Beipiaosaurus is a genus of therizinosauroid theropod dinosaurs that lived in China during the Early Cretaceous in the Yixian Formation. The first remains were found in 1996 and formally described in 1999. Before the discovery of Yutyrannus, Beipiaosaurus were among the heaviest dinosaurs known from direct evidence to be feathered. Beipiaosaurus is known from three reported specimens. Numerous impressions of feather structures were preserved that allowed researchers to determine the feathering color which turned out to be brownish.
They were relatively small-sized therizinosaurs, measuring long and weighing about in contrast to the advanced and giant Segnosaurus or Therizinosaurus. The necks of Beipiaosaurus were shorter than in most therizinosaurs, whose are characterized by elongated necks adapted for high-browsing. Also, their feet configuration differs from therizinosaurids, having a generic three-toed pes instead of four as seen in other members.
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.
History of discovery
In 1996, a farmer, Li Yinxian discovered a partial skeleton of a theropod dinosaur near the village of Sihetun. The following year, it was confirmed to have come from the lower beds of the Yixian Formation and represented a single individual. On 27 May 1999, the discovery was announced in the academic journal Nature and the type species Beipiaosaurus inexpectus 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 these animals".
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. It is housed in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology, in Beijing, China. It consists of a partial, sub-adult, skeleton that is largely disarticulated. A significant number of fossilized bones were recovered, including: cranial fragments, a mandible, teeth, three cervical vertebrae, four dorsal vertebrae, four dorsal ribs, two sacral vertebrae, twenty-five caudal vertebrae with a pygostyle, three chevrons, an incomplete furcula and scapula, both coracoids, both forelimbs, both ilia, an incomplete pubis, an incomplete ischium, a femur, both tibiae (one incomplete), an incomplete fibula, the astragalus and calcaneum, several tarsals, metatarsals, manual and pedal unguals, and skin impressions of the primitive plumage. The pelvic girdle and caudal vertebrae were discovered during a re-excavation of the fossil quarry where the first elements of the holotype were found. These rediscovered elements helped to complete the holotype specimen.
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 consists of a complete skull and mandible, a complete series of cervical vertebrae with several cervical ribs, some dorsal vertebrae with several dorsal ribs, both scapulae and coracoids, and much of the forelimbs. The rear of the skull of this specimen was badly crushed. Li et al. 2014 mentioned a third specimen labelled under the number BMNHC PH000911. 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. Traces of feather integument were extensively found around the neck area.
Description
Beipiaosaurus was a small therizinosaur, with the largest specimen having an estimated length of . In 2013, Lindsay E. Zanno and Peter Makovicky estimated its body mass at around , based on the length of its femur. In 2024, Gregory S. Paul estimated Beipiaosaurus' body length at , and its body mass at around . 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 measuring about same length as the femur. The neck appears to be shorter compared to other therizinosaurs. In 2003 the pygostyle, consisting of the fused five last vertebrae of the tail, was described in greater detail, suggesting that the original function of the pygostyle was not linked with pennaceous feathers.
Skull and dentition The skull of Beipiaosaurus was fairly large, proportionally, compared to other therizinosaurs. The premaxilla and maxilla are not known, and only one nasal is preserved, though it is not clear which side of the skull it belonged to. The transverse (side-to-side) arching of the nasal observed in other theropods was absent in Beipiaosaurus, though this may be taphonomic. The posterior (rear) half of the right frontal is preserved. The bone overall is inferred to have been subtriangular, as in other therizinosaurs. Unlike other therizinosaurs, the posterior portion is considerably broader than what is preserved of the anterior (front) portion; it is three times the width, as opposed to twice the width. It is flattened, as in Falcarius and Jianchangosaurus, as opposed to the domed shape seen in Erlikosaurus and most coelurosaurs. The parietals are both preserved. 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 postorbital differs from that of Erlikosaurus in having a proportionally shorter frontal process a longer accessory medial (midline) process, and a more dorsal (higher) rugose area. The posterior half of Beipiaosaurus' skull (the braincase) is represented exclusively by the laterosphenoid and prootic bones.
Beipiaosaurus' mandibles (lower jaws) are represented by a nearly complete right dentary, lacking the anterior and posterodorsal (top-rear) portions, the part of the left surangular, and part of the left angular. The dentary is around sixty-five percent as long as the femur, larger than that of any other therizinosaur. It is long and slender, subtriangular when seen from the side. At its lowest, the dentary is , whereas at its highest, it is . This size ratio is greater than that of most other therizinosaurs. The Meckelian groove is deep, narrow, and anterior, as opposed to the more dorsal one of Erlikosaurus. The left surangular forms most of the lateral surface of the mandible's posterior half, and bears two foramina on its lateral surface. The angular was very thin transversely, and formed the posteroventral (rear-bottom) and ventral (bottom) borders of the external mandibular fenestra.
Based on the amount of sockets in the dentary's alveolar margin, Beipiaosaurus likely had more than thirty-seven teeth in each dentary, similar to Alxasaurus and Eshanosaurus, but higher than most other therizinosaurs. The preserved teeth are fairly homodont (even in size and shape), and folidont (leaf-shaped with prominent denticles), though it is not clear whether those near the front of the dentary were enlarged and conidont (conical and with reduced denticles), as in other therizinosaurs. There denticles preserved are similar in size to those of Jianchangosaurus, with the carina of each crown bearing three denticles per millimetre. The tooth roots are slightly compressed mediolaterally. Resorption pits, containing replacement teeth, are associated with the medial surfaces of some functional teeth's roots, similar to Erlikosaurus.
Postcranial skeleton
Four of Beipiaosaurus' cervical (neck) vertebrae are preserved, all of them incompletely. Since they are not elongated, they likely belong to the posterior portion of the cervical series. The centra of the cervical vertebrae were , meaning that they were concave on either end. Based on Jianchangosaurus and STM 31-1, nine or ten were likely present. The neural arch of each vertebra, as seen from above, is wider than the corresponding centrum. The neural spines of the cervical vertebrae were low and undeveloped, like those of many other therizinosaurs. The postzygapophyseal facets had rounded, ventrally oriented articular surfaces. The posterior cervical vertebrae had ribs that were not fully attached, though this may reflect the ontogenetic stage of the holotype. Six (back) vertebrae are preserved, four of them articulated and exposed in lateral view, and the other two isolated and exposed in anterolateral (exposing the front and side) and lateral views. 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. The last four dorsal vertebrae were fused. The number of sacral vertebrae in Beipiaosaurus is unknown. The caudal (tail) series is represented by a total of thirty preserved vertebrae, though more were certainly present. Most of them are transversely compressed. All of them are amphicoelous, lacking pneumatisation. Anteriorly, the neural spines are subequal in height to the centra, though decrease in height posteriorly. The vertebrae towards the tip of the caudal column were fused into a pygostyle.
The pectoral girdle of Beipiaosaurus is known from a right scapula, both coracoids, and a partial furcula. Like other basal therizinosaurs (and some more derived genera), the scapula and coracoid were separate, though this may be ontogenetic. The coracoids were subrectangular, similar to most other therizinosaurs, except for Jianchangosaurus. The scapular blade was long and slender, slightly widening distally (far from the body axis). The glenoid fossa was oriented posteroventrally (downwards and towards the back), like other basal therizinosaurs. As in many maniraptorans, the forelimbs were somewhat longer than the hind limbs. The humerus was relatively straight, like some later therizinosaurs. A pointed internal tuberosity sat in the proximal end of the humerus, separated from the humeral head by a depression. Based on other basal therizinosaurs, the size ratio between the humerus and the ulna was roughly seventy-seven to seventy-eight percent. The ulnar shaft is oval-shaped medially, flatening distally. The radius was slightly larger than the ulna, though was more gracile. Nine carpal (wrist) elements are preserved in Beipiaosaurus, though their identification is difficult, as their morphologies and positions do not line up. The manus (hand) is fairly complete. It was overall slender, as were the phalanges (digit bones; finger bones, in this case). The manual unguals (claws) were long and recurved, roughly equal in length to the phalanges they articulated to. Unlike in other therizinosaurs, the middle manual ungual was the largest.