Tyrannosauridae

Phylogeny based on Holtz (in press a, b). Please refer to the page on Tyrannosaurid Systematics for synapomorphies supporting this tree. See Taxonomy for definitions of Aublysodontinae and Tyrannosaurinae.

Introduction

Tyrannosauridae is one of the best-represented clades of large bodied theropods in the fossil record. Five species in three to five genera are known from multiple relatively complete skeletons representing different ontogenetic stages: all five species include individuals with femoral lengths of over 1000 mm. Several other tyrannosaurid taxa are known from more incomplete remains.

Tyrannosaurid tooth remains were reported by Leidy in 1856; and partial skeletal material was described in the late 19th Century by Cope (1892) and Marsh (1890, 1892). Tyrant dinosaur anatomy has been described in more detail by Lambe (1904, 1917), Osborn (1905, 1906, 1912, 1916), Parks (1933), Russell (1970), Maleev (1974), Molnar et al. (1990), Molnar (1991), and Carr (1999).

Tyrant dinosaur paleobiology has been the subject of numerous technical and popular studies and speculations, due to the fact that they are known from relatively complete fossils of large to gigantic size and highly derived anatomy.

Some authors (Lambe 1917, Colinvaux 1978, Halstead and Halsted 1981, Barsbold 1983, Horner and Lessem 1993, Horner 1994, Horner and Dobb 1997) have suggested that tyrannosaurids in general or Tyrannosaurus rex in particular were obligate scavengers, incapable of taking down live prey. However, Farlow (1993) and Holtz (in press c) have found these theoretical, ecological and ecomorphological arguments wanting at present. Instead, as is the case with most large-bodied terrestrial carnivores, tyrant dinosaurs probably used both scavenging and predation in order to obtain their food. Carpenter (2000) has documented an adult specimen of the hadrosaurid Edmontosaurus annectens with a bite mark attributable by morphology and stratigraphy to Tyrannosaurus rex: because this injury shows subsequent regrowth of bone, this wound was inflicted on a living animal rather than a carcass.

Tyrannosaurid dentition is markedly heterodont, perhaps indicating a partition of different sectors of the jaw for different function: scraping for the incisiform premaxillary teeth versus tearing and crushing for the lateral teeth, for example. Despite statements to the contrary (Feduccia 1999), the dentition of tyrannosaurids is not similar to those of other theropods. Instead, the incrassate (labiolingually expanded) lateral teeth of tyrannosaurines have greater mediolateral and craniocaudal bending strength than those of other theropods (Farlow et al. 1991, Holtz in press c). Erickson and Olson (1996), Erickson et al. (1996), Molnar (2000), Hurum and Currie (2000), and Holtz (in press c) have argued on tooth mark and biomechanical evidence that tyrannosaurids were capable of producing a greater crushing bite than other large theropods, such as allosauroids and abelisaurids. A very large coprolite from the Scollard Formation, probably referable to Tyrannosaurus rex, is nearly half macerated ornithischian bone by volume (Chin et al. 1998), consistent with the crushing bite model.

Tyrannosaurid forelimbs are extremely reduced, both in proportion to the body and in terms of digit development. In contrast, the hindlimbs of tyrant dinosaurs (and in particular the tibiae and metatarsi) are elongate compared to those of most other theropods of the same body mass. Indeed, the hindlimb proportions of tyrannosaurids, ornithomimosaurs, and the gracile ceratosaur Elaphrosaurus form a continuous allometric trend, suggesting that the tyrant dinosaurs had a greater cursorial ability than other large-bodied theropods (such as allosauroids and spinosauroids) (Holtz 1995, Carrano 1998, Paul 2000). However, Farlow et al. (1995) have calculated that at extremely high speeds an adult Tyrannosaurus rex would be subjected to fatal damage if it fell, suggesting that very large individual tyrannosaurids may not have been as cursorial as younger or smaller specimens.

Currie (2000) reports the discovery of at least nine individuals of Albertosaurus sarcophagus from the same quarry; these were of different ontogenetic stages, suggesting that at least some tyrannosaurid taxa may have been gregarious.

Characteristics

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Nearly complete skeleton of a subadult Gorgosaurus libratus, from the Royal Tyrrell Museum of Palaeontology. Photograph � 1998, Thomas R. Holtz, Jr.

Tyrannosaurid anatomy is highly apomorphic (Holtz in press a, b). The following features are present in tyrannosaurids basally, and in some but not all of their potential outgroups. As such, depending on the phylogenetic position of the tyrant dinosaurs, these may be diagnostic features of Tyrannosauridae or synapomorphies with that taxon's closest relatives (Holtz 1994, 2000, in press a, b, Sereno 1999):

• Incisiform premaxillary teeth
• Nasals narrow caudally behind external nares
• Lacrimal-frontal contact present; prefrontals reduced
• Squamosal recess
• Quadrate height 40-50% of skull height at mid-orbit
• Coracoid dorsoventral length 5 times or more coracoid glenoid diameter
• Radial distal end flattened craniocaudally
• Distal carpals of adults poorly formed and lack trochlear surfaces
• Pollex ungual subequal to ungual of digit II in size
• Preacetabular notch on ilium
• Distal end of iliac pubic peduncle oriented horizontally
• Iliac supracetabular crest transverse width straight
• Dorsal surfaces of iliac blades very close to the midline
• Pronounced semicircular scar on the caudolateral surface of the ischium, just distal to the iliac process
• Ischial foot absent
• Femoral anterior trochanter as tall as greater trochanter
• Anterior and greater trochanter junction at distal end of anterior trochanter
• Tibiae and metatarsi elongate relative to other theropods of same femoral length (Holtz 1995)
• Calcaneum distal articular width 10% of maximum transverse width of astragalus
• Presence of an arctometatarsus (proximal end of metatarsal III reduced to a solid splint) (Holtz 1995)
• Metatarsals II and IV contact at mid-shaft on plantar surface

Additionally, the following features are diagnostic of Tyrannosauridae:

• Ventral ramus of premaxilla taller dorsoventrally than long rostrocaudally
• Nasals fused
• Marked ventral curvature of maxilla
• Squamosal-quadratojugal flange intruding rostrally into infratemporal fenestra, nearly contacting rostral margin
• Well-developed sagittal crest on dorsal surface of parietals
• Supratemporal fossa occupies at least the laterocaudal third of frontal
• Caudal surangular foramen very large
• Retroarticular process of articular absent
• Premaxillary teeth D- or U-shaped in cross-section with median ridge on caudal surface
• Premaxillary teeth smaller than lateral teeth
• Humerus greatly reduced; femur-humerus ratio between 2.8 and 3.5
• Fibular cranial tubercle distal to cranial expansion composed of two longitudinal ridges
• Dorsal surface of metatarsal III crescentic and restricted to caudal portion of the metatarsus dorsal surface

The following features are known in derived tyrannosaurids, are absent in potential outgroups (except some possible non-tyrannosaurid tyrannosauroids), but cannot be evaluated at present in the more fragmentary basal forms. Thus, these may represent synapomorphies of Tyrannosauridae, Tyrannosaurinae, or some subgroup thereof:

• Premaxillary tooth row arcade more mediolaterally than rostrocaudally oriented
• Lateral nuchal crest formed by parietals present, at least twice as tall as foramen magnum vertical height
• Pair of tab-like processes on supraoccipital wedge
• Basisphenoidal sphenoidal sinus deep, with large foramina
• Prominent muscular fossae on dorsal surface of palatine absent
• Rostral portion of fused vomers expanded to greater than twice shaft width
• Supradentary fused to coronoid, restricting lateral motion of intramandibular joint (Hurum and Currie 2000)
• Distal caudal neural spines axially elongate
• Acromial expansion well-developed, more than twice scapula midshaft width
• Scapula contributes more than half of the glenoid
• Scapular-humerus ratio between 2.2 and 2.5
• Metacarpal III very reduced and bears no digit
• Ilium long, slight shorter than femur
• Horizontal medial shelf from preacetabular blade to sacral ribs
• Broad ventral hook-like projection from preacetabular blade of ilium
• Pronounced midline crest on ilium
• Supracetabular crest on ilium reduced
• Pubic boot enlarged, approximately one-half pubis (or femur) length
• Ischium shaft long but more slender and shorter than pubis

Please see the page on Tyrannosaurid Systematics for characters supporting the subclades within the tyrant dinosaurs shown above.

Discussion of Phylogenetic Relationships

At the present time there are no published numerical phylogenetic analyses of tyrannosaurid relationships. The tree above is from work by Holtz in press (a, b). The synapomorphies supporting these relationship are discussed in detail on the Tyrannosaurid Systematics page.

Previous studies of tyrant dinosaur systematics have been concerned more with taxonomic issues of lumping and splitting rather than recovery of the phylogenetic relationships within the clade (e.g, Russell 1970, Maleev 1974, Paul 1988, Carpenter 1992). Russell (1970) presented one of the most extensive reviews of the Tyrannosauridae. His arrangement was as follows:

• Albertosaurus
  • A. libratus
  • A. sarcophagus
  • A. lancensis
• Daspletosaurus torosus
• Tarbosaurs bataar
• Tyrannosaurus rex

This study was the first to recognize Daspletosaurus and was influential in including the late Campanian Age Gorgosaurus libratus within the genus Albertosaurus. This practice was followed by various other authors (e.g., Paul 1988, Carpenter 1992, Holtz 1994, 1995, Carr 1999), although the justification for the union of Albertosaurus sarcophagus and A. libratus was the shared absence of the derived features found in Daspletosaurus, Tarbosaurus, and Tyrannosaurus. Holtz (in press a, b) found that a few derived cranial characters do support union of Albertosaurus and Gorgosaurus outside of all other tyrannosaurines, but that equally parsimonious trees unite Gorgosaurus and the more robust tyrannosaurines to the exclusion of Albertosaurus: see Tyrannosaurid Systematics.

Gorgosaurus lancensis, known only from a single skull from the Hell Creek Formation of Montana, was also provisionally moved into Albertosaurus by Russell (1970), Paul (1988), and Carpenter (1992). Bakker et al. (1988) referred this specimen to its own genus, Nanotyrannus. Carr (1999), however, demonstrated that this individual is most likely a juvenile specimen of Tyrannosaurus rex.

Carpenter's (1992) arrangement was largely similar to that of Russell, but included Tarbosaurus bataar in Tyrannosaurus and introduced the new taxon Maleevosaurus novojilovi (previously Gorgosaurus novojilovi) for a smaller specimen from Mongolia. The latter is regarded a subadult specimen of Tarbosaurus bataar by Carr (1999) and Holtz (in press a, b).

Paul (1988) differed with Russell (1970) and Carpenter (1992) in expanding Tyrannosauridae to include the more plesiomorphic aublysodontines and (provisionally) Indosuchus raptorius, a form now considered a member of the ceratosaurian clade Abelisauridae. Paul included Shanshanosaurus houyanshanensis as a species of Aublysodon, Daspletosaurus as a subgenus of Tyrannosaurus, and provisionally included Alectrosaurus olseni in Albertosaurus. Additionally, Paul recognized a distinct species Albertosaurus arctunguis for specimens regarded by other authors (Russell 1970, Carpenter 1992, Holtz in press a) as belonging to A. sarcophagus, and a new species Albertosaurus megagracilis for a specimen considered by Carr (1999) and Holtz (in press a, b) to be a subadult Tyrannosaurus rex. Paul's arrangement is as follows:

• Aublysodontinae
  • Aublysodon
    • A. mirandus
    • A. molnari
    • A. houyanshanensis
• Tyrannosaurinae
  • ?Indosuchus raptorius
  • Alioramus remotus
  • Albertosaurus
    • ?A. olseni
    • A. libratus
    • A. sarcophagus
    • A. arctunguis
    • A. megagracilis
    • ?A. lancensis
  • Tyrannosaurus
    • T. (Daspletosaurus) torosus
    • T. (Tyrannosaurus)
      • T. (T.) bataar
      • T (T.) rex

Horner et al. (1992) briefly mention a new taxon from the upper Two Medicine Formation of Montana. Based on a the distribution of a few derived features, they proposed the following topology:

     ====== Daspletosaurus torosus 
 
=====|
     |  === Two Medicine tyrannosaurine 
 
     ===|
        === Tyrannosaurus rex 
 

However, those authors did not include the Asian taxon Tarbosaurus bataar in their (admittedly preliminary) study; Holtz (in press a, b) found that when that Mongolian species is included, the relationship of the unnamed Two Medicine form becomes unresolved with regards to Daspletosaurus and the Tarbosaurus-Tyrannosaurus clade.

Aublysodontinae is included within Tyrannosauridae by some authors (Paul 1988, Currie et al. 1990, Holtz 1994, in press a, b, Kirkland et al. 1997, Cifelli et al. 1997, 1999), while others use a more restricted Tyrannosauridae and use the term "Aublysodontidae" for Aublysodon and its allies (Molnar and Carpenter 1989, Carpenter 1992): see Taxonomy for the taxonomic definitions used here.

Taxonomy

For the purposes of discussion here, the following provisional taxonomic definitions will be used:

• Tyrannosauridae, all descendants of the most recent common ancestor of Tyrannosaurus and Aublysodon
• Tyrannosaurinae, Tyrannosaurus and all taxa closer to Tyrannosaurus than to Aublysodon
• Aublysodontinae, Aublysodon and all taxa closer to Aublysodon than to Tyrannosaurus
• Tyrannosauroidea, all taxa closer to Tyrannosaurus than to Ornithomimus or to Deinonychus

However, these definitions must be provisional, as the type species of Aublysodon (A. mirandus) is known only from isolated premaxillary teeth, while A. molnari may represent a different genus. Material currently referred to Alectrosaurus olseni is under study; this material may eventually provide a much more useful anchor taxon for Tyrannosauridae.

(Note: the taxonomic definitions provided by Sereno (1998) are problematic, as in that taxonomy Tyrannosauridae represents Tyrannosaurus and all taxa closer to it than to Alectrosaurus, Aublysodon, and Nanotyrannus. The latter "taxon", however, appears to be a juvenile specimen of T. rex (Carr 1999).)

See Holtz (in press a, b) for further discussion of these taxonomic definitions.

Stratigraphic and Geographic Distribution

Several partially known theropod taxa have been considered closer to tyrannosaurids than to all other known clades of dinosaur: see Tyrannosauroidea. Zinke (1998) described premaxillary teeth similar in form to those of tyrant dinosaurs from the Late Jurassic (Kimmeridgian Age) of Portugal. These specimens differ in form from those of known tyrannosaurids in being squatter mediolaterally compared to crown height. The Portuguese teeth are considerably older than other tyrannosaurid taxa, and might be from non-tyrannosaurid tyrannosauroids, basal ornithomimosaurs, or some other as-yet unknown taxon.

Manabe (1999) has described a premaxillary tooth from the Jobu Formation (Early Cretaceous, ?Valanginian-?Barremian Age) of Japan, identical in form to those of typical tyrannosaurids. Teeth similar in form to those of Alectrosaurus are reported from the latest Albian-earliest Cenomanian Mussentuchit Member of the Cedar Mountain Formation of Utah (Kirkland et al. 1997; Cifelli et al. 1997, 1999). These teeth are thus older than all skeletal material unquestionably assignable to Tyrannosauridae. The latter are known at present from the Late Cretaceous Epoch. Alectrosaurus olseni represents the oldest named tyrannosaurid: depending on the age of the Iren Dabasu and Bayn Shire Formations, it may be as old as the Cenomanian to as young as the early Campanian Age (Currie and Eberth 1993). The remaining tyrannosaurid taxa are of late Campanian or Maastrichtian Age. Other than the Portuguese teeth, tyrannosaurid fossils are restricted to central and eastern Asia and North America.

Other Names for Tyrannosauridae

• tyrannosaurs
• Tyrant dinosaurs

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Information on the Internet

• Tyrannosaurus rex Exposition. UCMP Berkeley.
• Tyrannosaurus Was Not A Fast Runner. John R. Hutchinson and Mariano Garcia.
• Sue at the Field Museum. Information about Sue, the largest, most complete, best preserved Tyrannosaurus rex found to date.
• Tyrannosaurus Was Not A Fast Runner. John R. Hutchinson and Mariano Garcia.