Theropods are among the most iconic of dinosaurs, including beasts such as Tyrannosaurus rex. Clawed bipeds all with distinctive hollow bones, the earliest known dinosaurs were theropods, and it is theropods that are still living (as birds). Most of the Mesozoic forms had claws with a semi-opposable thumb on a grasping three-fingered hand; recurved, serrated, laterally compressed teeth; and were carnivorous.

They are a complex group, with a remarkable evolutionary history. The most primitive radiation of theropods is seen in the group Coleophysoidea. A lineage of particular interest (because it includes tyrannosaurs and to modern birds) is the tetanurans, a group of thero-pods whose zygapophysis-stiffened tails were used as dynamic counter-balances to grasping claws; it is among these that some of the most predatory dinosaurs reside. A subset of these, deinonychosaurs, developed eviscerating claws on the legs, grasping, powerful hands, large brains (and inferred high intelligence), and likely stereoscopic vision: pound for pound the most deadly carnivores ever evolved within Dinosauria. And deinonychosaurs are the most closely related non-avian dinosaurs to Aves (living birds).

Non-avian theropods were not strictly carnivorous, and a number of groups developed whose habits are still unknown. There were the long-armed, well-clawed oviraptorosaurids whose toothless mouths may have crushed mollusks or eggs. Then there were the ostrich-mimics; toothless, small-skulled forms that may have been among the fastest runners in all Dinosauria. Finally there were therizinosaurs whose long arms and massive claws are vaguely reminiscent of sloths.

Many theropods appear to be designed for aggressive, active behavior, and it was this aspect of their design that first suggested to researchers that deinonychosaur theropods in particular - and dinosaurs in general - might be endothermic. Deinonychosaurs remain among the best candidates for full-time mammalian-style endothermy.

With modern birds as living (if highly derived) examples, social behavior can be inferred in theropods. A variety of facial features such as hornlets adorned theropods, and some evidence suggests that many of them hunted in packs. Particularly bird-like is theropod maternal behavior: in those forms in which it is known, non-avian theropod mothers (?) incubated clutches of eggs very much like avian theropods.

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