| Scansoriopteryx Temporal range: Late Jurassic, 154 Ma |
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|---|---|
| Artist's reconstruction of a hatchling | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Dinosauria |
| Order: | Saurischia |
| Suborder: | Theropoda |
| Family: | †Scansoriopterygidae |
| Genus: | †Scansoriopteryx Czerkas & Yuan, 2002 |
| Species: | †S. heilmanni |
| Binomial name | |
| Scansoriopteryx heilmanni Czerkas & Yuan, 2002 |
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| Synonyms | |
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Epidendrosaurus ninchengensis |
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Scansoriopteryx ("climbing wing") is a genus of avialan 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.[1]
Most researchers regard this genus as a synonym of Epidendrosaurus, with some preferring to treat Scansoriopteryx as the junior synonym,[2][3] though it was the first name to be validly published.[4]
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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 "primitive" 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.[1]
A second specimen, the holotype of Epidendrosaurus ninchengensis (IVPP V12653), also shows features indicating it was a juvenile. The specimen is partially disarticulated, and most bones are preserved as impressions in the rock slab, rather than three-dimensional structures.[5] Impressions indicate a relatively long tail.
One distinctive feature of Scansoriopteryx is its elongated third finger, which is the longest on the hand (nearly twice as long as the second finger), and may be analogous to the insect-digging finger of the mammalian aye-aye (in most theropod dinosaurs, the second finger is the longest). Scansoriopteryx is also notable for its wide, rounded jaws. The lower jaw contained at least twelve teeth, larger in the front of the jaws than in the back. The lower jaw bones may have been fused together, a feature otherwise known only in the oviraptorosaurs. The tail was long, six or seven times the length of the femur, and ended in a fan of feathers.[5]
Because the only known specimens are juvenile, the size of a full-grown Scansoriopteryx is unknown–the type specimen is a tiny, sparrow-sized creature.[5]
Scansoriopteryx lent its name to the family Scansoriopterygidae. Studies of dinosaur relationships have found Scansoriopteryx to be a close relative of true birds and a member of the clade Avialae.[6]
The status of the name Scansoriopteryx has been controversial. The type specimen was described only a few months after a very similar specimen, Epidendrosaurus ninchengensis, was described online, though the name Epidendrosaurus was not published in print until after Scansoriopteryx.[1] These two specimens are so similar that they may be the same genus, in which case Article 21 of the International Code of Zoological Nomenclature (ICZN) would give priority to Scansoriopteryx. The journal in which Scansoriopteryx appeared has a very small circulation, but was distributed on roughly 2002-09-02, before the print appearance of Epidendrosaurus, but well after the later's appearance on the Internet, enough time for the name Epidendrosaurus to have come into wide use by experts. This situation was used as an example in a proposed amendment to the ICZN by Jerry Harris that would consider electronic articles with Digital Object Identifiers (DOIs) that are subsequently available in print to qualify as "publication" for naming purposes. Harris noted that while the name Epidendrosarus appeared first, Scansoriopteryx was the first to be published in print and is therefore the valid name, but the fact that the ICZN does not recognize online names as valid has led to confusion over which has priority.[4] In scientific literature, the genus Scansoriopteryx has been treated as a senior synonym of Epidendrosaurus by some scientists, such as Thomas R. Holtz, Jr.[7] and Alan Feduccia,[3] and as a junior synonym by other such as Kevin Padian.[2]
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.[1]
The provenance of the Scansoriopteryx type specimen is uncertain, as it was obtained from private fossil dealers who did not record exact geologic data. Czerkas and Yuan initially reported that it had likely come from the Yixian Formation, 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.[8], but this is hotly contested. See the Daohugou Beds article for details.
The holotype skeleton of Epidendrosaurus was recovered from the Daohugou fossil beds of northeastern China. In the past, there has been some uncertainty regarding the age of these beds. Various papers have placed the fossils here anywhere from the Middle Jurassic period (169 million years ago) to the Early Cretaceous period (122 ma).[9] The age of this formation has implications for the relationship between Epidendrosaurus and similar dinosaurs, as well as for the origin of birds in general. A Middle Jurassic age would mean that the bird-like dinosaurs in the Daohugou beds are older than the "first bird", Archaeopteryx, which was Late Jurassic in age. The provenance of Scansoriopteryx is uncertain, though Wang et al. (2006), in their study of the age of the Daohugou (see below), suggest that it probably hails from the same beds, and thus is likely a synonym of Epidendrosaurus.
A 2004 study by He et al. on the age of the Daohugou Beds found them to be Early Cretaceous, probably only a few million years older than the overlying Jehol beds of the Yixian Formation.[10] The 2004 study primarily used radiometric dating of a tuff within the Daohugou Bed to determine its age. However, a subsequent study by Gao & Ren took issue with the He et al. study. Gao and Ren criticized He et al. for not including enough specifics and detail in their paper, and also took issue with their radiometric dating of the Daohugou tuff. The tuff, Gao and Ren argued, contained crystals with a variety of diverse radiometric ages, some up to a billion years old, so using dates from only a few of these crystals could not determine the overall age of the deposits in which Epidendrosaurus (along with the other Daohugou fossils) were found. Gao and Ren went on to defend a Middle Jurassic age for the beds based on biostratigraphy (the use of index fossils) and the bed's relationship to a layer that is known to mark the Middle Jurassic-Late Jurassic boundary.[11]
Another study, published in 2006 by Wang et al., found that the Tiaojishan Formation (159-164 million years old) underlies, rather than overlies, the Daohugou Beds. After taking into account the great similarity between the Daohugou fauna and the fauna of the Yixian Formation, the authors concluded that the Daohugou probably represents the earliest evolutionary stages of the Jehol Biota, and that it "belongs to the same cycle of volcanism and sedimentation as the Yixian Formation of the Jehol Group."[8] Later in 2006, Liu et al. published their own study of the age of the Daohugou beds, this time using Zircon U-Pb dating on the volcanic rocks overlying and underlying salamander-bearing layers (salamanders are often used as index fossils). Liu et al. found that the beds formed between 164-158 million years ago, in the Middle to Late Jurassic.[12]
Several tiny fossil eggs discovered in the Sao Khua Formation of Phu Phok, Thailand (one of which contained an embryo) were originally thought to have been laid by a small dinosaur similar to Scansoriopteryx.[13] However, further study showed that the eggs actually belonged to an anguimorph lizard.[14]
In describing Scansoriopteryx, Czerkas & Yuan cited evidence for an arboreal (tree-dwelling) 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.[1]
The Epidendrosaurus was also interpreted as arboreal based on the elongated hand and specializations in the foot.[5] The describing 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 the Epidendrosaurus specimen 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.
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.[1] Scales were preserved near the base of the tail,[1] and the Epidendrosaurus specimen also preserved faint feather impressions at the end of the tail, similar to the pattern found in Microraptor.[5]