The coelurosaurs (Coelurosauria) make up the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. It is a diverse group that includes tyrannosaurs, ornithomimosaurs, and maniraptors; Maniraptora includes birds, the only descendants of coelurosaurs alive today. In the past, this classification included all the small-bodied theropods, while the carnosaurs included all the large-bodied theropods. This is not the case today—and although several alternate classifications have been proposed, none is universally accepted.

Because this group of theropods was extremely diverse, the group’s definition, the dinosaurs within the group, and the relationships between the various species are currently in a continual state of change. Presently, the major subgroups of coelurosaurs are the ornithomimosaurs (Ornithomimids), maniraptorans (Maniraptora), and a fairly recent addition to the coelurosaurs, the tyrannosaurs. Some of the dinosaurs in this group evolved during the Late Jurassic period, but the majority reached their peak in the Cretaceous period.

These carnivorous dinosaurs were more bird-like in characteristics and appearance than the large carnosaurs. In fact, birds are classified in the coelurosaur subgroup known as the maniraptorans (Maniraptora). Coelurosaurs had very long forelimbs and well-developed, hinge-like ankles, although later dinosaurs in this group may have lost these features or had modified versions. Some other characteristics of the coelurosaurs include some special bone structures, such as a triangular bulge on the lower part of the pelvis and a protrusion on the ankle bone.

The tyrannosaurs (also referred to as tyrannosaurids, depending on the text), including Tyrannosaurus rex, used to be classified by most scientists as carnosaurs. But recent cladistic (class) analysis shows that the tyrannosaurs are more closely related to the coelurosaurs. (Coelurosaurs are a class of dinosaurs related to birds; tyrannosaurs also belong to the larger group known as tetanurans, dinosaurs with “stiff tails.”) Although some probably evolved during the Late Jurassic period, most of them dominated the Cretaceous period.