derived traits (autapomorphies): a low ridge is present on the front of the thighbone shaft which extends upwards from the inner condyle; the last four dorsal vertebrae are fused; from at least the seventh vertebra onwards the tail vertebrae are fused into a pygostyle; a prominent triangular flange extends outwards, away from the body, from the underside of the first metacarpal; the skull is large, about as long as the thighbone; the first phalanx of the first digit has a joint surface with the claw, that is elongated in the direction of the finger tip on the outer side of its lower part; on the front edge of the ischium a projection, the obturator process, has a wavy profile with a tip that is curved downwards; the "boot" at the lower end of the ischium has twice the width, measured from the front to the rear, of the lower shaft of the ischium. In 2019 Liao and Xu redescribed the holotype skull in detail, noting new unique cranial traits for Beipiaosaurus. These consisted of the large postorbital joint of the frontal and its abrupt transition from the orbital edge, a long and keen anterior joint of the parietal, an elongate bottom branching of the squamosal articulation of the parietal, and the deeply external mandibular fenestra that is located towards the rear. Feathers and color The first feather impressions were found in the holotype specimen, consisting of short, slender filamentous feathers on the left arm. These impressions indicated that the body was predominately covered by downy feather-like fibers, similar to those of Sinosauropteryx, but longer, and oriented perpendicular to the arm. Xu et al. 1999 suggested that these downy feathers represent an intermediate stage between Sinosauropteryx and more advanced birds (Avialae). The tail was also covered in feathers between four and seven centimeters long, consisting of parallel filaments with a width of , without a trace of pennaceous feathers or a tail fan, as indicated by the preserved pygostyle. 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 (elongated broad filamentous feathers) were first described by Xu et al. 2009, based on specimen STM 31-1 consisting of the torso, head and neck. Xu and his team also found EBFFs in the tail of the holotype IVPP V11559, which were revealed by further