Tyrannosauroidea
Creators: name: Holtz, TR jr.
Publisher: name: Zenodo
Year: 2004
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Dinosauria

description: 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 <i>sensu stricto</i> but closer to this clade than to all other theropod groups. These include the newly described Early Cretaceous <i>Eotyrannus lengi</i> (Hutt et al. 2001; Naish et al. 2001), the Late Jurassic <i>Stokesosaurus</i> (Madsen 1974; Foster and Chure 2000; Rauhut 2000a), and the Late Cretaceous <i>Dryptosaurus aquilunguis</i> (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 <i>(Tyrannosaurus rex, Tarbosaurus bataar, Daspletosaurus torosus, Albertosaurus sarcophagus,</i> and <i>Gorgosaurus libratus)</i> of various ontogenetic stages. Among multi-tonne theropods only <i>Allosaurus fragilis</i> 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 <i>Stokesosaurus</i> 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 <i>Eshanosaurus</i> 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 <i>Tyrannosaurus rex</i> (and <i>Nanotyrannus lancensis,</i> 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 <i>(Tyrannosaurus rex, Tarbosaurus bataar, Daspletosaurus torosus, Albertosaurus sarcophagus,</i> and <i>Gorgosaurus libratus)</i> are all represented by individuals with femora more than 100 cm in length, reaching 138 cm in the largest <i>Tyrannosaurus rex</i> (Brochu 2002). The basal tyrannosauroid <i>Dryptosaurus</i> 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 <i>Tyrannosaurus, Tarbosaurus,</i> and <i>Daspletosaurus torosus;</i> the premaxilla-occipital condyle length of the largest <i>T. rex</i> skull (153 cm) exceeds the same measurement of the skull of <i>Giganotosaurus</i> (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 <i>Occurrence</i> <i>Age</i> <i>Material</i> <i>Occurrence</i> <i>Age</i> <i>Material</i> <i>(continued)</i> <i>Occurrence</i> <i>Age</i> <i>Material</i> <i>Nomina dubia</i> <i>Material</i> <i>Aublysodon amplus</i> Marsh, 1892 Tooth <i>Aublysodon cristatus</i> Marsh, 1892 Tooth <i>Aublysodon lateralis</i> Cope, 1876a Teeth <i>Aublysodon mirandus</i> Leidy, 1868 Premaxillary teeth <i>Deinodon horridus</i> Leidy, 1856 Teeth <i>Laelaps macropus</i> Cope, 1868 Fragmentary hindlimb elements <i>Tomodon horrificus</i> Leidy, 1865 Tooth (type of <i>Diplotomodon</i> Leidy, 1868a) <i>Tyrannosaurus lanpingensis</i> Yeh, 1975 Teeth <i>Tyrannosaurus turpanensis</i> 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 <i>Tyrannosaurus rex</i> and all taxa sharing a more recent common ancestor with <i>T. rex</i> than with <i>Ornithomimus velox, Deinonychus antirrhopus,</i> or <i>Allosaurus fragilis.</i> Tyrannosauroidea, or at least a phylogenetically secure subclade therein comprising <i>Stokesosaurus, Dryptosaurus, Eotyrannus,</i> 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 <i>Elaphrosaurus,</i> 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 <i>Tyrannosaurus</i> (Osborn 1912a; Gilmore 1946a; Bakker et al. 1988; Molnar 1991; Carr 1999; Brochu 2002; Carr and Williamson, in press), <i>Tarbosaurus</i> (Maleev 1974; Hurum and Sabath 2003), <i>Daspletosaurus</i> (Russell 1970a; Currie 2003b), <i>Gorgosaurus libratus</i> (Lambe 1917a; Carr 1999; Currie 2003b), and <i>Albertosaurus</i> (figs. 5.1–5.3; Lambe 1904a; Currie 2003b). Partial skulls are known for <i>Alioramus remotus</i> (Kurzanov 1976a; Currie 2003b) and the unnamed Alabama tyrannosaurid (Carr et al., in press). Cranial material is presently known for <i>Alectrosaurus olseni</i> (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 <i>Eotyrannus</i> (Hutt et al. 2001; Naish et al. 2001), <i>Dryptosaurus</i> (Cope 1866; Carpenter et al. 1997), and <i>Bagaraatan</i> (Osmólska 1996); cranial material referred to <i>Stokesosaurus</i> (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, <i>Tyrannosaurus rex</i> 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 <i>T. rex</i> 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 <i>Allosaurus,</i> for example, in which the comparable figure is only one-third of the skull base length, or the tyrannosaurid <i>Daspletosaurus,</i> 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 <i>Gorgosaurus libratus</i> [Carr 1999; Carr and Williamson, in press]). The eyes of most tyrannosaurids are laterally directed, but those of <i>Tyrannosaurus rex</i> 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 <i>W.</i> 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 <i>Allosaurus, Monolophosaurus,</i> and <i>Ornitholestes,</i> 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 <i>Eotyrannus</i> 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 <i>Tarbosaurus</i> and <i>Tyrannosaurus.</i> 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 <i>Allosaurus</i> [Witmer 1997a]). The number of maxillary teeth in tyrannosaurids varies: a maximum of 18 are reported in skulls referred to <i>Alectrosaurus olseni</i> and a minimum of 11 in some <i>Tyrannosaurus rex.</i> The number decreases with skull size in <i>Gorgosaurus libratus</i> and <i>T. rex</i> (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 <i>Tyrannosaurus,</i> 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 <i>Eotyrannus</i> 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 <i>Alioramus</i> 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 <i>L;</i> 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 <i>Tarbosaurus</i> and especially <i>Tyrannosaurus</i> this portion of the bone can have an inflated appearance. The dorsal surface bears a triangular cornual process in albertosaurines and <i>Daspletosaurus;</i> this prominence is lost in the <i>Tyrannosaurus-Tarbosaurus</i> clade. The postorbital is a triradiate bone with longer descending and caudal rami and a shorter medial process. In <i>Tyrannosaurus, Tarbosaurus,</i> and <i>Albertosaurus,</i> and to a much lesser degree in <i>Daspletosaurus,</i> 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 <i>Giganotosaurus</i> and <i>Carcharodontosaurus,</i> some <i>Allosaurus,</i> and some abelisaurids (Chure 1998c). In the case of <i>Gorgosaurus</i> 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 <i>Tarbosaurus</i> and <i>Tyrannosaurus.</i> The enlargement of the dorsal surface of the lacrimal excludes the prefrontals and frontals from view on the lateral aspect; in some <i>Tyrannosaurus</i> 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 <i>Tyrannosaurus rex</i> (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 prima
description: Published as part of <i>Holtz, TR jr., 2004, Tyrannosauroidea, pp. 111-136 in The Dinosauria, University of California Press</i> on pages 1-7, DOI: 10.5281/zenodo.3374526