Troodontidae

Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil discoveries of complete and articulated specimens (including specimens which preserve feathers, eggs, embryos, and complete juveniles), have helped to increase understanding about this group. Anatomical studies, particularly studies of the most primitive troodontids, like Sinovenator, demonstrate striking anatomical similarities with Archaeopteryx and primitive dromaeosaurids, and demonstrate that they are relatives comprising a clade called Paraves.

== Evolution == The oldest definitive troodontid known is Hesperornithoides from the Late Jurassic of Wyoming. The slightly older Koparion of Utah is only represented by a single tooth, and small maniraptoran teeth from the Middle Jurassic of England were identified as those of indeterminate troodontids in 2023. Over the Cretaceous, troodontids radiated throughout western North America, Asia, and Europe, suggesting a mostly Laurasian distribution for the group. However, in 2013, a single diagnostic tooth from the latest Cretaceous (Maastrichtian) Kallamedu Formation of southern India was identified as a troodontid, suggesting that troodontids either also inhabited Gondwana or managed to disperse to India from elsewhere prior to its separation as an island continent. The potential Gondwanan occurrence of troodontids is supported by the existence of Middle Jurassic remains, which suggest that they originated prior to the breakup of Pangaea. However, due to the lack of other remains from the region, it has been suggested that the existence of Gondwanan troodontids should be regarded as provisional.