Epidendrosaurus
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Extinct (fossil)
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| Epidendrosaurus ninchengensis Zhang et al., 2002 |
Epidendrosaurus ("upon-tree lizard") was a Middle Jurassic (or possibly Early Cretaceous, see "Provenance") maniraptoran dinosaur of 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. The only known specimen 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 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 Epidendrosaurus is unknown. The only specimen known so far is a tiny, sparrow-sized creature. The type specimens of Epidendrosaurus contains the fossilized impression of feathers.[1]
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[edit] Taxonomy
Epidendrosaurus (and possibly Scansoriopteryx, if it is a distinct genus) belongs to the family Scansoriopterygidae ("climbing wings"), though the exact taxonomic placement of this family is currently uncertain. 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.[2] 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.[3]
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.
[edit] 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).[4] 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.[5] 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.[6]
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."[7]
[edit] Paleobiology
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 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.
[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.
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- ^ Harris, J.D. (2004). "'Published works' in the Electronic Age: recommended amendments to Articles 8 and 9 of the Code." Bulletin of Zoological Nomenclature 61(3): 138-148.
- ^ Ren, D. et al. (2002). "On the biostratigraphy of the Jurassic fossil beds at Daohugou near Ningcheng, Inner Mongolia." Geol. Bull. China 21, 584-591.
- ^ He, H., Wang, X., Zhou, Z., Zhu, R., Jin, F., Wang, F., Ding, X. and Boven, A. (2004). "(^40)Ar/(^39)Ar dating of ignimbrite from Inner Mongolia, northeastern China, indicates a post-Middle Jurassic age for the overlying Daohugou Bed." Geophysical Research Letters 31, L20609.
- ^ Gao, K., and Ren, D. (2006). "Radiometric dating of ignimbrite from Inner Mongolia provides no indication of a post-Middle Jurassic age for the Daohugou Beds." Acta Geologica Sinica English Edition, 80(1): 42-45 (February 2006)
- ^ Wang, X., Zhou, Z., He, H., Jin, F., Wang, Y., Zhang, J., Wang, Y., Xu, X. & Zhang, F. (2005). "Stratigraphy and age of the Daohugou Bed in Ningcheng, Inner Mongolia." Chinese Science Bulletin, 50(20): 2369-2376.
[edit] External links
- Skeletal diagrams of scansoriopterygids from The Dinosauricon.
- Epidendrosaurus image gallery, from The Dinosauricon.
