Scansoriopteryx

Scansoriopteryx ("climbing wing") is a genus of maniraptoran dinosaur. Described from only a single juvenile fossil specimen found in Liaoning, China, Scansoriopteryx is a sparrow - sized animal that shows adaptations in the foot indicating an arboreal (tree-dwelling) lifestyle. It possessed an unusual, elongated third finger. The type specimen of Scansoriopteryx also contains the fossilized impression of feathers.

Description
The type specimen of Scansoriopteryx heilmanni (specimen number CAGS02-IG-gausa-1/DM 607) represents the fossilized remains of a hatchling maniraptoran dinosaur, similar in some ways to Archaeopteryx. It is notable for its "primitve" non-perforated hip socket, and pubis (hip bone) which points forward, unlike some advanced maniraptorans. Most distinctive is its long third finger, which is almost twice as long as the second finger. This is unlike the configuration seen in all other theropods, where the second finger is longest. It also has an unusually large first toe, or hallux.

Classification
Scansoriopteryx lent its name to the family Scansoriopterygidae, though the exact taxonomic placement of this family is currently uncertain. Also uncertain is the status of the name Scansoriopteryx. The type specimen was described only a few months after a very similar animal, Epidendrosaurus, was described online, though the name Epidendrosaurus was not published in print until after Scansoriopteryx. These two specimens are so similar that they may be the same genus, in which case the name published first would have priority. The body which sets the rules for the naming of animals, the ICZN, has seen an amendment to the code that would favor names published online, such as Epidendrosaurus, if/when formally adopted, but only if the ICZN rules such an amendment to be retroactive will Epidendrosaurus be given priority over Scansoriopteryx. See Epidendrosaurus for more information.

Implications
Czerkas and Yuan used the suite of primitive and birdlike characters in Scansoriopteryx to argue for an unorthodox interpretation of dinosaur evolution. They stated that Scansoriopteryx was "clearly more primitive than Archaeopteryx", based on its primitive, "saurischian-style" pubis and robust ischia. Scansoriopteryx also lacks a fully perforated acetabulum, the hole in the hip socket which is a key characteristic of Dinosauria and has traditionally been used to define the group. While the authors allowed that the hole may have closed secondarily, having evolved from a more traditional dinosaurian hip socket, they cited the other primitive features to argue that it is a true primitive trait, which would make Scansoriopteryx among the most birdlike and the most primitive known dinosaurs. Czerkas and Yuan called it a "proto-maniraptoran", supporting the hypothesis of Gregory S. Paul that the lager, ground-dwelling maniraptorans like Velociraptor evolved from small, flying or gliding forms that lived in trees. The authors took this idea further than Paul, however, and lent support to George Olshevsky's 1992 "birds came first" hypothesis, that all true theropods are secondarily flightless or at least secondarily arboreal, having evolved from small, tree-dwelling, Scansoriopteryx-like ancestors. Czerkas and Yuan also argued that, contrary to most phylogenetic trees, maniraptorans form a separate lineage from other theropods, and that this split occurred very early in theropod evolution.

Provenance
The provenance of Scansoriopteryx is uncertain, though Wang et al. (2006), in their study of the age of the Daohugou Beds, suggested that it probably hails from the same beds, and thus is likely a synonym of Epidendrosaurus. The Daohugou Beds supposedly date to the mid-late Jurassic Period. , but this is hotly contested. See the Daohugou Beds article for details.

Climbing
In describing Scansoriopteryx, Czerkas & Yuan cited 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 well-preserved foot, 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 the Scansoriopteryx specimen 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 digits, like iguanid lizards, also supports the tree-climbing hypothesis. Indeed, the hands of Scansoriopteryx are much better adapted to climbing than the modern tree-climbing hatchling of the Hoatzin.

Feathers and scales
Scansoriopteryx fossils preserve impressions of wispy, down-like feathers around select parts of the body, forming V-shaped patterns similar to those seen in modern down feathers. The most prominent feather impressions trail from the left forearm and hand. 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, Scansoriopteryx 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 maneuverability in leaping from branch to branch.

Scales were also preserved near the base of the tail.