Tyrannosauroidea

Tyrannosauroids varied widely in size, although there was a general trend towards increasing size over time. Early tyrannosauroids were small animals. One specimen of Dilong, almost fully grown, measured in length, and a fully grown Guanlong measured long. Teeth from Lower Cretaceous rocks (140 to 136 million years old) of Hyogo, Japan, appear to have come from an approximately long animal, possibly indicating an early size increase in the lineage. An immature Eotyrannus was over in length, and a subadult Appalachiosaurus was estimated at more than long, '', from the Field Museum of Natural History in Chicago. 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, 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 from 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. 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. As in most coelurosaurs, the second digit of the hand is the largest, even when the third digit is not present. The radius in tyrannosauroids the radius is distally flattened, just like members of Maniraptora but unlike the condition seen in ornithomimosaurs and alvarezsaurids. 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. 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 outside of the largest forms show proportions characteristic of fast-running animals, including elongated tibiae and metatarsals. and a probable inability to run (i.e move with an aerial phase where both feet are off the ground). The third metatarsal of tyrannosaurids, including in the very largest forms, was pinched at the top between the second and fourth, forming a structure known as the arctometatarsus that aided in energy conservation and, in smaller forms, increased speed. This structure was shared by derived ornithomimids, troodontids and caenagnathids, but was not present in basal tyrannosauroids like Dilong paradoxus, indicating convergent evolution. ==Classification==

'', a proceratosaurid tyrannosauroid from the Middle Jurassic of England.|left Tyrannosaurus was named by Henry Fairfield Osborn in 1905, along with the family Tyrannosauridae. The name is derived from the Ancient Greek words ' ' ('tyrant') and '''' ('lizard'). The superfamily name Tyrannosauroidea was first published in a 1964 paper by the British paleontologist Alick Walker. The suffix -oidea, commonly used in the name of animal superfamilies, is derived from the Greek ειδος '''' ('form'). Scientists have commonly understood Tyrannosauroidea to include the tyrannosaurids and their immediate ancestors. 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. 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. The Sereno definition was adopted in a 2010 review. However, other authors disputed the placement of megaraptorans within Tyrannosauroidea, and a study of megaraptoran hand anatomy published in 2016 caused even the original scientists suggesting their tyrannosauroid relationships to at least partly reject their prior conclusion. 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. Within this group, the allosaurids were often considered to be ancestral to tyrannosaurids. In the early 1990s, cladistic analyses instead began to place tyrannosaurids into the Coelurosauria, echoing suggestions first published in the 1920s. Tyrannosaurids are now universally considered to be large coelurosaurs. 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. A 2007 analysis found the family Coeluridae, including the Late Jurassic North American genera Coelurus and Tanycolagreus, to be the sister group of Tyrannosauroidea. Other early taxa include Stokesosaurus and Aviatyrannis, known from far less complete material. Alectrosaurus, a poorly known genus from Mongolia, is definitely a tyrannosauroid but its exact relationships are unclear. but is usually considered a carnosaur today. 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. Phylogeography In 2018 authors Rafael Delcourt and Orlando Nelson Grillo published a phylogenetic analysis of Tyrannosauroidea which incorporated taxa from the ancient continent of Gondwana (which today consists of the southern hemisphere), such as Santanaraptor and Timimus, whose placement in the group has been controversial. They have found that not only Santanaraptor and Timimus were placed as tyrannosaurs more derived than Dilong, but they have found in their analysis that tyrannosauroids were widespread in Laurasia and Gondwana since the Middle Jurassic. This reanalysis of phylogenetic relationships of tyrannosauroids in Appalachia has brought the rediscovery of the clade Dryptosauridae due to the similarities of metatarsals II and IV with the same bones in the Dryptosaurus holotype. However. the Merchantville taxon was found to still be different enough to separate it on the genus level from Dryptosaurus. In the phylogentic tree constructed Dryptosauridae is found to be a valid family of non tyrannosaurid eutyrannosaur. It currently sits in a polytomy with the Iren Dabasu taxon and more basal eutryannosaurs. ==Distribution==

The tyrannosauroids lived on the supercontinent Laurasia, which split from Gondwana in the Middle Jurassic. The earliest recognized tyrannosauroids lived in the Middle Jurassic, represented by the proceratosaurids Kileskus from the Western Siberia and Proceratosaurus from Great Britain. Upper Jurassic tyrannosauroids include Guanlong from China, Stokesosaurus from the western United States and Aviatyrannis and Juratyrant from Europe. Early Cretaceous tyrannosauroids are known from Laurasia, being represented by Eotyrannus from England and the Tetori Group of Japan. The Middle Cretaceous record of Tyrannosauroidea is rather patchy. Teeth and indeterminate postcrania of this interval are known from the Cenomanian-age Dakota Formation of western North America and Potomac Formation of New Jersey, as well as formations in Kazakhstan and Tajikistan; two genera, Timurlengia and Xiongguanlong, have been found in Asia, while the Brazilian Santanaraptor may belong to this group. 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. Non-tyrannosaurid tyrannosauroids like Alectrosaurus and possibly Bagaraatan were contemporaneous with tyrannosaurids in Asia, while they are absent from western North America. However, a response suggested that critical tyrannosauroid characters were absent from the fossil. The Australian taxon Timimus, known from a femur, and the Brazilian Santanaraptor, known from a partial juvenile skeleton, have also been suggested to be tyrannosaurs. ==Paleobiology==

Facial tissue A conference paper by Tracy Ford states that there was rough bone texture on the skulls of theropods and higher foramina frequency than lepidosaurs and mammals which would be evidential for a sensitive snout for theropods. A study in 2017 study on a new tyrannosaurid named Daspletosaurus horneri was published in the journal Scientific Reports, where paleontologist Thomas Carr analyzed the craniofacial texture of Daspletosaurus horneri and observed a hummocky rugosity which compared to crocodilian skulls, suggesting Daspletosaurus horneri and with it all tyrannosaurids have flat sensory scales. The subordinate regions were analyzed to have cornified epidermis. However, a 2018 presentation has an alternative interpretation. Crocodilians do not have flat sensory scales, but rather cracked, cornified epidermis due to growth. The hummocky rugosity in the skulls of lepidosaurs have correlation with scales which this bone texture is also present in tyrannosaurid skulls. The foramina frequency in theropod skulls does not exceed 50 foramina, which shows that theropods had lips. It's been proposed that lips are a primitive trait in tetrapods and the soft tissue present in crocodilians are a derived trait because of aquatic or semiaquatic adaptations. Body integument 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. These filaments have usually been interpreted as "protofeathers," homologous with the branched feathers found in birds and some non-avian theropods, although other hypotheses have been proposed. A skeleton of Dilong paradoxus was described in 2004 that included the first example of feathers in a tyrannosauroid. Similarly to down feathers of modern birds, the feathers found in Dilong were branched but not pennaceous, and may have been used for insulation. The presence of feathers in basal tyrannosauroids is not surprising since they are now known to be characteristic of coelurosaurs, found in other basal genera like Sinosauropteryx, Possibly, feathers were present on other areas of the body: preserved skin impressions are very small and come primarily from the legs, pelvic region, and underside of the tail, which either lack feathers or only covered in a light down in some modern large ground-dwelling birds. Alternatively, secondary loss of feathers 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 or even detrimental. However, other paleontologists argue that taphonomy is the possible cause of the lack of filamentous structures in tyrannosaurid fossils. Head crests '', a basal tyrannosauroid from China. 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. ==See also==