A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus

Anchiornis huxleyi : strictly for the troodontids The troodontids and dromaeosaurs are the dinosaur groups most closely related to birds. Anchiornis huxleyi from the Jurassic of China was thought to have been a stem-group bird — on the way to accumulating bird-like traits but retaining significant characteristics from earlier forebears. But the discovery of a new and spectacular specimen shows instead that it was a troodontid. As such, it is a very early member of that group (antedating Archaeopteryx ) and exhibits a rich plumage, especially on the legs and feet. The early history of the Paraves (troodontids, dromaeosaurs and birds) was clearly one of much evolutionary experimentation and ferment, only now being revealed as new fossil discoveries emerge. The early evolution of the major groups of derived non-avialan theropods is not well understood, resulting in the 'temporal paradox' argument against the theropod hypothesis of avian origins. Here, a small theropod specimen collected from the earliest Late Jurassic of China is recovered that is referable to the Troodontidae, which are among the theropods most closely related to birds, thus refuting the 'temporal paradox'. Furthermore, the extensive feathering of the specimen sheds new light on the early evolution of feathers. The early evolution of the major groups of derived non-avialan theropods is still not well understood, mainly because of their poor fossil record in the Jurassic. A well-known result of this problem is the ‘temporal paradox’ argument that is sometimes made against the theropod hypothesis of avian origins 1 . Here we report on an exceptionally well-preserved small theropod specimen collected from the earliest Late Jurassic Tiaojishan Formation of western Liaoning, China 2 . The specimen is referable to the Troodontidae, which are among the theropods most closely related to birds. This new find refutes the ‘temporal paradox’ 1 and provides significant information on the temporal framework of theropod divergence. Furthermore, the extensive feathering of this specimen, particularly the attachment of long pennaceous feathers to the pes, sheds new light on the early evolution of feathers and demonstrates the complex distribution of skeletal and integumentary features close to the dinosaur–bird transition.