Beipiaosaurus

Beipiaosaurus Temporal range: Early Cretaceous, 124.6Ma PreЄ Є O S D C P T J K Pg N ↓
Life restoration
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Theropoda
Superfamily:	†Therizinosauroidea
Genus:	†Beipiaosaurus Xu, Tang & Wang, 1999
Species:	† B. inexpectus
Binomial name
Beipiaosaurus inexpectus Xu, Tang & Wang, 1999

Beipiaosaurus /ˌbeɪpjaʊˈsɔrəs/ is a genus of therizinosauroid theropod dinosaur from the Early Cretaceous of China. Before the discovery of Yutyrannus, it was among the largest dinosaurs known from direct evidence to be feathered.

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.

Discovery

In 1996, the peasant Li Yinxian discovered a skeleton of a theropod dinosaur near the village of Sihetun. In the May 27, 1999, issue of the journal Nature the discovery was announced and the type species Beipiaosaurus rarus 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 this animal".^[1]

The type specimen of Beipiaosaurus inexpectus, holotype 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 Cretaceous period, approximately 125 to 124 million years ago.^[2] This specimen 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, a pygostyle, three chevrons, an incomplete furcula and scapula, two coracoids, a complete forelimb, a partial forelimb, both ilia, an incomplete pubis, and incomplete ischium, a femur, both tibiae (one incomplete), an incomplete fibula, an astragalus, a calcaneum, several tarsals, metatarsals, and unguals, and remains of the integument, including feathers.

Referred specimen

A second specimen, STM 31-1, a partial skeleton, was described by Xu et al. in 2009, which preserved a significant covering of unique, elongated feathers. This specimen consisted of a complete skull, a sclerotic ring, the mandible, the atlas and axis bones and nine additional cervical vertebrae, dorsal vertebrae, seventeen cervical ribs, twelve dorsal ribs, both scapulae and coracoids, one complete humerus and a proximal humerus, one complete radius and a distal radius, one complete ulna and a distal ulna, carpals, and some metacarpals. ^[3] The rear of the skull of this specimen was badly crushed.

Description

Beipiaosaurus measured 2.2 meters (7.3 ft) in length, and before the discovery of Yutyrannus is among the largest dinosaurs known from direct evidence to be feathered.^[1]

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. Beipiaosaurus had a toothless beak with cheek teeth. The head was large relative to other therizinosaurs, with the lower jaw measuring about same length as the femur.^[1] In 2003 the pygostyle, consisting of the fused five last vertebrae of the tail, was described in greater detail.^[4]

Distinguishing anatomical features

A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism.

According to Zanno (2010b), Beipiaosaurus can be distinguished from its direct relatives based on the following characteristics, which are its unique derived traits, or autapomorphies:^[5]

• A low ridge is present on the front of the thighbone shaft which extends upwards from the inner condyle (according to Zanno, 2008)
• The last four dorsal vertebrae are fused (according to Zanno, 2008)
• 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 (according to Zanno, 2008)
• The skull is large, about as long as the thighbone (according to Xu et al., 1999)
• 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 (according to Xu et al., 1999)
• On the front edge of the ischium a projection, the obturator process, has a wavy profile with a tip that is curved downwards (according to Zanno, 2008)
• 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 (according to Zanno, 2008)

Feathers

Skin impressions from the type specimen of B. inexpectus indicated that the body was covered predominately by downy feather-like fibers, similar to those of Sinosauropteryx, but longer, and are oriented perpendicular to the arm. Xu et al., who described the specimen, suggested that these downy feathers represent an intermediate stage between Sinosauropteryx and more advanced birds (Avialae).^[1] The tail was covered in feathers between four and seven centimeters long, consisting of parallel filaments with a width of 1.5 millimeters, without a trace of pennaceous feathers or a tail fan.^[4]

Unique among known theropods, Beipiaosaurus also possessed a secondary coat of much longer, simpler feathers that rose out of the down layer.^[6] These unique feathers (known as EBFFs, or elongated broad filamentous feathers) were first described by Xu et al. in 2009, based on a specimen consisting of the torso, head and neck. Xu and his team also found EBFFs in the original type specimen of B. inexpectus, revealed by further preparation.

The EBFFs differ from other feather types in that they consist of a single, unbranched filament. Most other primitive feathered 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 of Beipiaosaurus are also much longer than other primitive feather types, measuring about 100-150 millimeters (4-6 inches) long, roughly half the length of the neck. In Sinosauropteryx, the longest feathers are only about 15% of the neck length.

The EBFFs of Beipiaosaurus are also unusually broad, up to 3 mm wide in the type specimen. The broadest feathers of Sinosauropteryx are only 0.2 mm wide, and only slightly wider in larger forms such as Dilong. 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.

In a 2009 interview, Xu stated: "Both [feather types] are definitely not for flight, inferring the function of some structures of extinct animals would be very difficult, and in this case, we are not quite sure whether these feathers are for display or some other functions." He speculated that the finer feathers served as an insulatory coat and that the larger feathers were ornamental, perhaps for social interactions such as mating or communication.^[6]

Phylogeny and classification

Beipiaosaurus was first assigned to the Therizinosauroidea, in a basal position, by Xu et al. (1999). All subsequent phylogenetic analyses have confirmed this assignment. According to the definition by Paul Sereno of this group, Beipiaosaurus is even by definition the basal most member. Zanno (2010b) noted that Beipiaosaurus shares a sister-taxon relationship with a taxon that includes all the more derived therizinosauroids.^[5] Zanno also concluded that there is a genus that is less derived than Beipiaosaurus, the genus Falcarius, which is a basal therizinosaurian.

The following is a cladogram based on the phylogenetic analysis conducted by Lindsay E. Zanno in 2010, showing the relationships of Beipiaosaurus:^[5]

Paleoecology

Fauna and habitat

Studies suggest that the paleoenvironment of the Yixian Formation involved seasonal climate fluctuations, and was warm and humid, punctuated by dry seasons, in which the environment became more arid.^[7] The average yearly temperature during the time of Beipiaosaurus was 10 degrees celsius (50 degrees Fahrenheit), with relatively cold winters for the generally warm Mesozoic era.^[8] A study by Wu et al. (2013) concluded that orbital 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.^[9]

The Yixian Formation is well known for its great diversity of well preserved specimens and its dinosaurs, such as the large tyrannosauroids Yutyrannus and Dilong, the microraptorine Graciliraptor, the troodontid Mei, the dromaeosaurid Tianyuraptor, the large compsognathid Sinocalliopteryx and several non-theropod dinosaurs, such as Bolong and Dongbeititan.

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.^[10] 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.

See also

• Nothronychus and Falcarius for additional primitive therizinosaur discoveries.

References

External links

• Dinosauria - News of the first Beipiaosaurus discovery.
• Therizinosauroidea - UCMP, Berkeley