Pu et al. 2013 Beipiaosaurus /ˌbeɪpjaʊˈsɔːrəs/ is a genus of therizinosauroid theropod dinosaurs that lived in China during the Early Cretaceous in the Yixian Formation.

They were relatively small-sized therizinosaurs, measuring 2.2 m (7.2 ft) long and weighing about 27 kg (60 lb) in contrast to the advanced and giant Segnosaurus or Therizinosaurus.

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.

On May 27, 1999, the discovery was announced on the famous journal Nature and the type species Beipiaosaurus inexpectus named and described by Xu Xing, Tang Zhilu and Wang Xiaolin.

Beipiaosaurus is known from a single species, B. inexpectus, the specific name, meaning "unexpected" in Latin, referring to the "surprising features in these animals".

[2] The holotype (type specimen) of Beipiaosaurus inexpectus, IVPP V11559, was recovered in the Jianshangou Beds of the Yixian Formation in Liaoning Province, China.

The specimen was collected in sediment deposited during the Aptian stage of the Early Cretaceous period, approximately 125 million years ago.

[5] A second specimen, STM 31-1, a partial skeleton, was described by Xu et al. 2009, which preserves a significant covering of unique, elongated feathers.

This specimen hails from the Sihetun locality at the Beipiao County in Liaoning Province and compromises a partial individual preserving the skull (badly crushed), most of the vertebral column, both arms and other postcrania.

[2] In 2013, Lindsay E. Zanno and Peter Makovicky estimated its body mass at around 27 kg (60 lb), based on the length of its femur.

[8] In 2024, Gregory S. Paul estimated Beipiaosaurus' body length at 1.8 m (5.9 ft), and its body mass at around 50 kg (110 lb)[9] 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.

They were large and flat, and were separate bones, sutured along the midline; this is contrary to the condition in other therizinosaurs, like Erlikosaurus, where they were fused.

The angular was very thin transversely, and formed the posteroventral (rear-bottom) and ventral (bottom) borders of the external mandibular fenestra.

In the case of the isolated one preserved in lateral view, the centrum was subrectangular in shape, with slightly concave ventral borders.

The one preserved in anterolateral view, interpreted as the middle dorsal, had a gracile neural spine, almost as tall as the dorsoventral (top-to-bottom) height of the centrum, and upswept transverse processes.

A pointed internal tuberosity sat in the proximal end of the humerus, separated from the humeral head by a depression.

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 EBFFs of Beipiaosaurus are also much longer than other primitive feather types, measuring about 100–150 mm (10–15 cm) long, roughly half the length of the neck.

The broadest feathers of Sinosauropteryx are only 0.2 mm (0.020 cm) wide, and only slightly wider in larger forms such as Dilong.

[7] Jianchangosaurus is another primitive therizinosaur taxon known from the same formation that was found with impressions of a series of filamentous and unbranched feathers in its holotype specimen.

[14] The affinities of therizinosaurs were originally obscure and often problematic, giving rise to taxonomic debate since they feature similar adaptations to the unrelated sauropodomorphs and ornithischians.

The description of Beipiaosaurus helped to assemble the definitive placement of therizinosaurs within the Theropoda, especially as maniraptorans thanks to the numerous theropod features and irrefutable feather impressions in the holotype.

[15][13] The cladogram below is the result of the performed phylogenetic analysis of the Therizinosauria by Hartman et al. 2019 which is largely based on the data provided by the revision of Zanno in 2010.

In order to make comparisons with extant feathered dinosaurs, they analyzed several bird taxa such as Anas, Lonchura and Taeniopygia and found similar cell structures, but the fossil dinosaur corneocytes were more densely packed with keratin and lacking lipids (fat), suggesting that Beipiaosaurus and co-analyzed taxa did not get as warm as modern birds, mainly because they were ground-dwelling animals not able to fly.

[17] In addition, the identified corneocytes structures seem to indicate that non-avian dinosaurs had a similar way of shedding skin to extant birds and mammals.

[18] The average yearly temperature during the time of Beipiaosaurus was 10 °C (50 °F), with relatively cold winters for the generally warm Mesozoic era.

[3][21] Other contemporaries of Beipiaosaurus included ancient shrimp, snails and slugs, as well as a diverse group of insects, and fish such as Lycoptera.

Most vertebrates in this formation showed a tendency to become arboreal, including many tree-dwelling birds, and climbing mammals and lizards.

The flora was dominated by conifers related to modern species that are found mainly in subtropical and temperate upland forests, with the presence of ferns, cycads, and horsetails.