Tyrannosauroidea

Tyrannosauroidea (meaning 'tyrant lizard forms') is a superfamily (or clade) of coelurosaurian theropod dinosaurs that includes the family Tyrannosauridae as well as more basal relatives. Tyrannosauroids lived on the Laurasian supercontinent beginning in the Jurassic Period. By the end of the Cretaceous Period, tyrannosauroids were the dominant large predators in the Northern Hemisphere, culminating in the gigantic Tyrannosaurus itself. Fossils of tyrannosauroids have been recovered on what are now the continents of North America, Europe, Asia, and possibly Australia.

Tyrannosauroids were bipedal carnivores, as were most theropods, and were characterized by numerous skeletal features, especially of the skull and pelvis. Early in their existence, tyrannosauroids were small predators with long, three-fingered forelimbs. Late Cretaceous genera became much larger, including some of the largest land-based predators ever to exist, but most of these later genera had proportionately small forelimbs with only two digits. Primitive feathers have been found on Dilong, an early tyrannosauroid from China, and may have been present in other tyrannosauroid genera as well. Prominent bony crests in a variety of shapes and sizes on the skulls of many tyrannosauroids may have served display functions. {| class="toc" id="toc"

Contents
[hide]*1 Description
 * 2 Taxonomy and systematics
 * 2.1 Classification
 * 2.2 Phylogeny
 * 3 Distribution
 * 4 Paleobiology
 * 4.1 Feathers
 * 4.2 Head crests
 * 5 References
 * 6 External links
 * }

[edit] Description
Tyrannosauroids varied widely in size, although there was a general trend towards increasing size over time. Early tyrannosauroids were small animals.[1] One specimen of Dilong, almost fully grown, measured 1.6 meters (5.3 ft) in length,[2] and a full-grown Guanlong measured 3 meters (10 ft long).[3] Teeth from lower Lower Cretaceous rocks (140 to 136 million years old) of Hyogo, Japan, appear to have come from an approximately 5 metres (16 ft) long animal, possibly indicating an early size increase in the lineage.[4] An immature Eotyrannus was over 4 meters (13 ft) in length,[5] and a subadult Appalachiosaurus was estimated at more than 6 meters (20 ft) long,[1] indicating that both genera reached larger sizes. The Late Cretaceous tyrannosaurids ranged from the 9 meter (30 ft) Albertosaurus and Gorgosaurus to Tyrannosaurus, which exceeded 12 meters (40 ft) in length and may have weighed more than 6400 kilograms (7 short tons).[1]

Skulls of early tyrannosauroids were long, low and lightly constructed, similar to other coelurosaurs, while later forms had taller and more massive skulls. Despite the differences in form, certain skull features are found in all known tyrannosauroids. The premaxillary bone is very tall, blunting the front of the snout, a feature which evolved convergently in abelisaurids. The nasal bones are characteristically fused together, arched slightly upwards and often very roughly textured on their upper surface. The premaxillary teeth at the front of the upper jaw are shaped differently than the rest of the teeth, smaller in size and with a D-shaped cross section. In the lower jaw, a prominent ridge on the surangular bone extends sideways from just below the jaw joint, except in the basal Guanlong.[1] [2] [3]

Tyrannosauroids had S-shaped necks and long tails, as did most other theropods. Early genera had long forelimbs, about 60% the length of the hindlimb in Guanlong, with the typical three digits of coelurosaurs.[3] The long forelimb persisted at least through the Early Cretaceous Eotyrannus,[5] but is unknown in Appalachiosaurus.[6] Derived tyrannosaurids have forelimbs strongly reduced in size, the most extreme example being Tarbosaurus from Mongolia, where the humerus was only one-quarter the length of the femur.[1] The third digit of the forelimb was also reduced over time. This digit was unreduced in the basal Guanlong,[3] but in Dilong it was significantly more slender than the other two digits.[2] Eotyrannus still had three functional digits on each hand,<sup class="reference" id="cite_ref-huttetal2001_4-2">[5] but tyrannosaurids had only two, although the vestigial remnants of the third are found on some specimens.<sup class="reference" id="cite_ref-quinlanetal2007_6-0">[7] As in most coelurosaurs, the second digit of the hand is the largest, even when the third digit is not present.

Characteristic features of the tyrannosauroid pelvis include a concave notch at the upper front end of the ilium, a sharply defined vertical ridge on the outside surface of the ilium, extending upwards from the acetabulum (hip socket), and a huge "boot" on the end of the pubis, more than half as long as the shaft of the pubis itself.<sup class="reference" id="cite_ref-holtz2004_0-5">[1] These features are found in all known tyrannosauroids, including basal members Guanlong<sup class="reference" id="cite_ref-xuetal2006_2-4">[3] and Dilong.<sup class="reference" id="cite_ref-xuetal2004_1-3">[2] The pubis is not known in Aviatyrannis or Stokesosaurus but both show typical tyrannosauroid characters in the ilium.<sup class="reference" id="cite_ref-rauhut2003a_7-0">[8] The hindlimbs of all tyrannosauroids, like most theropods, had four toes, although the first toe (the hallux) did not contact the ground. Tyrannosauroid hindlimbs are longer relative to body size than almost any other theropods, and show proportions characteristic of fast-running animals, including elongated tibiae and metatarsals.<sup class="reference" id="cite_ref-holtz2004_0-6">[1] These proportions persist even in the largest adult Tyrannosaurus,<sup class="reference" id="cite_ref-brochu2003_8-0">[9] despite its probable inability to run.<sup class="reference" id="cite_ref-hutchinsongarcia2002_9-0">[10] The third metatarsal of tyrannosaurids was pinched at the top between the second and fourth, forming a structure known as the arctometatarsus.<sup class="reference" id="cite_ref-holtz2004_0-7">[1] The arctometatarsus was also present in Appalachiosaurus<sup class="reference" id="cite_ref-carretal2005_5-1">[6] but it is unclear whether it was found in Eotyrannus<sup class="reference" id="cite_ref-huttetal2001_4-3">[5] or Dryptosaurus.<sup class="reference" id="cite_ref-carpenteretal1997_10-0">[11] This structure was shared by derived ornithomimids, troodontids and caenagnathids,<sup class="reference" id="cite_ref-holtz1994_11-0">[12] but was not present in basal tyrannosauroids like Dilong, indicating convergent evolution.<sup class="reference" id="cite_ref-xuetal2004_1-4">[2]

[edit] Taxonomy and systematics
Tyrannosaurus was named by Henry Fairfield Osborn in 1905, along with the family Tyrannosauridae.<sup class="reference" id="cite_ref-osborn1905_12-0">[13] The name is derived from the Ancient Greek words τυραννος/tyrannos ('tyrant') and σαυρος/sauros ('lizard'). The superfamily name Tyrannosauroidea was first published in a 1964 paper by British paleontologist Alick Walker.<sup class="reference" id="cite_ref-walker1964_13-0">[14] The suffix -oidea, commonly used in the name of animal superfamilies, is derived from the Greek ειδος/eidos ('form').<sup class="reference" id="cite_ref-liddellscott_14-0">[15]

Scientists have commonly understood Tyrannosauroidea to include the tyrannosaurids and their immediate ancestors.<sup class="reference" id="cite_ref-walker1964_13-1">[14] <sup class="reference" id="cite_ref-bonaparteetal1990_15-0">[16] With the advent of phylogenetic taxonomy in vertebrate paleontology, however, the clade has received several more explicit definitions. The first was by Paul Sereno in 1998, where Tyrannosauroidea was defined as a stem-based taxon including all species sharing a more recent common ancestor with Tyrannosaurus rex than with neornithean birds.<sup class="reference" id="cite_ref-sereno1998_16-0">[17] To make the family more exclusive, Thomas Holtz redefined it in 2004 to include all species more closely related to Tyrannosaurus rex than to Ornithomimus velox, Deinonychus antirrhopus or Allosaurus fragilis.<sup class="reference" id="cite_ref-holtz2004_0-8">[1] Sereno published a new definition in 2005, using Ornithomimus edmontonicus, Velociraptor mongoliensis and Troodon formosus as external specifiers.<sup class="reference" id="cite_ref-sereno2005_17-0">[18]

[edit] Classification
SUPERFAMILY TYRANNOSAUROIDEA Timurlengia
 * ?Bagaraatan (Late Cretaceous, Mongolia)
 * ?Iliosuchus (Middle Jurassic, England)
 * ?Labocania (Late Cretaceous, western Mexico)
 * Alectrosaurus (Late Cretaceous, Mongolia)
 * Appalachiosaurus (Late Cretaceous, eastern North America)
 * Aviatyrannis (Late Jurassic, Portugal)
 * Bistahieversor (Late Cretaceous, New Mexico, United States)
 * Dilong (Early Cretaceous, eastern China)
 * Eotyrannus (Early Cretaceous, England)
 * Raptorex (Early Cretaceous, eastern China)
 * Stokesosaurus (Late Jurassic, western North America)
 * Xiongguanlong (Early Cretaceous, central China)
 * ?Family Coeluridae<sup class="reference" id="cite_ref-PS07_18-0">[19] 
 * Coelurus (Late Jurassic, western North America)
 * Tanycolagreus (Late Jurassic, western North America)
 * Family Dryptosauridae<sup class="reference" id="cite_ref-19">[20] 
 * Dryptosaurus (Late Cretaceous, eastern North America)
 * Family Proceratosauridae<sup class="reference" id="cite_ref-ORetal2009_20-0">[21]
 * Guanlong (Late Jurassic, western China)
 * Kileskus (Middle Jurassic, central Russia)<sup class="reference" id="cite_ref-AKI10_21-0">[22]
 * Proceratosaurus (Middle Jurassic, England)
 * ?Sinotyrannus (Early Cretaceous, eastern China)
 * Family Tyrannosauridae
 * Albertosaurus (Late Cretaceous, western North America)
 * Alioramus (Late Cretaceous, Mongolia)
 * Aublysodon (Late Cretaceous, western North America)
 * Daspletosaurus (Late Cretaceous, western North America)
 * Gorgosaurus (Late Cretaceous, western North America)
 * Nanotyrannus (Late Cretaceous, western North America)
 * Tarbosaurus (Late Cretaceous, Mongolia)
 * Tyrannosaurus (Late Cretaceous, western North America)

[edit] Phylogeny
While paleontologists have long recognized the family Tyrannosauridae, its ancestry has been the subject of much debate. For most of the twentieth century, tyrannosaurids were commonly accepted as members of the Carnosauria, which included almost all large theropods.<sup class="reference" id="cite_ref-romer1956_22-0">[23] <sup class="reference" id="cite_ref-gauthier1986_23-0">[24] Within this group, the allosaurids were often considered to be ancestral to tyrannosaurids.<sup class="reference" id="cite_ref-bonaparteetal1990_15-1">[16] <sup class="reference" id="cite_ref-molnaretal990_24-0">[25] In the early 1990s, cladistic analyses instead began to place tyrannosaurids into the Coelurosauria,<sup class="reference" id="cite_ref-novas1992_25-0">[26] <sup class="reference" id="cite_ref-holtz1994_11-1">[12] echoing suggestions first published in the 1920s.<sup class="reference" id="cite_ref-matthewbrown1922_26-0">[27] <sup class="reference" id="cite_ref-vonhuene1923_27-0">[28] Tyrannosaurids are now universally considered to be large coelurosaurs.<sup class="reference" id="cite_ref-holtz2004_0-9">[1] <sup class="reference" id="cite_ref-xuetal2006_2-5">[3] <sup class="reference" id="cite_ref-sereno1999_28-0">[29] <sup class="reference" id="cite_ref-rauhut2003b_29-0">[30] <sup class="reference" id="cite_ref-currieetal2003_30-0">[31] <sup class="reference" id="cite_ref-norelletal2006_31-0">[32] <sup class="reference" id="cite_ref-senter2007_32-0">[33]

In 1994, Holtz grouped tyrannosauroids with elmisaurids, ornithomimosaurs and troodonts into a coelurosaurian clade called Arctometatarsalia based on a common ankle structure where the second and fourth metatarsals meet near the tarsal bones, covering the third metatarsal when viewed from the front.<sup class="reference" id="cite_ref-holtz1994_11-2">[12] Basal tyrannosauroids like Dilong, however, were found with non-arctometatarsalian ankles, indicating that this feature evolved convergently.<sup class="reference" id="cite_ref-xuetal2004_1-5">[2] Arctometatarsalia has been dismantled and is no longer used by most paleontologists, with tyrannosauroids usually considered to be basal coelurosaurs outside Maniraptoriformes.<sup class="reference" id="cite_ref-holtz2004_0-11">[1] <sup class="reference" id="cite_ref-rauhut2003b_29-1">[30] <sup class="reference" id="cite_ref-norelletal2006_31-1">[32] One recent analysis found the family Coeluridae, including the Late Jurassic North American genera Coelurus and Tanycolagreus, to be the sister group of Tyrannosauroidea.<sup class="reference" id="cite_ref-senter2007_32-1">[33]

The most basal tyrannosauroid known from complete skeletal remains is Guanlong.<sup class="reference" id="cite_ref-xuetal2006_2-7">[3] Other early taxa include Stokesosaurus and Aviatyrannis, known from far less complete material.<sup class="reference" id="cite_ref-rauhut2003a_7-1">[8] The better-known Dilong is considered slightly more derived than Guanlong and Stokesosaurus.<sup class="reference" id="cite_ref-xuetal2004_1-6">[2] <sup class="reference" id="cite_ref-xuetal2006_2-8">[3] Dryptosaurus, long a difficult genus to classify, has turned up in several recent analyses as a basal tyrannosauroid as well, slightly more distantly related to Tyrannosauridae than Eotyrannus and Appalachiosaurus.<sup class="reference" id="cite_ref-holtz2004_0-12">[1] <sup class="reference" id="cite_ref-carretal2005_5-3">[6] <sup class="reference" id="cite_ref-holtzdml2005_33-0">[34] Alectrosaurus, a poorly known genus from Mongolia, is definitely a tyrannosauroid but its exact relationships are unclear.<sup class="reference" id="cite_ref-holtz2004_0-13">[1] Other taxa have been considered possible tyrannosauroids by various authors, including Bagaraatan, Labocania and a species erroneously referred to Chilantaisaurus, "C." maortuensis.<sup class="reference" id="cite_ref-holtz2004_0-14">[1] Siamotyrannus from the Early Cretaceous of Thailand was originally described as an early tyrannosaurid,<sup class="reference" id="cite_ref-buffetautetal1996_34-0">[35] but is usually considered a carnosaur today.<sup class="reference" id="cite_ref-rauhut2003b_29-2">[30] <sup class="reference" id="cite_ref-holtzetal2004_35-0">[36] Iliosuchus has a vertical ridge on the ilium reminiscent of tyrannosauroids and may in fact be the earliest known member of the superfamily, but not enough material is known to be sure.<sup class="reference" id="cite_ref-rauhut2003a_7-2">[8] <sup class="reference" id="cite_ref-holtzetal2004_35-1">[36]

[edit] Distribution
The earliest recognized tyrannosauroids lived in the Late Jurassic, including Guanlong from northwestern China,<sup class="reference" id="cite_ref-xuetal2006_2-9">[3] Stokesosaurus from the western United States and Aviatyrannis from Portugal. Some fossils currently referred to Stokesosaurus may instead belong to Aviatyrannis, given the great similarities in the dinosaur faunas of Portugal and North America during this time. If Iliosuchus from the Middle Jurassic of England is in fact a tyrannosauroid, it would be the earliest known genus and might suggest that the superfamily originated in Europe.<sup class="reference" id="cite_ref-rauhut2003a_7-3">[8]

Early Cretaceous tyrannosauroids are also found on all three northern continents. Eotyrannus from England<sup class="reference" id="cite_ref-huttetal2001_4-4">[5] and Dilong from northeastern China<sup class="reference" id="cite_ref-xuetal2004_1-7">[2] are the only two named genera of this age, while Early Cretaceous tyrannosauroid premaxillary teeth are known from the Cedar Mountain Formation in Utah<sup class="reference" id="cite_ref-kirklandetal1997_36-0">[37] and the Tetori Group of Japan.<sup class="reference" id="cite_ref-manabe1999_37-0">[38]

By the middle of the Cretaceous, tyrannosauroid fossils are no longer found in Europe, suggesting a localized extinction on that continent.<sup class="reference" id="cite_ref-holtz2004_0-15">[1] Tyrannosauroid teeth and possible body fossils are known from the North American Dakota Formation,<sup class="reference" id="cite_ref-kirklandetal1997_36-1">[37] as well as formations in Kazakhstan, Tajikstan and Uzbekistan,<sup class="reference" id="cite_ref-nesov1995_38-0">[39] from the middle of the Cretaceous. The first unquestionable remains of tyrannosaurids occur in the Campanian stage of the Late Cretaceous in North America and Asia. Two subfamilies are recognized. The albertosaurines are only known from North America, while the tyrannosaurines are found on both continents.<sup class="reference" id="cite_ref-holtz2004_0-16">[1] Tyrannosaurid fossils have been found in Alaska, which may have served as a land bridge allowing dispersal between the two continents.<sup class="reference" id="cite_ref-fiorillogangloff2000_39-0">[40] Non-tyrannosaurid tyrannosauroids like Alectrosaurus and possibly Bagaraatan were contemporaneous with tyrannosaurids in Asia, while they are absent from western North America.<sup class="reference" id="cite_ref-holtz2004_0-17">[1] Eastern North America was divided by the Western Interior Seaway in the middle of the Cretaceous and isolated from the western portion of the continent. The absence of tyrannosaurids from the eastern part of the continent suggests that the family evolved after the appearance of the seaway, allowing basal tyrannosauroids like Dryptosaurus and Appalachiosaurus to survive in the east as a relict population until the end of the Cretaceous.<sup class="reference" id="cite_ref-carretal2005_5-4">[6]

Basal tyrannosauroids may have also been present in what is now southeastern Australia during the Aptian of the Early Cretaceous. NMV P186069, a partial pubis (a hip bone) with a distinctive tyrannosauroid-like form, was discovered in Dinosaur Cove in Victoria, indicating that tyrannosauroids were not limited to the northern continents as previously thought.<sup class="reference" id="cite_ref-RBetal10_40-0">[41]

[edit] Feathers
Long filamentous structures have been preserved along with skeletal remains of numerous coelurosaurs from the Early Cretaceous Yixian Formation and other nearby geological formations from Liaoning, China.<sup class="reference" id="cite_ref-zhouetal2003_41-0">[42] These filaments have usually been interpreted as "protofeathers," homologous with the branched feathers found in birds and some non-avian theropods,<sup class="reference" id="cite_ref-chenetal1998_42-0">[43] <sup class="reference" id="cite_ref-xuetal2003_43-0">[44] although other hypotheses have been proposed.<sup class="reference" id="cite_ref-linghamsoliaretal2007_44-0">[45] A skeleton of Dilong was described in 2004 that included the first example of "protofeathers" in a tyrannosauroid. Similarly to down feathers of modern birds, the "protofeathers" found in Dilong were branched but not pennaceous, and may have been used for insulation.<sup class="reference" id="cite_ref-xuetal2004_1-8">[2]

The presence of "protofeathers" in basal tyrannosauroids is not surprising, since they are now known to be characteristic of coelurosaurs, found in other basal genera like Sinosauropteryx,<sup class="reference" id="cite_ref-chenetal1998_42-1">[43] as well as all more derived groups.<sup class="reference" id="cite_ref-zhouetal2003_41-1">[42] Rare fossilized skin impressions of large tyrannosaurids lack feathers, however, instead showing skin covered in scales.<sup class="reference" id="cite_ref-martinczerkas2000_45-0">[46] It is possible that "protofeathers" were present on areas of the body not preserved with skin impressions. Alternatively, secondary loss of "protofeathers" in large tyrannosaurids may be analogous with the similar loss of hair in the largest modern mammals like elephants, where a low surface area-to-volume ratio slows down heat transfer, making insulation by a coat of hair unnecessary.<sup class="reference" id="cite_ref-xuetal2004_1-9">[2]

[edit] Head crests
Bony crests are found on the skulls of many theropods, including numerous tyrannosauroids. The most elaborate is found in Guanlong, where the nasal bones support a single, large crest which runs along the midline of the skull from front to back. This crest was penetrated by several large foramina (openings) which reduced its weight.<sup class="reference" id="cite_ref-xuetal2006_2-10">[3] A less prominent crest is found in Dilong, where low, parallel ridges run along each side of the skull, supported by the nasal and lacrimal bones. This ridges curve inwards and meet just behind the nostrils, making the crest Y-shaped.<sup class="reference" id="cite_ref-xuetal2004_1-10">[2] The fused nasals of tyrannosaurid are often very rough-textured. Alioramus, a possible tyrannosaurid from Mongolia, bears a single row of five prominent bony bumps on the nasal bones; a similar row of much lower bumps is present on the skull of Appalachiosaurus, as well as some specimens of Daspletosaurus, Albertosaurus, and Tarbosaurus.<sup class="reference" id="cite_ref-carretal2005_5-5">[6] In Albertosaurus, Gorgosaurus and Daspletosaurus, there is a prominent horn in front of each eye on the lacrimal bone. The lacrimal horn is absent in Tarbosaurus and Tyrannosaurus, which instead have a crescent-shaped crest behind each eye on the postorbital bone.<sup class="reference" id="cite_ref-holtz2004_0-18">[1]

These head crests may have been used for display, perhaps for species recognition or courtship behavior.<sup class="reference" id="cite_ref-holtz2004_0-19">[1] An example of the handicap principle may be the case of Guanlong, where the large, delicate crest may have been a hindrance to hunting in what was presumably an active predator. If an individual was healthy and successful at hunting despite the fragile crest, it would indicate the superior quality of the individual over others with smaller crests. Similarly to the unwieldy tail of a male peacock or the outsized antlers of an Irish elk, the crest of Guanlong may have evolved via sexual selection, providing an advantage in courtship which outweighed any decrease in hunting ability.<sup class="reference" id="cite_ref-xuetal2006_2-11">[3]