| Beipiaosaurus | |
|---|---|
 | |
| Components of the holotype specimen (prior to the caudal vertebrae re-excavation) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | †Therizinosauria |
| Superfamily: | †Therizinosauroidea |
| Genus: | †Beipiaosaurus Xu et al.1999 |
| Type species | |
| †Beipiaosaurus inexpectus Xu et al. 1999 | |
| Synonyms[1] | |
Jianchangosaurus?Pu et al. 2013 | |
Beipiaosaurus/ˌbeɪpjaʊˈsɔːrəs/ is agenus oftherizinosauroidtheropoddinosaurs that lived inChina during theEarly Cretaceous in theYixian Formation. The first remains were found in 1996 and formally described in 1999. Before the discovery ofYutyrannus,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 2.2 m (7.2 ft) long and weighing about 27 kg (60 lb) in contrast to the advanced and giantSegnosaurus orTherizinosaurus. The necks ofBeipiaosaurus were shorter than in most therizinosaurs, whose are characterized by elongated necks adapted forhigh-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 theirprosauropod-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 derivedTherizinosauridae, to becoelurosaurian theropods, notsauropodomorph orornithischian relatives as once believed.
In 1996, a farmer, Li Yinxian discovered a partial skeleton of a theropod dinosaur near the village ofSihetun. The following year, it was confirmed to have come from the lower beds of theYixian Formation and represented a single individual. On May 27, 1999, the discovery was announced in the academic journalNature and thetype speciesBeipiaosaurus inexpectus named and described byXu Xing,Tang Zhilu andWang Xiaolin. The generic nameBeipiaosaurus translates as "Beipiao lizard" afterBeipiao, a city inChina near the location of its discovery.Beipiaosaurus is known from a single species,B. inexpectus, thespecific name, meaning "unexpected" inLatin, referring to the "surprising features in these animals".[2]
Theholotype (type specimen) ofBeipiaosaurus inexpectus,IVPP V11559, was recovered in the Jianshangou Beds of the Yixian Formation inLiaoning Province,China. The specimen was collected in sediment deposited during theAptianstage of theEarly Cretaceous period, approximately 125 million years ago.[2][3][4] It is housed in the collection of theInstitute of Vertebrate Paleontology and Paleoanthropology, inBeijing, China. It consists of a partial, sub-adult, skeleton that is largely disarticulated. A significant number of fossilized bones were recovered, including: cranial fragments, amandible,teeth, threecervical vertebrae, fourdorsal vertebrae, four dorsalribs, twosacral vertebrae, twenty-fivecaudal vertebrae with apygostyle, threechevrons, an incompletefurcula andscapula, bothcoracoids, both forelimbs, bothilia, an incompletepubis, an incompleteischium, afemur, bothtibiae (one incomplete), an incompletefibula, theastragalus andcalcaneum, severaltarsals,metatarsals, manual and pedalunguals, and skin impressions of the primitive plumage.[2][5] 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.[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 consisted of a complete skull, a sclerotic ring, the mandible, the atlas and axis bones, nine additional cervical vertebrae, dorsal vertebrae, seventeen cervical ribs, twelve dorsal ribs, both scapulae and coracoids, one complete humerus and proximal humerus, one complete radius and distal radius, one complete ulna and distal ulna, carpals, and some metacarpals. The rear of the skull of this specimen was badly crushed.[6] Li et al. 2014 mentioned a third specimen labelled under the numberBMNHC 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.[7]
Beipiaosaurus was a small therizinosaur, with the largest specimen having an estimated length of 2.2 m (7.2 ft).[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 estimatedBeipiaosaurus' 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 functionaltoes, but the feet ofBeipiaosaurus 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 thefemur. The neck appears to be shorter compared to other therizinosaurs.[2][6] In 2003 thepygostyle, 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.[5]
The skull ofBeipiaosaurus was fairly large, proportionally, compared to other therizinosaurs.[2] Thepremaxilla andmaxilla are not known, and only onenasal 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 inBeipiaosaurus, though this may betaphonomic. Theposterior (rear) half of the rightfrontal 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 theanterior (front) portion; it is three times the width, as opposed to twice the width. It is flattened, as inFalcarius andJianchangosaurus, as opposed to the domed shape seen inErlikosaurus and most coelurosaurs. Theparietals 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, likeErlikosaurus, where they were fused. Thepostorbital differs from that ofErlikosaurus in having a proportionally shorter frontal process a longer accessorymedial (midline) process, and a more dorsal (higher) rugose area. The posterior half ofBeipiaosaurus' skull (thebraincase) is represented exclusively by thelaterosphenoid andprootic bones.[10]
Beipiaosaurus' mandibles (lower jaws) are represented by a nearly complete rightdentary, lacking the anterior and posterodorsal (top-rear) portions, the part of the leftsurangular, and part of the leftangular. 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 5 mm (0.20 in), whereas at its highest, it is 20 mm (0.79 in). This size ratio is greater than that of most other therizinosaurs. TheMeckelian groove is deep, narrow, and anterior, as opposed to the more dorsal one ofErlikosaurus. The left surangular forms most of the lateral surface of the mandible's posterior half, and bears twoforamina on its lateral surface. The angular was very thin transversely, and formed the posteroventral (rear-bottom) andventral (bottom) borders of theexternal mandibular fenestra.[10]
Based on the amount of sockets in the dentary'salveolar margin,Beipiaosaurus likely had more than thirty-seven teeth in each dentary,[2][10] similar toAlxasaurus andEshanosaurus, but higher than most other therizinosaurs. The preserved teeth are fairly homodont (even in size and shape), and folidont[10] (leaf-shaped with prominentdenticles),[11] though it is not clear whether those near the front of the dentary were enlarged and conidont[10] (conical and with reduced denticles),[11] as in other therizinosaurs. There denticles preserved are similar in size to those ofJianchangosaurus, 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 toErlikosaurus.[10]
Four ofBeipiaosaurus'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. Thecentra of the cervical vertebrae wereamphicoelous, meaning that they were concave on either end. Based onJianchangosaurus and STM 31-1, nine or ten were likely present. Theneural arch of each vertebra, as seen from above, is wider than the corresponding centrum. Theneural 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. Sixdorsal (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 upswepttransverse processes.[12] The last four dorsal vertebrae were fused.[13] The number ofsacral vertebrae inBeipiaosaurus is unknown. Thecaudal (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, lackingpneumatisation. Anteriorly, the neural spines are subequal in height to the centra, though decrease in height posteriorly.[12] The vertebrae towards the tip of the caudal column were fused into apygostyle.[12][13]
The pectoral girdle ofBeipiaosaurus is known from a rightscapula, bothcoracoids, and a partialfurcula. Like other basal therizinosaurs (and some more derived genera), the scapula and coracoid were separate, though this may beontogenetic. The coracoids were subrectangular, similar to most other therizinosaurs, except forJianchangosaurus. The scapular blade was long and slender, slightly widening distally (far from the body axis). Theglenoid 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. Thehumerus was relatively straight, like some later therizinosaurs. A pointed internal tuberosity sat in the proximal end of the humerus, separated from thehumeral head by a depression. Based on other basal therizinosaurs, the size ratio between the humerus and theulna was roughly seventy-seven to seventy-eight percent. The ulnar shaft is oval-shaped medially, flatening distally. Theradius was slightly larger than the ulna, though was more gracile. Ninecarpal (wrist) elements are preserved inBeipiaosaurus, 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 thephalanges (digit bones; finger bones, in this case). The manualunguals (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.[12]
Theilium ofBeipiaosaurus was shaped like aparallelogram, similar to that of birds and dromaeosaurids, but unlike that of later therizinosaurs. The pubicpeduncle (to which thepubis attached) was longer than the ischiadic peduncle (to which theischium attached), unlike more basal taxa and like more derived taxa. The pubes lack the proximal and distant ends, while the right ischium preserves the proximal portion and the left ischium preserves the shaft and distal portions. The pubes and ischia were almost equal in size, with the pubes being slightly longer. Thefemur was gracile, intermediate between the curved femur ofFalcarius and the straight femur of later therizinosaurs. Thelesser trochanter was winglike, separated from thegreater trochanter by a narrow, deep cleft. Thefourth trochanter was thin and long. Thetibia was gracile and slender, with a relatively straight shaft. The fibula was slenderer and more gracile still.Tarsal (foot) elements are known in the form of the rightastragalus,calcaneum, and a distal tarsal. Themetatarsus is relatively complete, though only portions are visible through thematrix. The first metatarsal was robust. It did not contact the tarsus, and articulated with the second metatarsal on the mid-shaft. This differs from the condition seen in later therizinosaurs, which were functionallytetradactyl. The pedal (foot) phalanges are disarticulated and broken, so identification is difficult.[12]
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 ofSinosauropteryx, but longer, and oriented perpendicular to the arm. Xu et al. 1999 suggested that these downy feathers represent an intermediate stage betweenSinosauropteryx and more advanced birds (Avialae).[2] The tail was also covered in feathers between 4–7 cm (1.6–2.8 in), consisting of parallel filaments with a width of 1.5 mm (0.15 cm), without a trace ofpennaceous feathers or a tail fan, as indicated by the preservedpygostyle.[5] A secondary coat of much longer, simpler feathers rose out of the down layer. These 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 preparation. Some of these were damaged during preparation.[6]
The EBFFs differ from other feather types in that they consist of a single, unbranched filament. Most other primitivefeathered 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 ofBeipiaosaurus 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. InSinosauropteryx, the longest feathers are only about 15% of the neck length. The EBFFs ofBeipiaosaurus are also unusually broad, up to 3 mm (0.30 cm) wide in the holotype. The broadest feathers ofSinosauropteryx are only 0.2 mm (0.020 cm) wide, and only slightly wider in larger forms such asDilong. 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.[6]
Li et al. 2014 compared the color and shape of the melanosomes in 181 extant animal specimens, 13 fossil specimens (includingBeipiaosaurus) and previous data about the melanosome diversity usingscanning electron microscopes. They found that color in dinosaurs seem to be slightly connected with their physiology. While some species of living reptiles (lizards or crocodiles, which areectothermic) have less diversity in the shape of melanosomes and darker color ranges, some maniraptorans, birds and mammals (which areendothermic) have an increased diversity of melanosome shapes and more vivid colors. The examined specimen ofBeipaosaurus, BMNHC PH000911, preserves feather impressions which are located in the neck area. These are filamentous/sparse in structure and the sampled melanosomes were sphere-shaped and inferred to had a brownish colouration like those in modern reptiles which fall within the range of dark brownish colourations.[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. Only the distal ends of the feather impressions are visible and based on their morphology the feathers are considered to be EBFFs, bearing resemblance to those found along the specimens ofBeipiaosaurus. These findings suggest that they might have been used for visual display and were common among early therizinosaurs.[14]
The affinities of therizinosaurs were originally obscure and often problematic, giving rise to taxonomic debate since they feature similar adaptations to the unrelatedsauropodomorphs andornithischians. The description ofBeipiaosaurus helped to assemble the definitive placement of therizinosaurs within theTheropoda, especially asmaniraptorans thanks to the numerous theropod features and irrefutable feather impressions in the holotype.[2]Beipiaosaurus was first assigned to theTherizinosauroidea, in a very primitive position by Xu et al. 1999.[2] All subsequentphylogenetic analyses have confirmed this assignment. According to the definition byPaul Sereno of this group,Beipiaosaurus is even by definition the basal most member.Lindsay E. Zanno noted thatBeipiaosaurus shares a sister-taxon relationship withFalcarius, a taxon that includes all the more derived therizinosauroids, however, it appears to be thatFalcarius is more primitive thanBeipiaosaurus.[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.Beipiaosaurus occupied a more derived position thanFalcarius, as previously indicated by Zanno:[16]
| Therizinosauria |
| |||||||||||||||||||||||||||||||||
Cau (2024) suggested that the contemporaryJianchangosaurus represents ajunior synonym ofBeipiaosaurus, since the diagnosis distinguishing the taxa was based onontogenetically variable characters. As such,Jianchangosaurus would represent a less mature individual ofBeipiaosaurus.[1]
In 2018, McNamara and colleagues discovered the fossilised remains of skin flakes from numerous feathered dinosaurs from theJehol Biota and somebird species usingscanning electron microscope on the preserved feather impressions. The analyzed fossil taxa consisted ofConfuciusornis,Beipiaosaurus,Microraptor andSinornithosaurus. ForBeipiaosaurus, the specimen STM 31-1 was analyzed.[17]
By exposing the skin flakes under an electron microscope they foundcorneocytes, which arecells rich inkeratin. In order to make comparisons with extant feathered dinosaurs, they analyzed several bird taxa such asAnas,Lonchura andTaeniopygia and found similar cell structures, but the fossil dinosaur corneocytes were more densely packed with keratin and lackinglipids (fat), suggesting thatBeipiaosaurus and co-analyzed taxa did not get as warm as modern birds, mainly because they were ground-dwelling animals not able to fly. In the case of the primitive birdsConfuciusornis, they could not fly at all for long periods. In modern birds these structures, with the addition of fats, help to regulate body temperature during active flight.[17]
In addition, the identified corneocytes structures seem to indicate that non-avian dinosaurs had a similar way ofshedding skin to extant birds and mammals. Unlike many reptiles alive today (lizards or snakes) which shed their skin as a single piece or as several large pieces,Beipiaosaurus and other non-avian dinosaurs shed their skin asdandruff, likeConfuciusornis, modern birds or mammals.[17]
Studies suggest that thepaleoenvironment of theYixian Formation involved seasonal climate fluctuations, and was warm and humid, punctuated by dry seasons, in which the environment became more arid.[18] The average yearly temperature during the time ofBeipiaosaurus was 10 °C (50 °F), with relatively cold winters for the generally warm Mesozoic era.[19] A study by Wu et al. 2013 concluded thatorbital forcing, which is the effect on climate caused by shifts in the tilt of the Earth's axis and by the shape of the Earth's orbit, contributed to the climate fluctuations of this formation.[20]
The Yixian Formation is well known for its great diversity of well-preserved specimens and its dinosaurs, such as the tyrannosauroidsDilong andYutyrannus, the dromaeosauridsSinornithosaurus, oviraptorosaurs includingCaudipteryx, compsognathids includingSinocalliopteryx, avialans includingConfuciusornis and some non-theropod dinosaurs, such asPsittacosaurus andDongbeititan.[3][21]
Other contemporaries ofBeipiaosaurus included ancient shrimp, snails and slugs, as well as a diverse group of insects, and fish such asLycoptera. Most vertebrates in this formation showed a tendency to becomearboreal, including many tree-dwelling birds, and climbing mammals and lizards. The flora was dominated by conifers related to modern species that are found mainly insubtropical andtemperate upland forests, with the presence of ferns, cycads, and horsetails.[3][18][21]
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