Scansoriopterygidae

Taxobox
name = Scansoriopterygids
fossil_range = Jurassic - Early Cretaceous?

---

image_width = 200px
image_caption = Illustration of "Epidendrosaurus"
regnum = Animalia
phylum = Chordata
classis = Sauropsida
superordo = Dinosauria
ordo = Saurischia
subordo = Theropoda
unranked_familia = Maniraptora
familia = Scansoriopterygidae
familia_authority = Czerkas, 2002
subdivision_ranks = Genera
subdivision = "Epidendrosaurus" "Epidexipteryx" [Due to a pre-publication error, this is not a technically valid name] "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). The structure of their hands also bears some similarity to the feathered maniraptoran "Yixianosaurus".Xu, X., and Wang, X. (2003). "A new maniraptoran from the Early Cretaceous Yixian Formation of western Liaoning." "Vertebrata PalAsiatica," 41(3):195-202.] Because of their juvenile nature, the size of a full-grown scansoriopterygid dinosaur is unknown. Most specimens known so far are tiny, sparrow-sized creatures.

The type specimens of all described scansoripterygids contain the fossilized impression of feathers.Zhang, F., Zhou, Z., Xu, X. & Wang, X. (2002). "A juvenile coelurosaurian theropod from China indicates arboreal habits." "Naturwissenschaften", 89: 394-398. doi:10.1007 /s00114-002-0353-8.] Czerkas, S.A., and Yuan, C. (2002). "An arboreal maniraptoran from northeast China." Pp. 63-95 in Czerkas, S.J. (Ed.), "Feathered Dinosaurs and the Origin of Flight." The Dinosaur Museum Journal 1. The Dinosaur Museum, Blanding, U.S.A. [http://www.dinosaur-museum.org/featheredinosaurs/arboreal_maniraptoran.pdf PDF abridged version] ] Zhang, F., Zhou, Z., Xu, X., Wang, X., & Sullivan, C. (2008). "A bizarre Jurassic maniraptoran from China with elongate ribbon-like feathers." "Available from Nature Precedings", doi:10.1038/npre.2008.2326.1 [http://precedings.nature.com/documents/2326/version/1/files/npre20082326-1.pdf PDF full text] .]

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.

Taxonomy

"Epidendrosaurus" and "Epidexipteryx" (and possibly "Scansoriopteryx", if it is a distinct genus) comprise the family Scansoriopterygidae, though the exact taxonomic placement of this family is currently uncertain. Some scientists, such as Paul Sereno, initially considered the concept redundant because the group was originally monotypic and lacked a phylogenetic definition as a clade.Sereno, P. C. (2005). " [http://www.taxonsearch.org/dev/taxon_edit.php?Action=View&tax_id=495 Scansoriopterygidae] ." Stem Archosauria—TaxonSearch [version 1.0, 2005 November 7] ] However, in 2008 Zhang "et al." reported a second scansoriopterygid, "Epidexipteryx", and defined the clade as the most recent common ancestor of "Epidexipteryx" and "Epidendrosaurus" plus all its descendants.Zhang, Fucheng, Zhou, Zhonghe, Xu, Xing, Wang, Xiaolin, and Sullivan, Corwin. A bizarre Jurassic maniraptoran from China with elongate ribbon-like feathers. Available from Nature Precedings (2008)]

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.Senter, P. (2007). "A new look at the phylogeny of Coelurosauria (Dinosauria: Theropoda)." "Journal of Systematic Palaeontology", (doi|10.1017/S1477201907002143).] This view was supported by a second phylogenetic analysis performed by Zhang "et al." in 2008. These authors included the additional taxon "Epidexipteryx", and found formed a monophyletic clade with "Epidendrosaurus" at the base of Avialae. An abbreviated version of Zhang "et al."'s 2008 cladogram is presented below.

clade| style=font-size:100%;line-height:80%
label1=Maniraptora
1=clade
1=Therizinosauroidea
label2=unnamed
2=clade
1=Alvarezsauridae
2=Oviraptorosauria
label2=unnamed
2=clade
label1=unnamed
1=clade
1=Troodontidae
2=Dromaeosauridae
label2=Avialae
2=clade
label1=Scansoriopterygidae
1=clade
1="Epidendrosaurus"
2="Epidexipteryx"

2=Aves

Paleobiology

"Epidendrosaurus" is cited as being an arboreal (tree-dwelling) maniraptoran based on the elongated nature of the hand and specializations in the foot. 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.

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.

References

External links

* [http://dino.lm.com/images/display.php?id=294 Skeletal diagrams] of both specimens from The Dinosauricon.
* [http://dino.lm.com/taxa/display.php?name=Epidendrosaurus Epidendrosaurus image gallery] , from The Dinosauricon.
* [http://dino.lm.com/taxa/display.php?name=Scansoriopteryx Scansoriopteryx image gallery] , from The Dinosauricon.