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. If Megaraptora is part of Tyrannosauroidea, this would extend the distribution of the group to Australia and South America, and possible fragmentary remains of tyrannosauroids have also been reported from these continents. 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.
Description
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 1.6 meters (5.2 feet) in length, and a fully grown Guanlong measured 3 meters (9.8 feet) long. 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. An immature Eotyrannus was over 4 meters (13 feet) in length, and a subadult Appalachiosaurus was estimated at more than 6 meters (20 feet) long, 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). 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.
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. 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. The long forelimb persisted at least through the Early Cretaceous Eotyrannus, but is unknown in Appalachiosaurus. 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. The third digit of the forelimb was also reduced over time. This digit was unreduced in the basal Guanlong, while in Dilong it was more slender than the other two digits. Eotyrannus also had three functional digits on each hand. Tyrannosaurids had only two, although the vestigial metacarpal of the third are preserved in some well-preserved specimens. As in most coelurosaurs, the second digit of the hand is the largest, even when the third digit is not present. 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. These features are found in all known tyrannosauroids, including basal members Guanlong and Dilong. The pubis is not known in Aviatyrannis or Stokesosaurus but both show typical tyrannosauroid characters in the ilium. 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. Elongated metatarsals are present even in the largest adult Tyrannosaurus in spite of the tibiae being proportionately shorter than in smaller tyrannosauroids 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. The arctometatarsus was also present in Appalachiosaurus but it is unclear whether it was found in Eotyrannus or Dryptosaurus. This structure was shared by derived ornithomimids, troodontids and caenagnathids, but was not present in basal tyrannosauroids like Dilong paradoxus, indicating convergent evolution.
Classification
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