Scansoriopterygidae

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Scansoriopterygids Fossil range: Jurassic - Early Cretaceous?
Illustration of Epidendrosaurus
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Sauropsida Superorder: Dinosauria Order: Saurischia Suborder: Theropoda (unranked) Maniraptora Family: Scansoriopterygidae Czerkas, 2002
Genera
Epidendrosaurus Scansoriopteryx (type)

Scansoriopterygidae (meaning "climbing wings") may be a family of maniraptoran dinosaurs known from well-preserved fossils uncovered in Liaoning, China.

Scansoriopteryx and Epidendrosaurus were the first non-avian dinosaurs found that had clear adaptations to an arboreal or semi-arboreal lifestyle--it is likely that they spent much of their time in trees. All known specimens show features indicating they were juveniles, which makes it difficult to determine their exact relationship to other non-avian dinosaurs and birds. One distinctive feature of this group is their elongated third finger, which is the longest on the hand, and bears a vague resemblance to the mammalian aye-aye (in most theropod dinosaurs, the second finger is the longest). Because of their juvenile nature, the size of a full-grown scansoriopterygid dinosaur is unknown. The specimens known so far are tiny, sparrow-sized creatures.

The type specimens of both Epidendrosaurus and Scansoriopteryx contain the fossilized impression of feathers.^[1][2]

The age of these animals is not resolved. See the article about the Daohugou Beds for a summary of recent studies, which variously propose a date ranging between some 170 to about 120 mya. Currently, the available evidence seems to favor a later date during this period, but by no means conclusively so.

Contents 1 Taxonomy 2 Paleobiology 3 References 4 External links

[edit] Taxonomy

Epidendrosaurus (and possibly Scansoriopteryx, if it is a distinct genus) would comprise the family Scansoriopterygidae, though the exact taxonomic placement of this family is currently uncertain. Some scientists, such as Paul Sereno, consider this family invalid because is has not been given a phylogenetic definition and is redundant (as Sereno considers the two known species to be synonymous, an opinion that has never been formally published).^[3] They are definitely maniraptoran theropod dinosaurs, and share many features in common with birds. They may be close reletives of deinonychosaurs, or avians themselves. The structure of their hands bears some similarity to the feathered maniraptoran Yixianosaurus.^[4]

In his 2007 cladistic analysis of relationships among coelurosaurs, Phil Senter found Epidendrosaurus to be the closest dinosaurian relative of true birds, and a member of the clade Avialae.^[5] An abbreviated version of Senter's 2007 cladogram is presented below.

Maniraptora	Therizinosauroidea unnamed Alvarezsauridae unnamed Oviraptorosauria Paraves Deinonychosauria Avialae Epidendrosaurus Aves

[edit] Paleobiology

Epidendrosaurus is cited as being an arboreal (tree-dwelling) maniraptoran based on the elongated nature of the hand and specializations in the foot.^[1] The authors stated that the long hand and strongly curved claws are adaptations for climbing and moving around among tree branches. They viewed this as an early stage in the evolution of the bird wing, stating that the forelimbs became well-developed for climbing, and that this development later lead to the evolution of a wing capable of flight. They stated that long, grasping hands are more suited to climbing than to flight, since most flying birds have relatively short hands. Zhang et al. also noted that the foot of Epidendrosaurus is unique among non-avian theropods. While Epidendrosaurus does not preserve a reversed hallux, the backward-facing toe seen in modern perching birds, its foot was very similar in construction to more primitive perching birds like Cathayornis and Longipteryx. These adaptations for grasping ability in all four limbs makes it likely that Epidendrosaurus spent a significant amount of time living in trees.

In describing Scansoriopteryx, Czerkas & Yuan cited further evidence for an arboreal lifestyle. They noted that, unlike all modern bird hatchlings, the forelimbs of Scansoriopteryx are longer than the hind limbs. The authors argued that this anomaly indicates the forelimbs played an important role in locomotion even at an extremely early developmental stage. Scansoriopteryx has a better-preserved foot than the type of Epidendrosaurus, and the authors interpreted the hallux as reversed, the condition of a backward-pointing toe being widespread among modern tree-dwelling birds. Furthermore, the authors pointed to the short, stiffened tail of Scansoriopteryx as a tree-climbing adaptation. The tail may have been used as a prop, much like the tails of modern woodpeckers. Comparison with the hands of modern climbing species with elongated third digists, like iguanid lizards, also supports the tree-climbing hypothesis. Indeed, the hands of scansoriopterygids are much better adapted to climbing than the modern tree-climbing hatchling of the Hoatzin.^[2]

Both known scansoriopterygid specimens are juveniles, and preserve impressions of simple, down-like feathers, especially around the hand and arm. The longer feathers in this region led Czerkas and Yuan to speculate that adult scansoriopterygids may have had reasonably well-developed wing feathers which could have aided in leaping or rudimentary gliding, though they ruled out the possibility that Scansoriopteryx could have achieved powered flight. Like other maniraptorans, scansoriopterygids had a semilunate (half-moon shaped) bone in the wrist that allowed for bird-like folding motion in the hand. Even if powered flight was not possible, this motion could have aided manuverablitiy in leaping from branch to branch.^[2]

[edit] References

[edit] External links

• Skeletal diagrams of both specimens from The Dinosauricon.
• Epidendrosaurus image gallery, from The Dinosauricon.
• Scansoriopteryx image gallery, from The Dinosauricon.