Epidendrosaurus

Epidendrosaurus Temporal range: Late Jurassic/Early Cretaceous
A sketch of the hatchling Epidendrosaurus
Conservation status
Fossil
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Superorder:	Dinosauria
Order:	Saurischia
Suborder:	Theropoda
Infraorder:	Coelurosauria
(unranked):	Maniraptora
Family:	Scansoriopterygidae Czerkas, 2002
Genus:	Epidendrosaurus
Binomial name
Epidendrosaurus ninchengensis Zhang et al., 2002
Synonyms
Scansoriopteryx heilmanni Czerkas, 2002

Epidendrosaurus ("upon-tree lizard") was an Early Cretaceous (or possibly Middle Jurassic, see "Provenance") maniraptoran dinosaur and probably the sole genus in the family Scansoriopterygidae. It 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. 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 Epidendrosaurus is its 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 .

Taxonomy

Epidendrosaurus (and possibly Scansoriopteryx, if it is a distinct genus) comprise the family Scansoriopterygidae ("climbing wings"), 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 .

There has been some degree of uncertainty regarding the status of the name Epidendrosaurus. The type specimen was described online only a few months before a very similar specimen, named Scansoriopteryx heilmanni, and it 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 the name Epidendrosaurus if/when formally adopted .

Known specimens:

• Zhang Specimen (IVPP-V12653, the holotype): The type specimen of Epidendrosaurus ninchengensis ("Nincheng County branch lizard"). This specimen consists mostly of bone imprints in both part and counter part. Impressions indicate a relatively long tail.
• Czerkas Specimen (CAGS02-IG-gausa-1/DM 607): The type specimen of Scansoriopteryx heilmanni (Heilmann's climbing feather"). The paper describes wispy, hair-like impressions seen 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 tail is short compared to the Zhang Specimen, but part of the tail may have been sheared off in the separating of the rock slabs. If the tail turns out to be truly short, it would confirm that this specimen represents a distinct genus and species.

Implications

Czerkas and Yuan use the suite of primitive and birdlike characters in scansoriopterygids to argue for an unorthodox interpretation of dinosaur evolution. They state that scansoriopterygids are "clearly more primitive than Archaeopteryx", based on their primitive, "saurischian-style" pubis and robust ischia. Scansoriopterygids also lack 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 allow that the hole may have closed secondarily, having evolved from a more traditional dinosaurian hip socket, they cite the other primitive features to argue that it is a true primitive trait, which would make scansoriopterygids among the most birdlike and the most primitive known dinosaurs. Czerkas and Yuan term them "proto-maniraptorans", 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 take this idea further than Paul, however, and lend 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, Epidendrosaurus-like ancestors. Czerkas and Yuan also argue 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

Epidendrosaurus hails 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) . 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 criticize He et al. for not including enough specifics and detail in their paper, and also take issue with their radiometric dating of the Daohugou tuff. The tuff, Gao and Ren argue, contains 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 cannot determine the overall age of the deposits in which Epidendrosaurus (along with the other Daohugou fossils) were found. Gao and Ren go 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."

Paleobiology

Arboreality

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.

In describing their scansoriopterygid specimen, Czerkas & Yuan cite further evidence for an arboreal lifestyle. They note that, unlike all modern bird hatchlings, the forelimbs of scansoriopterygids are longer than the hind limbs. The authors argue that this anomaly indicates the forelimbs played an important role in locomotion even at an extremely early developmental stage. The Scansoriopteryx specimen has a better-preserved foot than the type of Epidendrosaurus, and the authors interpret the hallux as reversed, the condition of a backward-pointing toe being widespread among modern tree-dwelling birds. Furthermore, the authors point 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 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.

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.

External links

• Skeletal diagrams of both specimens from The Dinosauricon.
• Epidendrosaurus image gallery, from The Dinosauricon.
• Scansoriopteryx image gallery, from The Dinosauricon.