 Teeth

Tyrannosaurus turpanensis Zhai, Zheng, et Tong, 1978 Teeth and sacral vertebrae

Most modern authors have followed D. A. Russell (1970a) in preferring Osborn’s (1906) name Tyrannosauridae , while many earlier workers (e.g., Matthew and Brown 1922, 1923; Huene 1923, 1926c; Maleev 1974) used Deinodontidae Brown, 1914 , itself an emendation of Dinodontidae Cope, 1866 .

Definition and Diagnosis

Tyrannosauroidea as used herein is defined as the clade composed of Tyrannosaurus rex and all taxa sharing a more recent common ancestor with T. rex than with Ornithomimus velox , Deinonychus antirrhopus , or Allosaurus fragilis . Tyrannosauroidea, or at least a phylogenetically secure subclade therein comprising Stokesosaurus , Dryptosaurus , Eotyrannus , and Tyrannosauridae , may be diagnosed by the following synapomorphies: the main body of the premaxilla taller dorsoventrally than it is long rostrocaudally (also in Abelisauridae ); a prominent and laterally extending horizontal shelf on the lateral surface of the surangular, rostral and ventral to the mandibular condyle; a large opening for the caudal surangular foramen; the retroarticular process reduced; the dorsal surface of the iliac blades converging closely on the midline (also found in ornithomimosaurs and mononykine alvarezsaurids); the dorsal portion of the cranial margin of the preacetabular blade of the ilium concave cranially and the ventral portion convex; an accessory broad, ventral hooklike projection from the preacetabular blade of ilium; a straight supracetabular crest on the lateral margin of the ilium in dorsal view (also found in various maniraptorans); a median vertical ridge on the external surface of the ilium; a ventral flange on the puboischial peduncle; the cranial margin of the pubic peduncle concave; the pubis shaft having a marked concave curvature cranially (convergent with some oviraptorosaurs); the shaft of the ischium much more slender than the pubis shaft; a semicircular scar on the caudolateral surface of ischium, just distal to the iliac process (also in ornithomimosaurs); a sheet of bone extending from the obturator process continuing down at least half the length of the ischium; the fibular cranial tubercle distal to the cranial expansion composed of two longitudinal ridges; elongate metatarsals relative to those of other theropods having femora of the same length (also in Elaphrosaurus , ornithomimosaurs, and troodontids); and the proximal surface of metatarsal III crescentic and limited to the plantar half of the proximal surface of the metatarsus.

Anatomy

Tyrannosauroid anatomy has been described by Lambe (1904a, 1917a), Osborn (1905, 1906, 1912a, 1917), Parks (1933), Gilmore (1946a), Russell (1970a), Madsen (1974), Maleev (1974), Bakker et al. (1988), Molnar et al. (1990), Molnar (1991), Carpenter et al. (1997), Chure and Madsen (1998), Carr (1999), Carr and Williamson (2000, in press), Foster and Chure (2000), Carpenter and Smith (2001), Hutt et al. (2001), Naish et al. (2001), Brochu (2002), Carr et al. (in press), Currie (2003b), and Hurum and Sabath (2003).

Skull and Mandible

Complete or near-complete skulls are known for Tyrannosaurus (Osborn 1912a; Gilmore 1946a; Bakker et al. 1988; Molnar 1991; Carr 1999; Brochu 2002; Carr and Williamson, in press), Tarbosaurus (Maleev 1974; Hurum and Sabath 2003), Daspletosaurus (Russell 1970a; Currie 2003b), Gorgosaurus libratus (Lambe 1917a; Carr 1999; Currie 2003b), and Albertosaurus ( figs. 5.1–5.3; Lambe 1904a; Currie 2003b). Partial skulls are known for Alioramus remotus (Kurzanov 1976a; Currie 2003b) and the unnamed Alabama tyrannosaurid (Carr et al., in press). Cranial material is presently known for Alectrosaurus olseni (Gilmore 1933a; Mader and Bradley 1989), although more recently discovered skulls may be referable to this taxon (Perle 1977). Only fragmentary cranial material is known for Eotyrannus (Hutt et al. 2001; Naish et al. 2001), Dryptosaurus (Cope 1866; Carpenter et al. 1997), and Bagaraatan (Osmólska 1996) ; cranial material referred to Stokesosaurus (Madsen 1974; Chure and Madsen 1998) cannot be definitely referred to this taxon.

Tyrannosauroid skulls are long and low among small individuals (adults of small-bodied taxa and juveniles of larger species), deepening dorsoventrally in the adults of the largebodied taxa ( figs. 5.1–5.3; Carr 1999; Carr and Williamson, in press; Currie 2003b). In general form the skulls of most tyrannosaurids resemble those of other large carnivorous theropods and nondinosaurian archosaurs (e.g., poposaurids and rauisuchids) in being considerably deeper dorsoventrally than wide transversely; however, Tyrannosaurus rex has a more rounded rostrum, proportionately wider at the base than in more primitive tyrannosaurids. Where the skulls are complete, tyrannosaurids have a proportionately wider rostrum than do carnosaurs of similar size. In T. rex this width is greatly increased postorbitally, so that the maximum skull width is approximately two-thirds of the premaxilla-occipital condyle length of the skull (compared with Allosaurus , for example, in which the comparable figure is only one-third of the skull base length, or the tyrannosaurid Daspletosaurus , in which it is slightly less than half of the skull base length in). The cranial and some postdentary bones have extensive internal chambers. Tyrannosaurids have variable cranial ornamentation, including rugosities on the nasals, postorbitals, and jugals and hornlets on the lacrimals.

The nares are rostrocaudally elongate and bounded by the premaxillae, the maxillae, and the nasals. The orbits are oval, with the dorsoventral axis becoming proportionately longer as the skull size increases (except in Gorgosaurus libratus [Carr 1999; Carr and Williamson, in press]). The eyes of most tyrannosaurids are laterally directed, but those of Tyrannosaurus rex are more rostrally directed, reoriented due to the postorbital expansion of the skull.The infratemporal fenestra of tyrannosaurids is distinctive, with a vertical rostral margin and a caudal margin shaped like a sideways W. The central constriction divides the fenestra in half due to a synapomorphic extension of both the squamosal and the quadratojugal, with the suture between the two subhorizontal and down the longest part of the protrusion. This differs from the constriction of the infratemporal fenestrae of basal tetanurans such as Allosaurus, Monolophosaurus , and Ornitholestes , in which the squamosal forms the majority of the protrusion. The supratemporal fenestra is smaller than the infratemporal fenestra, situated entirely dorsally, and bounded by the frontal, parietal, supraoccipital, squamosal, and postorbital. The posttemporal fenestrae are greatly reduced. The choanae are located about halfway along the length of the skull and are surrounded by the maxilla, the palatine, and the vomer. The palatal surface is roofed by extensive medial projections from the premaxillae and maxillae.

The premaxilla is short and deep in tyrannosauroids; the ventral ramus of the premaxilla is taller dorsoventrally than it is long rostrocaudally, and it is positioned so that it is more obscured by the maxilla in lateral view than in other theropods. There are only four premaxillary teeth. The premaxillary teeth of Eotyrannus and tyrannosaurids are arranged in an arcade that is much more transversely oriented than rostrocaudally oriented, so that the rostralmost surface of the last tooth is rostral to the caudalmost surface of the first tooth (in contrast to the case of most theropods, in which the premaxillary tooth arcade is arranged more closely to the main skull axis).

The maxilla is an elongate, roughly triangular element with a rounded rostral apex and two caudal rami separated by the internal antorbital fenestra. The ventral margin of large tyrannosaurids has a convex curvature, most pronounced in the Tarbosaurus and Tyrannosaurus . The maxillary antorbital fossa bears both promaxillary and maxillary fenestrae, the former often obscured in lateral view in larger individuals by a shelf on the lateral surface of the maxilla (Carr et al., in press). In tyrannosaurines the promaxillary fenestra rotates to face rostrally, and the rostral margin of the maxillary fenestra contacts the rostral rim of the antorbital fossa. Medially the maxillae of tyrannosaurids house a series of internal chambers (maxillary and promaxillary antra and pneumatic interalveolar recesses); as preserved these chambers are not separated from each other by shelves of bone (similar to the condition in Allosaurus [Witmer 1997a]). The number of maxillary teeth in tyrannosaurids varies: a maximum of 18 are reported in skulls referred to Alectrosaurus olseni and a minimum of 11 in some Tyrannosaurus rex . The number decreases with skull size in Gorgosaurus libratus and T. rex (accompanied by a proportional increase in the crosssectional size of the teeth during ontogeny, so that larger teeth exclude the cranialmost tooth position [see Currie 2003b for a dissenting view]). The interdental plates are large and rugose. The palatal shelves of the maxillae of tyrannosaurids are much more extensive then those of carnosaurs or other large theropods ( fig. 5.4A). In most tyrannosaurids these shelves contact the vomers along the midline for the rostral half of the length of the tooth row: they angle back toward the tooth row forming a diagonal contact with the palatines along their caudomedial surface. In Tyrannosaurus , however, these projections are much more extensive, contacting the vomers for more than threequarters of the length of the tooth row.

The elongate nasals of Eotyrannus and tyrannosaurids are fused along the midline of the dorsal surface, although the rostralmost and caudalmost ends are not entirely fused and the suture line is visible along the ventral surface. In adult tyrannosaurids the dorsal surface is rugose, and it can have an almost barklike texture in some cases.The nasals are penetrated by many foramina, suggesting that a horny covering was present. In Alioramus there is a row of five vertical blades along the midline, while in the Alabama tyrannosaurid there are six small bumps.

The lacrimal is shaped like an inverted L; the rostral and descending rami of the lacrimals form about half of the dorsal and all of the caudal surface, respectively, of the internal antorbital fenestra. The rostral ramus contains internal pneumatic chambers; in adult Tarbosaurus and especially Tyrannosaurus this portion of the bone can have an inflated appearance. The dorsal surface bears a triangular cornual process in albertosaurines and Daspletosaurus ; this prominence is lost in the Tyrannosaurus-Tarbosaurus clade.

The postorbital is a triradiate bone with longer descending and caudal rami and a shorter medial process. In Tyrannosaurus , Tarbosaurus , and Albertosaurus , and to a much lesser degree in Daspletosaurus , a prong of the postorbital constricts the orbit and demarcates a dorsal (eyeball-bearing) and ventral portion of the orbit; similar constrictions are found in the carnosaurs Giganotosaurus and Carcharodontosaurus , some Allosaurus , and some abelisaurids (Chure 1998c). In the case of Gorgosaurus this suborbital flange extends to the descending of the ramus of the lacrimal, creating a circular orbit in the largest adults. Smaller individual tyrannosaurids have a slight bump on the thickened dorsal rim above and behind the orbit that becomes greatly expanded in larger individuals. This structure forms a rugose postorbital boss in most adult Tarbosaurus and Tyrannosaurus . The enlargement of the dorsal surface of the lacrimal excludes the prefrontals and frontals from view on the lateral aspect; in some Tyrannosaurus this exclusion is completed by intergrowth of the lacrimal and postorbital rugosities.

The squamosal is pneumatized (Witmer 1997a, 1997b) and forms a complex, four-pronged structure. The ventral process forms the dorsal half of the prong that constricts the infratemporal fenestra; the suture with the quadratojugal is nearly as long as the squamosal is tall in most taxa. The quadratojugal is a complex element, wide at the dorsal and ventral surfaces and narrowest dorsal to the juncture of the ascending and caudoventral rami. The dorsal portion of the ascending process is closely appressed medially to the lateral margin of the quadrate.

The jugal is a large triradiate element. The dorsal surface of the rostral prong participates in the internal antorbital fenestra and contains an enlarged rostral foramen. This aperture opens to a complex of internal chambers that also exit via a smaller foramen below the lacrimal articulation in some Tyrannosaurus rex (Molnar 1991) . The lacrimal articulation contributes only slightly to the orbital margin but forms most of the rostral margin of the antorbital fenestra. In tyrannosaurids a low cornual process extends caudoventrally from the middle portion of the ventrolateral surface of the jugal; in large individuals this jugal horn can be rugose.

The prefrontal is greatly reduced in all tyrannosaurids. It is bordered primarily by the lacrimal and the frontal, with a small contribution from the nasal. The frontals of tyrannosaurids vary in shape relative to skull size, starting as triangular bones but expanding caudally to become rectangular in tyrannosaurids having the largest skulls (Currie 2003a). In the largest tyrannosaurids there is a sagittal crest along most of the midline of the paired frontals. In all tyrannosaurids the frontal is firmly sutured medially.

Tyrannosaurids possess a sagittal crest on the fused parietals (formed along the midline where the supratemporal fossae are confluent). Caudally the parietals bear a large transverse nuchal crest, which rises above the supraoccipital ( fig. 5.5). The crest is more pronounced in tyrannosaurids than in other large-bodied theropods, particularly in Alioramus , Tarbosaurus , and Tyrannosaurus . In these latter taxa the crest is often rugose on the dorsal surface. Fossae for the ligamentum nuchae are present dorsally on the caudal surface of the transverse nuchal crest.

The parasphenoid is shaped as a thin rostral prong. It forms the rostral wall of the hypophyseal fossa, is fused with the basisphenoid, and contacts the orbitosphenoid and the laterosphenoid dorsally. The sphenethmoid is known for Gorgosaurus and Tyrannosaurus , in which it is a boxlike structure housing the passages for the olfactory nerves. A median septum separates the two passages. Just behind the sphenethmoid lies the subquadrate orbitosphenoid, bearing the optical foramen. Almost horizontal, it is sutured to the frontal laterally and to the laterosphenoid caudally. The prootic, which is caudal to the laterosphenoid and partly overlaps the opisthotic, is penetrated by the fenestra ovalis and the jugular foramen. The laterosphenoid is tall and forms much of the lateral wall of the braincase. Dorsolaterally it contacts the postorbital by an apparently immobile condyloid joint. The laterosphenoid floors the endocranial cavity and forms the dorsal part of the sella turcica. The oculomotor and trochlear nerves pass through the base of the pila antotica of the laterosphenoid ( Tyrannosaurus ) or the laterosphenoid-orbitosphenoid junction ( Gorgosaurus and Daspletosaurus ). The trigeminal foramen penetrates the laterosphenoid. The opisthotic bears the foramen of the facial nerve. It is extended caudolaterally as a vertical plate and forms, with the exoccipital, the paroccipital process. The paroccipital process of tyrannosaurids is hollow and projects more horizontally and transversely than in carnosaurs and other large tetanurans, in which it is more ventrally and rostrocaudally oriented. The two branches of the vestibulocochlear nerve are separate from the facial nerve. Three tympanic recesses connect to the inner ear, the caudal invading the base of the paroccipital process through a large foramen. The occipital region is caudally oriented in Gorgosaurus and Daspletosaurus ; however, in Alioramus , Albertosaurus , Tarbosaurus , and Tyrannosaurus it is more ventrally oriented, suggesting that the skull was oriented with the rostrum directed slightly ventrally in life.

The occipital elements of tyrannosaurids are often extensively fused ( fig. 5.5). The supraoccipital forms a broad rectangular structure that bears a pair of tablike processes. Internally the supraoccipital houses a tripartite sinus arching over the endocranial cavity (Brochu 2002). The basioccipital forms about half of the occipital condyle and most of the occipital plate below, with extensive contributions from the exoccipitals. Deep subcondylar recesses are situated immediately lateral and slightly ventral to the occipital condyle (Witmer 1997a, 1997b). The basisphenoid region is expanded transversely relative to that in most other large theropods. The basisphenoid floors the braincase. The ventrolaterally directed basipterygoid processes extend from the base of the basisphenoid to contact the pterygoids at well-defined articular surfaces. The basisphenoid is excavated into a deep, ventrally opened recess (Witmer 1997a, 1997b) surrounded by a basipterygoid web (connecting the basipterygoids), a basituberal web (connecting the basal tubera), and webs connecting the basal tubera and the basipterygoid processes on each side (Bakker et al. 1988). The basisphenoid recess is penetrated by a pair of large foramina in tyrannosaurids (Witmer 1997a, 1997b). Although these foramina are reported as a single opening (Russell 1970a) or paired but much reduced (Bakker et al. 1988) in Daspletosaurus torosus , new evidence suggests that they were similarly formed in Daspletosaurus as in other tyrannosaurids (Carr 1999). The basal tubera are plesiomorphically large in Gorgosaurus and Daspletosaurus but reduced in Alioramus , Albertosaurus , Tarbosaurus , and Tyrannosaurus .

The quadrate of tyrannosaurids and probably Eotyrannus (Hutt et al. 2001) is pneumatic. The jaw articulation of tyrannosaurids lies no farther caudally than the caudalmost point of the occipital condyles and rostral to the caudalmost extent of the paroccipital processes; this is unlike the condition in other large-skulled theropods, in which the jaw articulation is well caudal to the occipital condyle and to the caudalmost point of the paroccipital processes.

The pterygoid has two main components: a horizontal, flat palatal plate and a vertical, thin quadrate ramus. The main body of the ectopterygoid has several interconnecting chambers, which open caudoventrally ( fig. 5.4B). Two foramina open to the ventral ectopterygoid sinus in Albertosaurus and Gorgosaurus ; the ectopterygoid chambers of Daspletosaurus , Tarbosaurus , and Tyrannosaurus are inflated.

The palatine in most tyrannosaurids ( Gorgosaurus , Albertosaurus , Daspletosaurus , and subadult Tarbosaurus and Tyrannosaurus ) is triradiate ( fig. 5.4C), with maxillary, pterygoid, and vomerine processes but lacking the jugal process of noncoelurosaurian tetanurans. In adult Tarbosaurus and Tyrannosaurus the palatine is a pneumatic trapezoidal element with a flat ventral surface. The long maxillary process (or the long side of the trapezoid in Tyrannosaurus ) contacts the palatal shelf of the maxilla, and the maxillary and vomerine processes form the caudal margin of the choana. The lateral surface of the tyrannosaurid palatine bears a recess, and a large foramen or foramina (one in the Alabama tyrannosaurid, Daspletosaurus , and some Tyrannosaurus rex ; two or more in Gorgosaurus , Albertosaurus , Tarbosaurus , and other T. rex ) lead to a strutted cavity within the bone (Witmer 1997a, 1997b).

The vomer comprises two portions, a rostral rhomboid plate and a caudal laterally compressed stem (Molnar 1999). The rostral plate is expanded in tyrannosaurids, unlike in most other theropods, and is either trapezoidal or diamond-shaped. The vomer has extensive contact with the palatal shelves of the maxilla, and the rostral end of the plate may contact the ventral surface of the palatal shelves of the premaxillae.

To date, the epipterygoid has been described only in Albertosaurus , Alioramus , and Tyrannosaurus . It is a teardropshaped bone linking the quadrate process of the pterygoid to the laterosphenid.

The endocranial cavity is larger in tyrannosaurids than in carnosaurs of equivalent skull size (Larsson et al. 2000; Larsson 2001). The use of computer tomography (CT) on various tyrannosaurids (Brochu 2000, 2002) has greatly enhanced previous knowledge, which was drawn mostly from endocranial molds (Osborn 1912a; Maleev 1965; Hopson 1979). The cavity is short but tall. There is a transversely expanded rostral region followed by a transversely constricted but dorsoventrally expanded region that tapers toward the foramen magnum. The rostral region presumably corresponds to the forebrain, while the dorsoventrally expanded region housed the mid- and hindbrains.The floor of the cavity is flexed upward between the jugular and trigeminal foramina. Just rostral to the trigeminal foramen, the cavity is widened to accommodate the optic lobes or the cerebrum. Brochu (2000, 2002) has demonstrated the greatly enlarged olfactory lobes of Tyrannosaurus rex . However, without a comparative series of this region in theropods of intermediate size it is not certain whether these are apomorphically enlarged in T. rex (or Tyrannosauridae ) relative to other nonavian theropods or instead are the size expected by allometry; hence, any paleobiological interpretation for this taxon based on the absolute size of its olfactory lobe remains spurious at present.

The mandibles of primitive tyrannosauroids, as well as small individuals of tyrannosaurids, tend to be slender ( fig. 5.3B), while those of adult tyrannosaurids (and adult and juvenile Tyrannosaurus ) are more robust and deeper dorsoventrally than in more primitive taxa ( fig. 5.6; Carr 1999; Currie 2003b). The mandibular fenestra is small, while the caudal surangular foramen is large in most tyrannosaurids, although it is secondarily reduced in Tarbosaurus bataar . The dentary bears as many as 18 teeth ( Alectrosaurus olseni ) and as few as 11 ( Tyrannosaurus rex ). The interdental plates of tyrannosaurids are large, as in the maxilla; in Eotyrannus they are small spikes. The caudal end of the dentary flares dorsally and ventrally in adult tyrannosaurids, marking the deepening of the postdentary region compared with that in more primitive forms. The supradentary of tyrannosaurids is fused to the coronoid ( fig. 5.6C; Hurum and Currie 2000). Indeed, it is not at all certain that these represent two separate bones in any theropod (Currie 2003b).The splenial is large, flat, and triangular and bears a large rostral mylohyoid foramen and a caudal notch forming the rostral margin for the internal mandibular fenestra.The prearticular forms a large, flat crescent (the curvature that is greatest in those taxa with the deepest postdentary region) and forms the caudal, ventral, and rostroventral margins of the adductor fossa; the small triangular coronoid forms the rostrodorsal margin. The surangular is a large, thin, curved vertical plate that is dorsoventrally deep on the rostral margin. The rostral external surangular foramen is small, while the caudal surangular foramen is large (except in Tarbosaurus ). Dorsal to the caudal surangular foramen is a horizontal shelf dorsal to which is a flat facet, presumably for the attachment of the superficial mandibular adductor musculature. The articular is roughly tetrahedral and bears a large foramen aerosum and a large central sinus. Tyrannosaurids do not possess a retroarticular process; instead, a shallow concavity occupies the whole of the caudal surface of the articular, presumably for the insertion of the mandibular depressor muscle. The shape of the glenoid in tyrannosaurids closely conforms to that of the mandibular condyles of the quadrate.

Tyrannosauroid premaxillary ( fig. 5.7A) and “lateral” (maxillary and dentary) teeth ( fig. 5.7B) are distinct from each other, more so than in most theropod taxa; in fact, Marsh (1892) considered isolated premaxillary teeth of tyrannosaurids to be mammalian. The incisiform premaxillary teeth of Eotyrannus and tyrannosaurids have a D- or U-shaped basal cross section that is mesiodistally as deep as or deeper than it is labiolingually ( fig. 5.7A).Both carinae lie along the caudal surface of the tooth, on a plane perpendicular to the main axis of the skull. These teeth often have a strongly developed vertical ridge on their distal surface. Some tyrannosaurid premaxillary teeth lack serrations on their carinae; however, it is not at all certain that these “ Aublysodon ” teeth (Molnar and Carpenter 1989; Currie et al. 1990; Lehman and Carpenter 1990; Holtz 2001b) are not simply the product of wear, digestion, or postmortem erosion (Brochu 2002; Carr and Williamson, in press). In some tyrannosaurid teeth the carinae are bifurcated, perhaps due to trauma or aberrant tooth replacement, but this is also consistent with genetic factors (Erickson 1995). In some tyrannosaurids the rostralmost maxillary teeth are incisiform or subconical. Otherwise, the lateral teeth of tyrannosaurids are larger than the premaxillary teeth, and the carinae are slightly offset from the rostral and caudal margins of the tooth (more so in more robust teeth than in slender forms). In primitive tyrannosauroids ( Dryptosaurus and Eotyrannus ) and in juveniles of tyrannosaurids the teeth are ziphodont (bladelike), although the crosssectional diameter is greater labiolingually than found in the teeth of other theropods of the same crown height. In larger advanced tyrannosaurid taxa, however, the teeth are incrassate, so that the labiolingual width at the tooth base sometimes is equal to or exceeds the mesiodistal length. The tooth root can constitute almost two-thirds of the total length in some tyrannosaurids. Denticles on the lateral teeth are wider labiolingually than they are tall proximodistally. In Eotyrannus and some tyrannosaurids the denticulations extend over the tip of the tooth. In some of the largest tyrannosaurid teeth, wrinkles in the enamel extend from the carinae.

Tyrannosaurid teeth show three kinds of wear: slight rounding of the tip, development of flat, oblique facets adjacent to the top both labially and lingually, and abrasion of the serrations both mesially and distally (Molnar et al. 1990; Abler 1992; Farlow and Brinkman 1994). The facets do not correspond in the upper and lower dentitions, so it is unlikely that they were formed by tooth-to-tooth contact.

To date, ceratobranchials have only been reported in a probable juvenile Tyrannosaurus rex (Gilmore 1946a; Bakker et al. 1988). These bones are elongate rods with a curved segment near the middle of the shaft.

Postcranial Skeleton

The majority of the postcranium of Tyrannosaurus (Osborn 1917; Brochu 2002), Tarbosaurus (Maleev 1974) , Daspletosaurus (Russell 1970a) , Gorgosaurus (Lambe 1917a) , and Albertosaurus (Parks 1933) is known ( fig. 5.8). More fragmentary remains have been recovered for the Alabama tyrannosaurid (Carr et al., in press), Alioramus (Kurzanov 1976a) , Alectrosaurus (Mader and Bradley 1 989), Eotyrannus (Hutt et al. 2001; Naish et al. 2001), Dryptosaurus (Carpenter et al. 1997) , and Bagaraatan (Osmólska 1996) .

AXIAL SKELETON

In tyrannosaurids there are 10 cervicals, 13 dorsals, 5 sacrals (although the caudalmost dorsal is slightly sacralized), and 35–44 caudals. Cervicals, dorsals, and some sacrals of tyrannosaurids (and, as far as can be ascertained, Eotyrannus ) bear pleurocoels, and the centra have a complex camellate structure (Britt 1993, 1997). The spinous processes of tyrannosaurids are also highly pneumatized (Brochu 2002).

As in most theropods, the atlas is composed of only the neurapophyses, the intercentrum, and the odontoid ( fig. 5.9). The crescentic intercentrum is axially compressed, twice as long as it is high. The axial pleurocentrum is pleurocoelous, generally having only a single pneumatopore. The axial spinous process has only a low vertical component projecting dorsally. Pre- and postzygapophyses are present, and the latter bear epipophyses.

Postaxial cervical centra are amphiplatyan, amphicoelous, or at most weakly opisthocoelous ( fig. 5.9B); this differs from the condition in other large-bodied theropods, in which the centra are markedly opisthocoelous. The spinous processes, zygapophyses, and rib articulations are well developed. Facets for the zygapophyses are dorsally (rather than craniodorsally) oriented in Eotyrannus and tyrannosaurids. The cervical central of adult tyrannosaurines are craniocaudally compressed, only about half as long as they are tall; however, those of juveniles and of other tyrannosauroids are as long as or longer than they are tall. Although tyrannosaurids are often characterized as being short-necked (cf. Steel 1970), this description is only applicable to Tyrannosaurinae (Holtz 2001b; Currie 2003b); the cervical series of Gorgosaurus and Albertosaurus are almost as long as the dorsal series. Tyrannosaurids’ appearing to be short-necked may be a result of their enlarged skulls rather than the proportions of the sections of the vertebral column.

Dorsal vertebrae have powerfully developed spinous processes and transverse processes, the latter being horizontally directed ( fig. 5.10). The centra are amphicoelous or amphiplatyan and become longer caudally in the series.All dorsals have hyposphenehypantrum articulations. Dorsal ribs are well developed and broadly curved cranially, suggesting a barrel-shaped chest region. The trunk becomes shallower and narrower in front of the sacrum.

The sacrum proper consists of five vertebrae ( fig. 5.11); however the caudalmost dorsal is completely surrounded laterally by the ilia, with which it has broad contact via the transverse processes, and is thus functionally sacralized. In at least some material the sacrum is compressed, so that the centra are much deeper dorsoventrally than they are wide transversely, although this may be due in part to postmortem distortion. The sacral spinous processes are fused to form a continuous lamina. The sacral ribs and transverse processes are robust and have broad contact with the ilia.

The transition point between the proximal and distal parts of the caudal series (as marked by the presence or absence of transverse processes and spinous processes, respectively) is located at about caudal 15. The distal centra are more elongate in form than the proximal ( fig. 5.12A); the caudal centra are amphiplatyan or amphicoelous ( fig.5.12B).The transverse processes are broad and horizontal. The distal spinous processes are axially elongate. The cranial zygapophyses of the distal series are elongate, sometimes extending more than half of the length of the preceding centrum.

The hemal processes in tyrannosaurids show a transition from proximal L-shaped forms through caudal 10 ( fig.5.13C, D); hatchet-shaped forms, with the distal portion longer craniocaudally than the proximal portion and a ventrally convex margin, from caudal 11 to 15 ( fig. 5.13B); and boat-shaped hemal processes with cranial and caudal projections, and more than twice as long craniocaudally as tall dorsoventrally, from caudal 15 onward ( fig. 5.13A). Osborn (1917) miscalculated the length and transitional sequence of the (then incompletely known) tail of Tyrannosaurus rex . He thus restored this tyrannosaurid, using Allosaurus , with 53 caudals, as a model. He placed the only discovered hatchet-shaped hemal process between caudals 21 and 23 (ten or more vertebrae distal to the actual position) and interpolated a gradual sequence from the elongate proximal hemal processes. This incorrect model of the tyrannosaurid tail has been used to restore other incomplete skeletons since and may have been the source for some misinterpretation of the actual caudal anatomy of Tyrannosauridae (e.g., Forster et al. 1998).

Gastralia have saddle-shaped facets between the medial and lateral segments and between interlocking portions of each sequential medial segment ( fig. 5.14; Lambe 1917a; Parks 1933; Maleev 1974; Brochu 2002). The median segment of each gastralium is longer than the lateral segment. In Gorgosaurus the series had 19 segments (Lambe 1917a); Brochu (2002) estimates that 18 were present in Tyrannosaurus . The cranialmost two segments of the gastral basket of Tyrannosaurus (Brochu 2002) and Tarbosaurus (Maleev 1974) are fused into a platelike mass; given the similarity of this element to the supposed sternum of Gorgosaurus (Lambe 1917a) , that latter structure is more simply explained as also being a fused gastral plate.

APPENDICULAR SKELETON

The scapula is long and straplike. The caudal end is only slightly expanded in Eotyrannus , Gorgosaurus , Daspletosaurus , and Tarbosaurus but much more so in Tyrannosaurus and Albertosaurus , the latter having more expansion dorsally than ventrally. The acromial expansion is proportionately large. The scapula contributes more of the glenoid than does the coracoid. The coracoid is an oval plate approximately one-quarter of the scapular length and has a well-developed caudoventral process. The coracoid foramen is large. The biceps tubercle is present and moderately well developed.

Furculae are known for Albertosaurus , Gorgosaurus , and Daspletosaurus (Makovicky and Currie 1998). Tyrannosaurid furculae have a broad curvature. They have a short hypocleideum, and larger individuals have expanded epicleidial facets. Furculae articulate with the acromial processes, and the apex lies cranial and ventral to the cranial margins of the coracoids. Tyrannosaurid furculae can be distinguished from the fused cranialmost medial gastral segments of some theropods in that the latter are more massive, have more gradually tapering and straight (rather than sigmoid) rami, and do not have epicleidial facets (Currie and Makovicky 1998).

The sternum was reported for Gorgosaurus (Lambe 1917a) , but based on new observations from Tyrannosaurus (Brochu 2002) that element is more likely a fused gastral plate. Thus, no ossified sternum has been confirmed for any member of Tyrannosauroidea.

The tyrannosaurid forelimb is greatly reduced relative to the hindlimb and the scapula. Ratios of femoral to humeral length range from approximately 2.8 in Daspletosaurus to 4.0 in Tarbosaurus . Ratios of scapular to humeral length range from 2.2 in Daspletosaurus to 2.9 in Tarbosaurus . The antebrachium is shorter than the humerus, while the manus is intermediate in length between the two. The forelimb of Dryptosaurus is also reduced, but that of Eotyrannus is plesiomorphically elongate.

The humerus is slender, and the deltopectoral crest is weakly developed in all tyrannosauroids except Eotyrannus ( fig. 5.15). The crest is largest in Dryptosaurus , Albertosaurus , and Tyrannosaurus . The humeral head is not offset from the lateral tuberosity by a cleft. The shaft is straighter than in most large tetanurans, although within Tyrannosaurus rex two different forms are present, one with a more pronounced medial curvature and one with a much straighter shaft; Carpenter and Smith (2001) suggest that this difference might be a product of sexual dimorphism. The distal end is weakly expanded. The radius is a simple cylinder ( fig. 5.16B). The length of the ulna is approximately 60% of the length of the humerus in most tyrannosaurids, and only 45% in Tyrannosaurus bataar . The olecranon process is moderately well developed but short ( fig. 5.16A).

The carpus of tyrannosaurids is only poorly preserved. Five elements (three proximal, two distal) have been recovered for Gorgosaurus and Albertosaurus , four for Tarbosaurus , three for Daspletosaurus , and only two for Tyrannosaurus (Holtz 2001a; Chure, in press). However, this might well be an artifact of preservation rather than a taxonomic feature. Homologies are hindered by the apomorphically reduced nature of these elements, which lack well-developed facets or trochlea. A single large distal element caps parts of the proximal ends of metacarpals I and II. In fact, in a juvenile tyrannosaurid from the Horseshoe Canyon Formation of Alberta, Canada (Russell 1970a; Holtz 2001a) and in Eotyrannus (Naish, pers. comm.) this element does in fact possess transverse trochlea and a semilunate shape. In adult tyrannosaurids this element is poorly formed and lacks these features, perhaps reflecting a shift away from manual prehension during tyrannosauroid evolution and tyrannosaurid ontogeny.

The manus of Eotyrannus is tridactyl and well developed (Hutt et al. 2001); in contrast, the manus of tyrannosaurids was functionally didactyl, with only metacarpals I and II possessing phalanges ( fig. 5.17).The condition in Dryptosaurus is uncertain: at least one digit bore an immense trenchant claw, but which digit this was is uncertain (Carpenter et al. 1997). Metacarpal III in tyrannosaurids is greatly reduced in length and thickness relative to all other theropods and lacks a distal articulation. Metacarpal II is the longest element in the manus: it is about twice as long as metacarpal I in most tyrannosaurids but only about 70% longer in Tyrannosaurus and only 30%–60% longer in Tarbosaurus . Metacarpal III is longer than metacarpal I (the primitive condition in Theropoda ) in Gorgosaurus , Albertosaurus , and Daspletosaurus , but in Tarbosaurus metacarpal III is reduced, so that it is shorter than metacarpal I. Metacarpal III is present in Tyrannosaurus rex , as attested by the notch in the proximal end of metacarpal II for its articulation, but this element has not yet been found (Carpenter and Smith 2001), and it is thus uncertain whether it was as reduced as in Tarbosaurus . These proportions might reflect the general trend in theropod evolution of reduced size of manual elements from digit V toward digit I (Wagner and Gauthier 1999), with metacarpals III and II more reduced in the derived tyrannosaurines relative to the other, more primitive tyrannosauroids. The phalangeal formula of tyrannosaurids is 2-3-0-X-X. Phalanx 1 of digit I is the longest, being longer than metacarpal II in Albertosaurus , Daspletosaurus (incorrectly restored as phalanx 1 of digit II in Russell 1970a), and Tarbosaurus but subequal to metacarpal II in Gorgosaurus and shorter than it in Tyrannosaurus . The phalanges are robust and bear smooth ginglymoid articular facets. The unguals are proportionately shorter and stockier in tyrannosaurids than in Dryptosaurus or Eotyrannus . The distal end tapers to a point in these two basal forms, as well as in Gorgosaurus , Albertosaurus , and Dryptosaurus , but is thicker throughout its length, with a blunter tip, in species of Tyrannosaurus . The manual unguals are more laterally compressed than those of the pes and bear prominent grooves for the claw sheath. The flexor tubercles are less well developed in tyrannosaurids than in Dryptosaurus or Eotyrannus .

The ilium is long and bladelike in tyrannosauroids, only slightly shorter than the femur in albertosaurines and longer than the femur in tyrannosaurines ( fig. 5.18; Holtz 2001b; Currie 2003a). The dorsal portion of the cranial edge of the preacetabular blade is concave in Stokesosaurus and Tyrannosauridae . The preacetabular blade is deep in lateral view due to an apomorphic hooklike ventral projection extending from the preacetabular blade.This ventral projection is supported at its base by a medial shelf contacting the cranial sacral ribs ( fig. 5.18A). The ventral projection and the preacetabular blade meet at a notch on the cranial margin of the ilium. The dorsal margins of the ilia converge more closely along the midline in Stokesosaurus and Tyrannosauridae than in most other theropods, sometimes contacting the sacral spinous processes and nearly contacting each other. On the lateral surface of the ilium, dorsal to the acetabulum, is a prominent vertical midline crest. The pubic peduncle is more massive than the ischial and longer craniocaudally than wide transversely. In Bagaraatan , Stokesosaurus , and tyrannosaurids the cranial surface of the pubic peduncle is concave (Rauhut 2003), and there is a ventral flange extending from it (Hutchinson 2001a).The supracetabular crest is reduced. The postacetabular blade is longer than the preacetabular blade and has a squared end (more tapered in Tarbosaurus ).

The pubis is long and rodlike and extends subvertically relative to the axis of the sacrum ( fig. 5.18B). The shaft curves cranially in Dryptosaurus and tyrannosaurids, and a cranially projecting, crestlike pubic tubercle is present near the proximal end of the shaft (Hutchinson 2001a). The pubic boot is large in tyrannosaurids, about half as long craniocaudally as the pubis is long dorsoventrally in albertosaurines and two-thirds as long in tyrannosaurines. The pubic boot is equally long cranially as caudally and narrow transversely, and it tapers caudally to a point (less so cranially). The obturator foramen opens ventrally to form an obturator notch.The pubic shafts are closely appressed throughout the distal two-thirds of the pubis length; there is no cranial pubic foramen.

The ischium is rodlike, slender, points caudoventrally, and tapers to a point ( fig. 5.18B). The obturator process is triangular and proximally placed. In tyrannosaurids a ventrally extending sheet of bone continues the obturator process down at least half of the length of the ischial shaft. There is a prominent semicircular scar on the caudolateral surface of the proximal end of the ischium, caudal to the obturator process and ventral to the iliac articulation (Holtz 2001b). Hutchinson (2001a) and Carrano and Hutchinson (2002) reconstruct this as an attachment for M. flexor tibialis internus part 3.

The hindlimb of tyrannosauroids is long relative to their body size (Holtz 1995b; Currie 2003a). As in all nonavian theropods, the tibia becomes proportionately shorter relative to the femur as the body size increases (Holtz 1995b); consequently, using the proportions of the hindlimb elements without reference to overall body size is problematic for taxonomy, particularly for forms (e.g., tyrannosaurids) that ranged in size from about 1 m for juveniles to 14 m in adults. In adult tyrannosauroids of small body size and in juveniles of the larger taxa the tibia is longer than the femur; in larger individuals the tibia is often as short as the femur or even shorter (Holtz 1995b; Currie 2000). However, in tyrannosauroids the tibia is proportionately longer than in any other nonavian theropod (other than ornithomimosaurs and Elaphrosaurus ) of the same femur length, and the point at which tibia length is less than femur length occurs in much larger individuals than in carnosaurs, ceratosaurs, and other taxa (Holtz 1995b). Similarly, the metatarsi of tyrannosaurids are much longer and more gracile than those of other theropods of the same femur length, again with the exception of ornithomimosaurs and Elaphrosaurus [Holtz 1995b; Paul 2000]).

The femur is a robust element, straight in cranial view ( fig. 5.19A, B). As in most theropods, it is curved, so that it is concave caudally; this curvature is greater in even the largest tyrannosaurids than in carnosaurs. The well-defined, hemispherical head projects medially and dorsally. The greater trochanter is cleft from and lower than the femoral head. The alariform cranial trochanter is as high as or higher than the greater trochanter and can equal or exceed the height of the dorsalmost point of the femoral head. The fourth trochanter is powerfully developed. In the distal condyles the flexor sulcus is well developed and the extensor groove is deep and prominent. The scar for M. tibialis cranialis is well defined (a triangular shape) just above the tibial condyle on the cranial face of the shaft.

The tibia, although powerfully built, is never as robust and stocky as in carnosaurs and other basal tetanurans ( fig. 5.19C, D). The cnemial process is well developed and curves out of the lateral surface of the tibia. The incisura tibialis (excavation on the cranial portion) occupies more than two-thirds of the lateral surface of the proximal tibia. The proximolateral condyle is pinched cranially and caudally, forming a waist between the condyle and the main body of the tibia. The crista fibularis is well developed and proximally placed. On the craniodistal surface there is a prominent facet for the large ascending process of the astragalus. The fibula is slender. The proximal end is expanded to 75% or more of the craniocaudal length of the tibia. The medial surface of the proximal half of the fibula is deeply excavated. There is a distinct cranial tubercle distal to the cranial expansion, formed by two parallel longitudinal ridges, the lateral one being larger. The distal end is tapered but contacts the tarsus.

The astragalus of tyrannosauroids bears a tall, triangular ascending process that extends for one-quarter to one-third of the vertical distance of the femur. Some individuals bear a deep fossa at the base of the ascending fossa (Mader and Bradley 1989). The calcaneum is small but supports the distal end of the fibula and contributes to the lateral condyle.Two distal tarsals, disklike elements nearly equal in size, are present.

The metatarsus of tyrannosauroids is long and slender ( figs. 5.20, 5.21). Metatarsals II and IV are subcylindrical and nearly equal in length, although IV is longer; metatarsal III is longer than the others. Metatarsal III is pinched proximally to form a slender, solid bony splint and is wedge-shaped distally. Buttressing surfaces from metatarsals II and IV contact the sides of metatarsal III. Metatarsal III has not been recovered for nontyrannosaurid tyrannosauroids, so it is not certain whether it was fully pinched in these taxa. Metatarsal III contributes only slightly to the proximal surface of the metatarsus, forming a crescentic shape (never an oval, contra Holtz 1995b) limited to the caudal half of that surface. This compact, slender metatarsal structure has been termed the arctometatarsus (Holtz 1994, 1995b). The arctometatarsi of tyrannosaurids are never proximally co-ossified. There is a contact on the dorsal part of the proximal ends of metatarsals II and IV, and metatarsals II and IV contact at midshaft on the plantar surface. Juvenile or small adult tyrannosaurid metatarsi might be confused with those of large ornithomimids (Currie 2000), but in the latter metatarsal III has an oval rather than a crescentic proximal surface. The distal articular surfaces of metatarsals II and IV are elongate and condylar, while that of metatarsal III has a nearly rectangular ginglymoid form. Metatarsal II can be distinguished from IV in that the distal articulation of the former is nearly square in outline, while that of the latter is narrower and more triangular; in dorsal view metatarsal II generally has a greater area and is wider mediolaterally than metatarsal IV.Metatarsal I is greatly reduced in size and positioned slightly distal to midshaft and slightly shifted plantarly on metatarsal II (but not fully retroverted as in avialans). Metatarsal V has a cranially facing, rugose tuberosity (Brochu 2002).

The arctometatarsalian condition is as well developed in Tyrannosaurus rex as in other tyrannosaurids: metatarsal III is reduced and compressed proximally, and the metatarsus is long, slender, and compact (Holtz 1995b; Brochu 2002). However, outdated restorations of incorrectly mounted hindlimbs of T. rex (with a thicker metatarsal III visible for its entire length throughout the metatarsus and subequal in length to the other weight-bearing metatarsals) continue to be published in modern vertebrate paleontological literature (e.g., Carroll 1988, 1997; Benton 1997).

The pedal phalangeal formula is 2-3-4-5-0. Digit III is the longest, and digits II and IV are subequal in length. Digit I is reduced but does bear an ungual. The subcylindrical phalanges are wider than tall. The pedal unguals are stouter and less laterally compressed than the manual unguals and considerably larger. The grooves for the claw sheaths are well developed. The unguals are blunter and less tapered than those of carnosaurs such as Sinraptor and Allosaurus .

Systematics and Evolution

Tyrannosaurids were considered members of Carnosauria for many decades (Huene 1932; Colbert 1955; Romer 1956; Walker 1964; Steel 1970; Gauthier 1986; Bonaparte et al. 1990; Molnar et al. 1990). However, as early as the 1920s Huene (1923, 1926c) and Matthew and Brown (1922) suggested that these giant theropods shared a closer phylogenetic position with small coelurosaurian dinosaurs, in particular Ornitholestes and Ornithomimidae , than with other large-bodied theropods, such as Allosaurus and Megalosaurus . This hypothesis has since been supported in subsequent numerical phylogenetic analyses (Novas 1992b; Pérez-Moreno et al. 1993, 1994; Holtz 1994, 1998a, 2001a; Sereno et al. 1994, 1996; Forster et al. 1998; Makovicky and Sues 1998; Sereno 1999a; Currie and Carpenter 2000; Norell et al. 2000; Rauhut 2003). Subsequently, Tyrannosauridae and Tyrannosauroidea are now included within the taxon Coelurosauria rather than in Carnosauria.