Epidendrosaurus

Epidendrosaurus ("upon-tree lizard") is an extinct genus of mid-Mesozoic (see "Provenance") maniraptoran dinosaur of the family Scansoriopterygidae. Epidendrosaurus was the first non-avian dinosaur found that had clear adaptations to an arboreal or semi-arboreal lifestyle--it is likely that it spent much of its time in trees. The only known specimen (IVPP V12653) shows features indicating it was a juvenile. One distinctive feature of Epidendrosaurus is its elongated third finger, which is the longest on the hand, and may be analogous to the insect-digging finger of the modern-day mammalian aye-aye (in most theropod dinosaurs, the second finger is the longest). Because the specimen is a juvenile, the size of a full-grown Epidendrosaurus is unknown--the type specimen is a tiny, sparrow-sized creature. The type specimens of Epidendrosaurus contains the fossilized impression of feathers.

Taxonomy
Epidendrosaurus belongs to the family Scansoriopterygidae ("climbing wings"), though the exact taxonomic placement of this family is uncertain. Studies of dinosaur relationships have found Epidendrosaurus to be a close relative of true birds and a member of the clade Avialae.

There has been some degree of uncertainty regarding the status of the name Epidendrosaurus. The type specimen was described online in the online version of the journal Naturwissenschaften on August 21, 2002, while the identical print version (except for the date) was not published until September 30, 2002. However a very similar specimen named Scansoriopteryx heilmanni was described in the August 1, 2002 issue of The Dinosaur Museum Journal.

These two specimens are so similar that they may be the same genus, in which case a strict interpretation of Article 21 of the International Code of Zoological Nomenclature (ICZN) would give priority to Scansoriopteryx. However, the journal in which Scansoriopteryx appeared has a very small circulation and the issue was not distributed until roughly September 2, 2002, well after the electronic appearance of Epidendrosaurus. The conflict is used as an example in a proposed amendment to the ICZN that would consider electronic articles with Digital Object Identifiers (DOIs) that are subsequently available in print to qualify as "publication" for naming purposes, which if formally adopted would favor Epidendrosaurus. In scientific literature, the genus Scansoriopteryx has been treated as a senior synonym of Epidendrosaurus by Alan Feduccia and as a junior synonym by Kevin Padian.

The holotype specimen of Epidendrosaurus ninchengensis (catalogue number IVPP-V12653) consists mostly of bone imprints in both part and counter part. Impressions indicate a relatively long tail, unlike the apparently short tail seen in Scansoriopteryx.

Provenance
The fossilized 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 mya). 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 Daogugou (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, where Scansoriopteryx was found. 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.

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." 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.

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 state that the long hand and strongly curved claws are adaptations for climbing and moving around among tree branches. They view 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 state that long, grasping hands are more suited to climbing than to flight, since most flying birds have relatively short hands.

Zhang et al. also note 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.

The type specimen of Epidendrosaurus also preserved faint feather impressions at the end of the tail, similar to the pattern found in the dromaeosaurid Microraptor. While the reproductive strategies of Epidendrosaurus itself remain unknown, several tiny fossil eggs discovered in Phu Phok, Thailand (one of which contained the embryo of a theropod dinosaur) may have been laid by a small dinosaur similar to Epidendrosaurus or Microraptor. The authors who described these eggs estimated the dinosaur they belonged to would have had the adult size of a modern Goldfinch.