Temporal range: Early Cretaceous, 124.6Ma
Xu, Tang & Wang, 1999
|Species:||† B. inexpectus|
Xu, Tang & Wang, 1999
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.
In 1996, the peasant Li Yinxian discovered a skeleton of a theropod dinosaur near the village of Sihetun. In the May 27, 1999, issue of the journal Nature the discovery was announced and the type species Beipiaosaurus rarus named and described by Xu Xing, Tang Zhilu and Wang Xiaolin. The generic name Beipiaosaurus translates as "Beipiao lizard" after Beipiao, a city in China near the location of its discovery. Beipiaosaurus is known from a single species, B. inexpectus, the specific name, meaning "unexpected" in Latin, referring to "the surprising features in this animal".
The fossil, holotype IVPP V11559, was uncovered in the Jianshangou Beds of the Yixian Formation in Liaoning Province, China, which has been dated to the early Aptian stage of the early Cretaceous Period, 124.6 million years ago. It consists of a partial skeleton that is largely disarticulated, of a subadult individual. A significant number of fossilized bones were recovered, including: cranial fragments, a mandible, three cervical vertebrae, four dorsal vertebrae, twenty-five caudal vertebrae, a pygostyle, chevrons, a furcula, the scapula, two coracoids, a complete forelimb, a partial forelimb, a complete pelvis, a hindlimb, a partial hindlimb and remains of the integument.
In 2003 the pygostyle, consisting of the fused five last vertebrae of the tail, was described in greater detail.
A second specimen, STM31-1, a partial skeleton, was described by Xu et al. in 2009, which preserved a complete skull as well as a significant covering of unique, elongated feathers.
Beipiaosaurus measured 2.2 meters (7.3 ft) in length, and before the discovery of Yutyrannus is among the largest dinosaurs known from direct evidence to be feathered. Beipiaosaurus had a toothless beak with cheek teeth. More advanced therizinosaurids have four functional toes, but the feet of Beipiaosaurus' still 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 having about the length of the femur.
According to Zanno (2010b), Beipiaosaurus can be distinguished based on the following characteristics:
- a low ridge is present on the cranial femoral shaft which extends proximally from the medial condyle
- four fused caudal dorsal vertebrae are present, and a pygostyle that incorporates up to seven caudal vertebrae
- a prominent triangular flange extends from the ventrolateral surface of metacarpal I
- possesses a large skull, about equal to the length of the femur (according to Xu et al., 1999)
- the lateral articular surface is proximodistally elongate on the flexor side of manual phalanx I-1. (according to Xu et al., 1999)
- four fused posterior dorsal vertebrae are present (according to Zanno, 2008)
- the lateral buttress of metacarpal I are triangular (according to Zanno, 2008)
- the obturator process of the ischium is sinusoidal with a ventrally deflected distal portion (according to Zanno, 2008)
- the ischial boot is twice the width (craniocaudal depth) of the distal shaft (according to Zanno, 2008)
- a mediodistal ridge is present on anterior femur (according to Zanno, 2008)
Skin impressions from the type specimen of B. inexpectus indicated that the body was covered predominately by downy feather-like fibers, similar to those of Sinosauropteryx, 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). The tail was covered in feathers between four and seven centimetres long, consisting of parallel filaments with a width of 1.5 millimetres, without a trace of pennaceous feathers or a tail fan.
Unique among known theropods, Beipiaosaurus also possessed a secondary coat of much longer, simpler feathers that rose out of the down layer. 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.
Phylogeny and classification 
Beipiaosaurus was first assigned to the Therizinosauroidea, in a basal position, by Xu et al. (1999). All subsequent phylogenetic analyses have confirmed this assignment. According to the definition by Paul Sereno of this group, Beipiaosaurus is even by definition the basal most member. Zanno (2010b) noted that Beipiaosaurus shares a sister-taxon relationship with a taxon that includes all the more derived therizinosauroids. Zanno also concluded that there is a genus that is less derived than Beipiaosaurus, the genus Falcarius, which is a basal therizinosaurian.
See also 
- Xu, X., Tang, Z-L., and Wang, X-L. (1999). "A therizinosauroid dinosaur with integumentary structures from China." Nature, 399(6734): 350-354.
- Zhou, Z. (2006). "Evolutionary radiation of the Jehol Biota: chronological and ecological perspectives." Geological Journal, 41: 377-393.
- Xu, X.; Cheng, Y., Wang, X-L., Chang C. (2003). "Pygostyle-like structure from Beipiaosaurus (Theropoda, Therizinosauroidea) from the Lower Cretaceous Yixian Formation of Liaoning, China". Acta Geologica Sinica 77 (3): 294–298.
- Xu X., Zheng X.-t. and You, H.-l. (2009). "A new feather type in a nonavian theropod and the early evolution of feathers." Proceedings of the National Academy of Sciences (Philadelphia), 106(3): 832-834. doi:10.1073/pnas.0810055106
- Bryner, Jeanna (2009). "Ancient Dinosaur Wore Primitive Down Coat." http://www.foxnews.com/story/0,2933,479875,00.html
- L. E. Zanno. 2010. Osteology of Falcarius utahensis (Dinosauria: Theropoda): characterizing the anatomy of basal therizinosaurs. Zoological Journal of the Linnean Society 158(1):196-230