Beipiaosaurus

Beipiaosaurus is a genus of therizinosauroid theropod dinosaur. The discovery of Beipiaosaurus (pronounced /ˌbeɪpjaʊˈsɔrəs/), 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 the Jianshangou bed of the Yixian Formation in Liaoning Province, China, which has been dated to the early Cretaceous Period, 124.6 million years ago.^[1] It is known from a single species, B. inexpectus, named for "the surprising features in this animal.".^[2] 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 completepelvis with hindlimb. A second specimen was described by Xu et al. in 2009, which preserved a complete skull as well as a significant covering of unique, elongated feathers.^[3]

Beipiaosaurus measured 2.2 meters (7.3 ft) in length, and is among the largest known feathered dinosaurs.^[2] Beipiaosaurus had a toothless beak with cheek teeth. More advanced therizinosaurids have four functional toes, but the feet of Beipiaosaurus' have reduced inner toes, showing that the derived therizinosaurid condition may have evolved from a three-toed therizinosauroid ancestor. The head was large relative to other therizinosaurs, with the lower jaw over half the length of the femur.^[2]Skin impressions from the type specimen of B. inexpectus indicated that the body was covered predominately by downy feather-like fibers, similar to those ofSinosauropteryx, but longer, and are oriented perpendicular to the arm. Xu et al., who described the specimen, suggested that these downy feathers represent an intermediate stage between Sinosauropteryx and more advanced birds (Avialae).^[2]

Unique among known theropods, Beipiaosaurus also possessed a secondary coat of much longer, simpler feathers that rose out of the down layer.^[4] These unique feathers (known as EBFFs, or elongated broad filamentous feathers) were first described by Xu et al. in 2009, based on a specimen consisting of the torso, head and neck. Xu and his team also found EBFFs in the original type specimen of B. inexpectus, revealed by further preparation.

The EBFFs differ from other feather types in that they consist of a single, unbranched filament. Most other primitive feathered dinosaurs have down-like feathers made up of two or more filaments branching out from a common base or along a central shaft. The EBFFs of Beipiaosaurus are also much longer than other primitive feather types, measuring about 100-150 millimeters (4-6 inches) long, roughly half the length of the neck. In Sinosauropteryx, the longest feathers are only about 15% of the neck length.

The EBFFs of Beipiaosaurus are also unusually broad, up to 3 mm wide in the type specimen. The broadest feathers of Sinosauropteryx are only 0.2 mm wide, and only slightly wider in larger forms such as Dilong. Additionally, where most primitive feather types are circular in cross section, EBFFs appear to be oval-shaped.

None of the preserved EBFFs were curved or bent beyond a broad arc in either specimen, indicating that they were fairly stiff. They were probably hollow, at least at the base.

In a 2009 interview, Xu stated: "Both [feather types] are definitely not for flight, inferring the function of some structures of extinct animals would be very difficult, and in this case, we are not quite sure whether these feathers are for display or some other functions." He speculated that the finer feathers served as an insulatory coat and that the larger feathers were ornamental, perhaps for social interactions such as mating or communication.^[4]

The exact classification of therizinosaurs had 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 that all therizinosauroids, including the more derived Therizinosauridae, to be coelurosaurian theropods, not sauropodomorph or ornithischian relatives as once believed.