Theropoda (; from ancient Greek [θηρίον, (therion) "wild beast"; πούς, ποδός (pous, podos) "foot"]) is one of the three major clades of dinosaurs, alongside Ornithischia and Sauropodomorpha. Theropods, both extant and extinct, are characterized by hollow bones and three toes and claws on each limb. They are generally classed as saurischian dinosaurs, placing them closer to sauropodomorphs than to ornithischians. They were ancestrally carnivorous, although a number of theropod groups evolved to become herbivores and omnivores. Many members of the subgroup Coelurosauria were covered with feathers, and it is possible that they were also present in other theropods. In the Jurassic, birds evolved from small specialized coelurosaurian theropods. There are around 11,000 living bird species, making theropods the only known living group of dinosaurs.
Theropods first appeared during the Carnian age of the Late Triassic period 231.4 million years ago (Ma) and included the majority of large terrestrial carnivores from the Early Jurassic until the end of the Cretaceous, about 66 Ma, including the largest terrestrial carnivorous animals ever, such as Tyrannosaurus and Giganotosaurus, though non-avian theropods exhibited considerable size diversity, with some non-avian theropods like scansoriopterygids being no bigger than small birds.
Biology
Traits Various synapomorphies for Theropoda have been proposed based on which taxa are included in the group. For example, a 1999 paper by Paul Sereno suggests that theropods are characterized by traits such as an ectopterygoid fossa (a depression around the ectopterygoid bone), an intramandibular joint located within the lower jaw, and extreme internal cavitation within the bones. However, since taxa like Herrerasaurus may not be theropods, these traits may have been more widely distributed among early saurischians rather than being unique to theropods.
Instead, taxa with a higher probability of being within the Theropoda may share more specific traits, such as a prominent promaxillary fenestra, cervical vertebrae with pleurocoels in the anterior part of the centrum leading to a more pneumatic neck, five or more sacral vertebrae, enlargement of the carpal bone, and a distally concave portion of the tibia, among a few other traits found throughout the skeleton. Like the early sauropodomorphs, the second digit in a theropod's hand is enlarged. Theropods also have a very well developed ball and socket joint near their neck and head.
Most theropods belong to the clade Neotheropoda, characterized by the reduction of several foot bones, thus leaving three toed footprints on the ground when they walk (tridactyl feet). Digit V was reduced to a remnant early in theropod evolution and was gone by the late Triassic. Digit I is reduced and generally do not touch the ground, and greatly reduced in some lineages. They also lack a digit V on their hands and have developed a furcula which is otherwise known as a wishbone. Early neotheropods like the coelophysoids have a noticeable kink in the upper jaw known as a subnarial gap. Averostrans are some of the most derived theropods and contain the Tetanurae and Ceratosauria. While some used to consider coelophysoids and ceratosaurs to be within the same group due to features such as a fused hip, later studies showed that it is more likely that these were features ancestral to neotheropods and were lost in basal tetanurans. Averostrans and their close relatives are united via the complete loss of any digit V remnants, fewer teeth in the maxilla, the movement of the tooth row further down the maxilla and a lacrimal fenestra. Averostrans also share features in their hips and teeth.
Diet and teeth
Theropods exhibit a wide range of diets, from insectivores to herbivores and carnivores. Strict carnivory has always been considered the ancestral diet for theropods as a group, and a wider variety of diets was historically considered a characteristic exclusive to the avian theropods (birds). However, discoveries in the late 20th and early 21st centuries showed that a variety of diets existed even in more basal lineages. All early finds of theropod fossils showed them to be primarily carnivorous. Fossilized specimens of early theropods known to scientists in the 19th and early 20th centuries all possessed sharp teeth with serrated edges for cutting flesh, and some specimens even showed direct evidence of predatory behavior. For example, a Compsognathus longipes fossil was found with a lizard in its stomach, and a Velociraptor mongoliensis specimen was found locked in combat with a Protoceratops andrewsi (a type of ornithischian dinosaur). This was likely caused by a sand dune blowing over the two animals mid combat, resulting in fossilization.
The first confirmed non-carnivorous fossil theropods found were the therizinosaurs, originally known as "segnosaurs". First thought to be prosauropods, these enigmatic dinosaurs were later proven to be highly specialized, herbivorous theropods. Therizinosaurs possessed large abdomens for processing plant food, and small heads with beaks and leaf-shaped teeth. Further study of maniraptoran theropods and their relationships showed that therizinosaurs were not the only early members of this group to abandon carnivory. Several other lineages of early maniraptorans show adaptations for an omnivorous diet, including seed-eating (some troodontids) and insect-eating (many avialans and alvarezsaurs). Oviraptorosaurs, ornithomimosaurs and advanced troodontids were likely omnivorous as well, and some theropods (such as Masiakasaurus knopfleri and the spinosaurids) appear to have specialized in catching fish.
Diet is largely deduced by the tooth morphology, tooth marks on bones of the prey, and gut contents. Some theropods, such as Baryonyx, Lourinhanosaurus, ornithomimosaurs, and birds, are known to use gastroliths, or gizzard-stones.
The majority of theropod teeth are blade-like, with serration on the edges, called ziphodont. Others are pachydont or folidont depending on the shape of the tooth or denticles. The morphology of the teeth is distinct enough to tell the major families apart, which indicate different diet strategies. An investigation in July 2015 discovered that what appeared to be "cracks" in their teeth were actually folds that helped to prevent tooth breakage by strengthening individual serrations as they attacked their prey. The folds helped the teeth stay in place longer, especially as theropods evolved into larger sizes and had more force in their bite.
As a general rule, predatory theropods above between 16 and 32 kg preyed predominantly on prey as large or larger than themselves, while those below this body mass threshold preyed primarily on prey smaller than themselves. This is similar to the pattern observed in predatory mammals, where taxa above ~21.5 kg predominantly prey on animals larger than themselves, because the energy animals above this body mass threshold gain from catching small prey is insufficient to offset the bioenergetic expenditure required to capture them.
Integument (skin, scales and feathers)
Mesozoic theropods were also very diverse in terms of skin texture and covering. Feathers or feather-like structures (filaments) are attested in most lineages of coelurosaurs (see feathered dinosaur). However, outside the coelurosaurs, feathers may have been confined to the young, smaller species, or limited parts of the animal. Many larger theropods had skin covered in small, bumpy scales. In some species, these were interspersed with larger scales. This type of skin is best known in the ceratosaur Carnotaurus, which has been preserved with extensive skin impressions. Osteoderms, scales with a bony core, are known from Ceratosaurus, which was discovered with segments of osteoderms on top of its neck and tail, probably forming a continuous row in life. In carnosaurs rectangular scutate scales are known from the top of the feet and the underside of the tail in Concavenator, and from the underside of the neck in Allosaurus.
There is evidence of some lineages of theropods being ancestrally feathered but losing them in favor of scales in later members. In tyrannosauroids, the early members Dilong and Yutyrannus are preserved with evidence of feathers, while in the later tyrannosaurids, like Tyrannosaurus, Tarbosaurus, Albertosaurus, Gorgosaurus, and Daspletosaurus, there is evidence of scales, though it is unknown if they lost all feathers entirely.
The coelurosaur lineages most distant from birds had feathers that were relatively short and composed of simple, possibly branching filaments. Simple filaments are also seen in therizinosaurs, which also possessed large, stiffened "quill"-like feathers. More fully feathered theropods, such as dromaeosaurids, usually retain scales only on the feet. Some species may have mixed feathers elsewhere on the body as well. Scansoriopteryx preserved scales near the underside of the tail, and Juravenator may have been predominantly scaly with some simple filaments interspersed. On the other hand, some theropods were completely covered with feathers, such as the anchiornithid Anchiornis, which even had feathers on the feet and toes.
Based on relationships between tooth size and skull length and also a comparison of the degree of wear of the teeth of non-avian theropods and modern lepidosaurs, it is concluded that theropods had lips that protected their teeth from the outside. Visually, the snouts of such theropods as Daspletosaurus had more similarities with lizards than crocodilians, which lack lips.
Size
Tyrannosaurus was for many decades the largest and best-known theropod to the general public. Since its discovery, however, a number of other, comparably-sized giant carnivorous dinosaurs have been described, including Spinosaurus, Carcharodontosaurus, and Giganotosaurus. These large theropod dinosaurs are estimated to exceed Tyrannosaurus in length, especially in the case of Spinosaurus, though more recent studies and reconstructions show that Tyrannosaurus, although shorter, was more heavily built and thus still comparable to them in mass and overall size. The largest known Tyrannosaurus specimens such as Sue and Scotty are currently estimated to be the most massive individual theropod specimens known to science, though by only a narrow margin; a considerable bias in sample size resulting from the much greater number of adult specimens in Tyrannosaurus compared to all other theropods of similar mass, combined with the majority of individual adult giant theropods (even with Tyrannosaurus) never having entered the fossil record, mean it is difficult to judge which, if any, of these theropods was the largest by mass in reality. There is still no clear explanation for exactly why giant theropods grew to be so massive compared to any land predators that came before and after them.
The largest extant theropod is the common ostrich, up to 2.74 m (9 ft) tall and weighing between 90 and 130 kg (200 – 290 lb). The smallest non-avian theropod known from adult specimens is the anchiornithid Anchiornis huxleyi, at 110 grams in weight and 34 centimeters (1 ft) in length. When modern birds are included, the bee hummingbird (Mellisuga helenae) is smallest at 1.9 g and 5.5 cm (2.2 in) long.
Recent theories propose that theropod body size shrank continuously over a period of 50 million years, from an average of down to , eventually evolving into over 11,000 species of modern birds. This was based on evidence that theropods were the only dinosaurs to get continuously smaller, and that their skeletons changed four times as fast as those of other dinosaur species.
Growth rates