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 . Fossils of tyrannosauroids have been recovered on what are now the continents of North America, Europe and Asia, with fragmentary remains possibly attributable to tyrannosaurs also known from South America and 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 identified in fossils of two species and may have been present in other tyrannosauroids as well. Prominent bony crests in a variety of shapes and sizes on the skulls of many tyrannosauroids may have served display functions.

Tyrannosauroids varied widely in size, although there was a general trend towards increasing size over time. Early tyrannosauroids were small animals.^[3] One specimen of Dilong, almost fully grown, measured 1.6 meters (5.2 feet) in length,^[4] and a fully grown Guanlong measured 3 meters (9.8 feet) long.^[5] Teeth from 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.^[6] An immature Eotyrannus was over 4 meters (13 feet) in length,^[7] and a subadult Appalachiosaurus was estimated at more than 6 meters (20 feet) long,^[3] indicating that both genera reached larger sizes. The Late Cretaceous tyrannosaurids ranged from the 9 meters (30 feet) Albertosaurus and Gorgosaurus to Tyrannosaurus , which exceeded 12 meters (39 feet) in length and may have weighed more than 6,400 kilograms (7 short tons).^[3] A 2010 review of the literature concluded that tyrannosaurs were "small- to mid-sized" for their first 80 million years but were "some of the largest terrestrial carnivores to ever live" in their last 20 million years.^{[8] [9]}

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.^{[3] [4] [5]}

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.^[5] The long forelimb persisted at least through the Early Cretaceous Eotyrannus,^[7] but is unknown in Appalachiosaurus.^[10] 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.^[3] The third digit of the forelimb was also reduced over time. This digit was unreduced in the basal Guanlong,^[5] while in Dilong it was more slender than the other two digits.^[4] Eotyrannus also had three functional digits on each hand.^[7] Tyrannosaurids had only two, although the vestigial metacarpal of the third are preserved in some well-preserved specimens.^[11] 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.^[3] These features are found in all known tyrannosauroids, including basal members Guanlong^[5] and Dilong.^[4] The pubis is not known in Aviatyrannis or Stokesosaurus but both show typical tyrannosauroid characters in the ilium.^[12] 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.^[3] These proportions persist even in the largest adult Tyrannosaurus,^[13] despite its probable inability to run.^[14] The third metatarsal of tyrannosaurids was pinched at the top between the second and fourth, forming a structure known as the arctometatarsus.^[3] The arctometatarsus was also present in Appalachiosaurus^[10] but it is unclear whether it was found in Eotyrannus^[7] or Dryptosaurus .^[15] This structure was shared by derived ornithomimids, troodontids and caenagnathids,^[16] but was not present in basal tyrannosauroids like Dilong paradoxus , indicating convergent evolution.^[4]

Tyrannosaurus was named by Henry Fairfield Osborn in 1905, along with the family Tyrannosauridae.^[17] 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 the British paleontologist Alick Walker.^[18] The suffix -oidea, commonly used in the name of animal superfamilies, is derived from the Greek ειδος eidos ('form').^[19]

Scientists have commonly understood Tyrannosauroidea to include the tyrannosaurids and their immediate ancestors.^{[18] [20]} 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.^[21] 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 .^[3] Sereno published a new definition in 2005, using Ornithomimus edmontonicus, Velociraptor mongoliensis and Troodon formosus as external specifiers.^[22] The Sereno definition was adopted in a 2010 review.^[8]

Some studies have suggested that the clade Megaraptora, usually considered to be allosauroids, are basal tyrannosauroids.^{[23] [24]} However, other authors disputed the placement of megaraptorans within Tyrannosauroidea,^{[25] [26]} 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.^[27]

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.^{[28] [29]} Within this group, the allosaurids were often considered to be ancestral to tyrannosaurids.^{[20] [30]} In the early 1990s, cladistic analyses instead began to place tyrannosaurids into the Coelurosauria,^{[16] [31]} echoing suggestions first published in the 1920s.^{[32] [33]} Tyrannosaurids are now universally considered to be large coelurosaurs.^{[3] [5] [34] [35] [36] [37] [38]}

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.^[16] Basal tyrannosauroids like Dilong, however, were found with non-arctometatarsalian ankles, indicating that this feature evolved convergently.^[4] Arctometatarsalia has been dismantled and is no longer used by most paleontologists, with tyrannosauroids usually considered to be basal coelurosaurs outside Maniraptoriformes.^{[3] [36] [38]} While many place tyrannosauroids as basal coelurosaurs, Paul Sereno in his 1990s analysis of theropods would find the Tyrannosaurs to be sister taxa to the Maniraptora with them being closer to birds than Ornithomimosaurs were. He called this group Tyrannoraptora (which in the absence of papers that recover a Tyrannosaur-maniraptoran clade), is a clade which contains most Coelurosaurs.^[39] A 2007 analysis found the family Coeluridae, including the Late Jurassic North American genera Coelurus and Tanycolagreus , to be the sister group of Tyrannosauroidea.^[34]

The most basal tyrannosauroid known from complete skeletal remains is Guanlong, a representative of the family Proceratosauridae.^{[5] [40]} Other early taxa include Stokesosaurus and Aviatyrannis, known from far less complete material.^[12] The better-known Dilong is considered slightly more derived than Guanlong and Stokesosaurus.^{[4] [5]} 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.^{[3] [10] [41]} Alectrosaurus , a poorly known genus from Mongolia, is definitely a tyrannosauroid but its exact relationships are unclear.^[3] Other taxa have been considered possible tyrannosauroids by various authors, including Bagaraatan and Labocania .^[3] Siamotyrannus from the Early Cretaceous of Thailand was originally described as an early tyrannosaurid,^[42] but is usually considered a carnosaur today.^{[36] [43]} 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.^{[12] [43]}

Below on the left is a cladogram of Tyrannosauroidea from a 2022 study by Darren Naish and Andrea Cau on the genus Eotyrannus , and on the right is a cladogram of Eutyrannosauria from a 2020 study by Jared T. Voris and colleagues on the genus Thanatotheristes :^{[44] [45]}