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Beipiaosaurus inexpectus is a therizinosauroid, a member of a bizarre group of theropod dinosaurs. The discovery of Beipiaosaurus (bay-peow-SAWR-us), which translates as "Beipiao lizard" after a city in China near the location of its discovery, was announced in the May 27, 1999, issue of the journal Nature. These fossils were found in Liaoning Province, China and have been dated to the Middle Cretaceous period, about 125 million years ago. A significant number of fossilized bones for this species were recovered, including: cranial fragments, a mandible, three cervical vertebrae, four dorsal vertebrae, a caudal vertebra, the scapula and scapulacoracoid, a complete forelimb, and a complete pelvis with hindlimb.
The exact classification of therizinosaurs has in the past been hotly debated, since their prosauropod-like teeth and body structure indicate that they were generally herbivorous, unlike typical theropods. Beipiaosaurus, being considered to be a primitive therizinosauroid, has features which suggest all therizinosauroids, including the more derived Therizinosauridae, to be coelurosaurian theropods, not sauropodomorph or ornithischian relatives as once believed.
Beipiaosaurus measured 2.2 metres (7.3 ft) in length and .88 metres (2.9 ft.) tall at the hip, and is among the largest known feathered dinosaurs. Its weight is estimated as about 85 kg (187 lb.) Beipiaosaurus had a toothless beak with cheek teeth. Therizinosauridae have four functional toes, but Beipiaosaurus' feet have reduced inner toes, showing that the derived therizinosaurid condition may have evolved from a three-toed therizinosauroid ancestor. The head was relatively larger than that of other therizinosaurs, and it had some features similar to the related Oviraptorosauria (perhaps the sistergroup). The fossil's skin impressions indicate its body was covered by downy feather-like fibres (protofeathers), which are very similar to those of Sinosauropteryx. As only theropod dinosaurs are so far known to have been feathered, this discovery is further evidence that therizinosaurs were indeed theropods.
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