''Tianyuraptor'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of short-armed dromaeosaurid

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

('running lizard'; a type of small dinosaur considered to be closely related to birds) that lived during the Early Cretaceous, about 122 million years ago. Its remains have been found in western Liaoning, China. It was similar to other dromaeosaurids found in Liaoning, with the exception of being somewhat more primitive. The

type specimen In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralizes th ...

, formally named in 2009, shows features not seen in previously known Northern Hemisphere ( Laurasian) dromaeosaurids, but present in Southern Hemisphere ( Gondwanan) species and early birds. Because of this, the scientists who first studied ''Tianyuraptor'' described it as a " transitional species", bridging the gap between northern and southern types of dromaeosaurid. ''Tianyuraptor'' also differs from previously known dromaeosaurids in that it possesses a relatively small

furcula The (Latin for "little fork") or wishbone is a forked bone found in most birds and some species of non-avian dinosaurs, and is formed by the fusion of the two pink clavicles. In birds, its primary function is in the strengthening of the thoracic ...

("wishbone"), and unusually short forelimbs.

Etymology

The generic name of ''Tianyuraptor'' combines ''Tianyu'', referring to the Shandong Tianyu Museum of Nature where the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

specimen is stored, with ''raptor'', the Latin word for 'robber', referring to the action of grabbing prey, often used in naming dromaeosaurids. The specific epithet, ''ostromi'', is in honor of

John Ostrom John Harold Ostrom (February 18, 1928 – July 16, 2005) was an American paleontologist who revolutionized modern understanding of dinosaurs in the 1960s. As first proposed by Thomas Henry Huxley in the 1860s, Ostrom showed that dinosaurs were ...

, who contributed greatly to the study of dromaeosaurid fossils, including ''

Deinonychus ''Deinonychus'' ( ; ) is a genus of dromaeosaurid theropod dinosaur with one described species, ''Deinonychus antirrhopus''. This species, which could grow up to long, lived during the early Cretaceous Period, about 115–108 million y ...

'' and

feathered dinosaurs A feathered dinosaur is any species of dinosaur possessing feathers. While this includes all species of birds, there is a hypothesis that many, if not all non-avian dinosaur species also possessed feathers in some shape or form. It has been su ...

Description

''Tianyuraptor'' is a medium-sized dromaeosaurid that has several derived features that separate it from other dromaeosaurids. These include the length of the middle caudal (tail) vertebrae being more than twice that of the dorsal (back) vertebrae, a small and extremely slender

, and an unusually long hindlimb that is roughly three times as long as the entire series of dorsal vertebrae. As in other dromaeosaurid fossils discovered in Liaoning, the tail is relatively long at , nearly 4.8 times as long as the femur. The

is STM1–3, a nearly complete and fully articulated skeleton that is only missing the extreme distal end of the tail. A total of 25 fully articulated caudal vertebrae are preserved and at the end three at most are estimated to be missing. The

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

was discovered in the Dawangzhangzi Bed of the

Yixian Formation The Yixian Formation (; formerly transcribed as Yihsien Formation) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the late Barremian and early Aptian stages of the Early Cretaceous. It is known for its exq ...

( Jehol Group), located in Lingyuan in western Liaoning, China. The Yixian Formation is an Early Cretaceous rock unit, dated to between approximately 129.7 and 122.1 million years old, in the

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 129.4 ± 1.5 Ma (million years ago) and 121.4 ± 1.0 Ma). It is a subdivision of the Early Cretaceous Epoch (or Lower Cretaceous Series). It is precede ...

and Aptian faunal stages. The Dawangzhangzi Bed specifically has been dated to about 122 million years ago. STM1–3 is believed to be a sub-adult, with features including the incomplete fusion of skeletal parts during

ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the s ...

. The holotype of ''Tianyuraptor'' preserves no soft tissues, unlike many other

theropod Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...

specimens from the Jehol Group.

Limbs

The forelimbs are comparatively short, being only 53% of the hindlimbs' length. This differs greatly from the known skeletal elements of other dromaeosaurids, most of which have relatively long forelimbs that are more than 70% of the hindlimbs' length. While ''Tianyuraptor'' is larger in size than all other known microraptorines, it also has relatively elongated lower hindlimbs, like other microraptorines. In this regard, it is different from most other dromaeosaurids, which have relatively short lower legs. For example, ''Tianyuraptor'' has a

tibiotarsus The tibiotarsus is the large bone between the femur and the tarsometatarsus in the leg of a bird. It is the fusion of the proximal part of the tarsus with the tibia. A similar structure also occurred in the Mesozoic Heterodontosauridae. These s ...

/femur length ratio of greater than 1.30, while ''

Velociraptor mongoliensis ''Velociraptor'' (; ) is a genus of small dromaeosaurid dinosaur that lived in Asia during the Late Cretaceous epoch, about 75 million to 71 million years ago. Two species are currently recognized, although others have been assigned in the pa ...

'', a creature of similar size, exhibits a ratio of less than 1.10. Aside from elongate hind limbs, ''Tianyuraptor'' is different from other members of Microraptorinae in regards to the relative lengths of the forelimb elements. The forelimbs of Tianyuraptor are proportionally much shorter than those of larger dromaeosaurids. For example, a similar-sized ''

Velociraptor ''Velociraptor'' (; ) is a genus of small dromaeosaurid dinosaur that lived in Asia during the Late Cretaceous epoch, about 75 million to 71 million years ago. Two species are currently recognized, although others have been assigned in the p ...

'' specimen shows an arm/leg length ratio of approximately 0.75., while ''Tianyuraptor'' has an arm/leg ratio of 0.53.

Paleobiology

The stark difference in arm length compared to other dromaeosaurids implies that the function of the arms in ''Tianyuraptor'' was different from that of other dromaeosaurs. Members of the subfamily Microraptorinae, such as ''

Microraptor ''Microraptor'' (Greek, μικρός, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China. They ...

'', have been suggested to have been aerodynamic and may have glided. Microraptorines are usually noted for their long and robust forelimbs and large, asymmetrical flight feathers. However, the shortened forelimbs, small furcula, and the transversely wide

coracoid A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...

in ''Tianyuraptor'' suggest that it was not suited for aerodynamic gliding or flight.

Classification

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

analysis performed by Zheng ''et al.'' showed ''Tianyuraptor'' to be a basal member of a group containing Laurasian dromaeosaurids. ''Tianyuraptor'' seems to possess several features that are unknown in other Laurasian dromaeosaurids, but which are seen in basal avialans and Gondwanan dromaeosaurids, including '' Austroraptor'', ''

Buitreraptor ''Buitreraptor'' (meaning "La Buitrera seizer") is a genus of dromaeosaurid dinosaurs that lived during the Late Cretaceous of Argentina at the Candeleros Formation. ''Buitreraptor'' was described in 2005 and the type species is ''Buitreraptor go ...

'', '' Neuquenraptor'', ''

Rahonavis ''Rahonavis'' is a genus of bird-like theropods from the Late Cretaceous (Maastrichtian, about 70 mya) of what is now northwestern Madagascar. It is known from a partial skeleton ( UA 8656) found by Catherine Forster and colleagues in Maevaran ...

'', and ''

Unenlagia ''Unenlagia'' (meaning "half-bird" in Latinized Mapudungun) is a genus of dromaeosaurid theropod dinosaur that lived in South America during the Late Cretaceous period. The genus ''Unenlagia'' has been assigned two species: ''U. comahuensis'', ...

''. Zheng and colleagues also noted that ''Tianyuraptor'' shares some features with the monophyletic subfamily

Microraptorinae Microraptoria (Greek, μίκρος, ''mīkros'': "small"; Latin, ''raptor'': "one who seizes") is a clade of basal dromaeosaurid theropod dinosaurs. The first microraptorians appeared 125 million years ago in China. Many are known for long feather ...

, though they went on to say that this mixture of features suggests a basal placement for ''Tianyuraptor'' within Microraptorinae, as evidenced by their phylogenetic analysis which indicated maximum parsimony in six of the 30 results recovered by the analysis. The authors then go on to suggest that since ''Tianyuraptor'' is considered a short-armed microraptorine, more derived long-armed microraptorines might have independently evolved flight capability. However, it is also equally possible, as argued by Zheng ''et al.'', that ''Tianyuraptor'' may in fact be a basal member of a clade containing all other Laurasian dromaeosaurids with the exception of Microraptorinae. This is indicated by the other 24 out of 30 most parsimonious trees recovered from the analysis. The discovery of ''Tianyuraptor'' sheds new light on the early evolution of dromaeosaurs and further exemplifies the great diversity this group enjoyed at an early stage.

References