Today sees the formal publication of the bizarre little Chinese maniraptoran theropod Epidexipteryx hui Zhang et al., 2008 from the Daohugou Formation of Ningcheng County, north-eastern China. Unfortunately the publication of this new species is not quite the surprise it should be, as the authors inadvertently submitted their manuscript to the wrong venue a few weeks ago, thereby making the article visible to the whole world some time before it was ready to be published. Anyway, we’ll just have to pretend that never happened. Belonging to a recently discovered group called the scansoriopterygids, Epidexipteryx is tiny (less than 20 cm long I think), and very, very weird (Zhang et al. 2008). Its skull is short-snouted with a truncated antorbital fenestra, a nostril positioned in a relatively high position, and strongly procumbent, proportionally large anterior teeth (the teeth get smaller further back). Its arms and hands were very elongate (see figure below, from Zhang et al. 2008).
What makes it particularly odd – and this is probably the one ‘take home’ feature that everyone will be talking about – is that it preserves four super-long, strap-like structures growing from its distal-most ten tail vertebrae (these are termed ETFs by the authors, for ‘Elongate ribbon-like Tail Feathers’). These structures are likely to have played a role in display (Zhang et al. 2008), and if they’re homologous with feathers (which is likely), then here is possible evidence that feathers initially evolved for display (unlike birds and other feathered maniraptorans, Epidexipteryx does not have complex feathers on its arms). However…
… what’s likely to be widely overlooked is that the body of Epidexipteryx is covered with ‘non-ETF’ structures that are even more interesting (see figure below, from Zhang et al. 2008). Looking something like flattened, closed flowers with their petals pointing upwards in parallel fashion, the ‘non-ETFs’ have their distal parts composed of filamentous parallel barbs, and they emerge from a membranous base. Epidexipteryx was hence covered in complex, filamentous structures. The potential significance of this will not have been lost on you if you’re familiar with the development of feathers in modern birds, or with the model of feather evolution developed by Prum & Brush (2002, 2003). The latter authors proposed that feathers started out (‘stage 1′) as hollow cylinders, then (‘stage 2′) became unbranched barbs attached to a calamus. By stage 3, feathers were planar structures with the barbs diverging from a central rachis, and from hereon we get the complex vaned feathers that we know and love. It is conceivable the ‘non-ETFs’ of Epidexipteryx represent stage 2 structures, but – given that the structures on Epidexipteryx have a far wider base than the narrow calamus inferred by Prum & Brush (2002, 2003) – perhaps this new fossil hints at a more complicated sequence of steps. Also worth noting is that the pennaceous feathers of birds begin growth as a tubular structure: from a broad ‘follicle collar’ that extends around the base, the rachis extends upwards, the barbs emerging both from it and from the edges of the collar. Might the membranous bases of the ‘non-ETFs’ of Epidexipteryx be homologous with the follicle collar? This is all very speculative, but these are interesting questions that need to be examined.
The short-snouted skull and big, procumbent anterior teeth of Epidexipteryx are very interesting as the same features seem to be cropping up quite a bit among maniraptorans. Basal oviraptorosaurs (like Caudipteryx and Incisivosaurus) were like this, as were at least some basal birds. Based on the behaviour of some extant mammals with procumbent anterior teeth (notably the shrew-opossums or caenolestoids), the procumbency of the teeth in Epidexipteryx suggest that it was grabbing small prey, perhaps insects and/or their larvae. We know from the other scansoriopterygid specimens (read on) that at least some members of this group had very elongate third fingers: were they extracting insects in aye-aye fashion, and then grabbing the exposed prey with procumbent teeth? Having said all that, note that we don’t yet have a taxon that combines both long third fingers and procumbent teeth: it’s not possible to determine whether Epidexipteryx had a long third finger, and Epidendrosaurus (read on) lacks teeth. By the way, is it coincidental that we mostly see enlarged or procumbent anterior teeth in feathered theropod taxa? There is a ver 1 article on this very observation here.
By curious coincidence, I was writing about scansoriopterygids just yesterday (for a current book project) – now I can add the information on this new taxon. Two scansoriopterygids have previously been named, but both are now regarded as synonymous. The first of them is Epidendrosaurus ningchengensis Zhang et al., 2002 and, like Epidexipteryx, it’s from the Daohugou Formation. The age of the Daohugou Formation is controversial: it might be as old as Middle Jurassic or as young as Early Cretaceous, with the most recent studies suggesting the latter. Epidendrosaurus was also tiny (less than 20 cm long), its near-complete skeleton revealing elongate hands and the impression of a long tail. It appears to be a juvenile, possibly a hatchling (Zhang et al. 2002) [adjacent Epidexipteryx by Qiu Ji and Xing Lida, from Carl Zimmer's blog at Discover magazine here].
The second animal is Scansoriopteryx heilmanni Czerkas & Yuan, 2002 from Liaoning Province in north-eastern China. Known from a somewhat more complete skeleton than Epidendrosaurus, its generic name means ‘climbing wing’ while the specific name honours Gerhard Heilmann, the Danish artist and author whose 1927 book The Origin of Birds had been the standard work on avian origins until the 1960s (for more on Heilmann see the discussion here). The Scansoriopteryx specimen preserves a far better skull than Epidendrosaurus, and it also possesses an articulated hand and more complete pelvic girdle (Czerkas & Yuan 2002). The skull was short, with enormous eye sockets and no evidence for teeth. The three-fingered hand was very unusual in that the third finger was much longer than either the first or second (usually, the second finger is the longest in theropods). In the foot, the first toe (the hallux) was positioned very low down and close to the other three toes, and its position on the foot suggested that it might have helped this animal to grab onto twigs and branches when climbing. In contrast to opisthopubic birds and dromaeosaurs, the Scansoriopteryx pelvis is propubic. Filament-like feathers appeared to be preserved adjacent to the hand and elsewhere on the specimen, and what was identified as a small patch of scales were present near the end of the tail. Like the Epidendrosaurus specimen, Scansoriopteryx also appeared to be a hatchling [skeletal reconstruction of Scansoriopteryx below by John Conway].
Epidendrosaurus and Scansoriopteryx appear to be essentially identical and it is now generally agreed that they are the same animal. As has been much discussed, the fact that both appeared in the literature at about the same time made it difficult to be sure which name was older (and thus which had technical priority). It seems that Epidendrosaurus was published first, so this is the name we now use for this animal (Harris 2004).
How did Epidendrosaurus live? The tiny size, very long arms and hands, and low position of the first toe all suggest that it was a climber. Its remarkable third finger might have been used in grasping branches, but it was also noted that the finger recalls that of the aye-ayes Daubentonia (Zhang et al. 2002), where the very long, slim third finger is used to extract grubs from their burrows. Given that both Epidendrosaurus specimens are very young juveniles, and given that substantial changes can occur in anatomy and skeletal proportions during growth, it is possible that adults were very different. Now that we have Epidexipteryx, is it just a grown-up Epidendrosaurus? Epidexipteryx and Epidendrosaurus would seem to differ in that the former reportedly has a shortened tail, whereas the latter had a long one including 22 vertebrae at least (Czerkas & Yuan 2002). However, a break in the Epidexipteryx tail leaves room for doubt: maybe it originally had a far longer, Epidendrosaurus-like tail [short-tailed reconstruction of Epidexipteryx shown below]. More work is needed to resolve this.
Previous work has established that Epidendrosaurus is a maniraptoran close to the clade that includes Archaeopteryx and other birds (Zhang et al. 2002, Senter 2007). Zhang et al. (2008) find that Epidexipteryx and Epidendrosaurus form a clade (for nomenclatural reasons this has to be called Scansoriopterygidae Czerkas & Yuan, 2002, even though the generic name Scansoriopteryx is not currently recognised), and that this clade is the sister-taxon to the Archaeopteryx + other bird clade. The name Aves has been used by some authors as node-based for Archaeopteryx + other birds (Chiappe et al. 1996), while the branch-based clade that includes all maniraptorans closer to modern birds than to other taxa is called Avialae (Gauthier 1986). If we follow these recommendations, the scansoriopterygids are the most basal known members of Avialae, but they’re not part of Aves. Given that scansoriopterygids recall oviraptorosaurs and members of other maniraptoran clades in some of their characters, it is conceivable that this position might change in future however.
And with its short skull, procumbent teeth, propubic pelvis and bizarre integument, here is more evidence that the sequence of character acquisition among maniraptorans was more complicated, and more interesting, than conventionally thought.
PS – thoughts from one of the authors will be posted on Dave’s Hone blog here, some time today, hopefully. Huh, he can make the promises: but can he deliver?
Refs – –
Chiappe. L. M., Norell, M. A. & Clark, J. M. 1996. Phylogenetic position of Mononykus (Aves: Alvarezsauridae) from the Late Cretaceous of the Gobi Desert. Memoirs of the Queensland Museum 39, 557-582.
Czerkas, S. A. & Yuan, C. 2002. An arboreal maniraptoran from northeast China. In Czerkas, S. J. (ed) Feathered Dinosaurs and the Origin of Flight. The Dinosaur Museum (Blanding, Utah), pp. 63-95.
Gauthier, J. 1986. Saurischian monophyly and the origin of birds. Memoirs of the California Academy of Science 8, 1-55.
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, 138-148.
Prum, R,. O. & Brush, A. H. 2002. The evolutionary origin and diversification of feathers. The Quarterly Review of Biology 77, 261-295.
– . & Brush, A. H. 2003. Which came first, the feather or the bird? Scientific American 286 (3), 84-93.
Senter, P. 2007. A new look at the phylogeny of Coelurosauria (Dinosauria: Theropoda). Journal of Systematic Palaeontology 5, 429-463.
Zhang, F., Zhou, Z., Xu, X. & Wang, X. 2002. A juvenile coelurosaurian theropod from China indicates arboreal habits. Naturwissenschaften 89, 394-398.
– ., Zhou, Z., Xu, X., Wang, X. & Sullivan, C. 2008. A bizarre Jurassic maniraptoran from China with elongate ribbon-like feathers. Nature 455, 1105-1108.