Theropoda (; from ancient Greek θηρίο- ποδός [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, to about 11,000 living species, making theropods the only group of dinosaurs that remain. 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
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