Scansoriopterygidae
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 A sketch of Scansoriopteryx
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Extinct (fossil)
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Scansoriopterygidae ("climbing wings") is a family of maniraptoran dinosaurs known from well-preserved fossils uncovered in Liaoning, China. They 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].
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[edit] Taxonomy
Epidendrosaurus (and possibly Scansoriopteryx, if it is a distinct genus) comprise the family Scansoriopterygidae, though the exact taxonomic placement of this family is currently uncertain. 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 [3].
[edit] Paleobiology
[edit] Arboreality
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]
[edit] Flight
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 wings, and leave open the possibility that adults of this family could achieve some form of powered or gliding 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
- ^ a b 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.
- ^ a b c 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. PDF abridged version
- ^ Xu, X., and Wang, X. (2003). "A new maniraptoran from the Early Cretaceous Yixian Formation of western Liaoning." Vertebrata PalAsiatica, 41(3):195-202.
[edit] External links
- Skeletal diagrams of both specimens from The Dinosauricon.
- Epidendrosaurus image gallery, from The Dinosauricon.
- Scansoriopteryx image gallery, from The Dinosauricon.
