Beipiaosaurus or “Beipiao lizard” is a genus of theropod dinosaur from the Early Cretaceous Period, 124 million years ago. It was named after a city in China near the location of discovery. The fossils were found in the Jianshangou bed of the Yixian Formation in Liaoning Province, China. It is known only from one species, B. inexpectus. A significant number of fossils have been recovered including skull fragments, scapula, a complete forelimb, and complete hind limb and pelvis. A second specimen was described in 2009 by Xu, which had a complete preserved skull and well preserved feathers.

Impressions of feathers found on the remains indicate that the body was covered mostly by downy feather-like fibers that were placed perpendicular to the arm. It is believed that these feather-like fibers were an intermediate stage between early dinosaurs and more advanced birds. Unique among theropods, Beipiaosaurus possessed a secondary coat of longer, simple feathers that rose out of the downy layer. These EBFFs (elongated broad filamentous feathers) were described by Xu in 2009. Xu and his team found EBFFs in the original type specimen as well as their own.

The EBFFs differ from other types of feathers in that they consist of a single, unbranched filament. Most other primitive feathered dinosaurs had downy-like feathers made up of two or more filaments branching out from a common base. In Beipiaosaurus, these EBFFs were much longer than the feathers of other dinosaurs of the time. Its feathers measured up to 6 inches long (about half the length of the neck). The feathers of the Beipiaosaurus were also straight and stiff and most likely hollow, at least at the base. The feathers were not meant for flight, but it is unclear if they were used more for display or some other unknown function (possibly a form of insulation from cold).

Beipiaosaurus measured 7 feet 4 inches in length and was one of the largest known feathered dinosaurs. It had a toothless beak with cheek teeth. The feet of the Beipiaosaurus had reduced inner toes that possibly evolved from three-toed dinosaurs, rather than the common four-toed therizinosaurs. The head was large relative to other therizinosaurs, with the lower jaw over half the length of the femur.

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