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Feathered Dinos.Pdf NATURE|Vol 461|1 October 2009 NEWS & VIEWS light, but the transmitted light also undergoes any number of them. The challenge now will be was that ancestor. Thus, there is no temporal a geometric Pancharatnam–Berry phase delay to extend the droplet approach to yield larger paradox. At the same time, the idea that some — a change in phase that depends on the ori- numbers of intertwined wavefronts, and to Cretaceous theropods might be flightless entation of the optic axis of the liquid crystal. construct a robust, miniature converter that descendants of early birds can now be assessed Because of the way in which the optic axes are can be used in practical applications. Given on the basis of evidence rather than being man- orientated within the droplets, laser beams the apparent purity of the beams produced dated by temporal congruence. But Anchiornis emerge with a helical wavefront (Fig. 1b), and using Brasselet and colleagues’ strategy, this is a does more than refute the temporal paradox, hence with an orbital angular momentum. challenge well worth pursuing. ■ in that the distribution of feathers on its body Pancharatnam–Berry phase delays have pre- Miles Padgett is in the Department of Physics suggests that we now need to revise our viously been used in macroscopic light-mode and Astronomy, University of Glasgow, thoughts on the evolution of flight. converters based on liquid crystals7, but never Glasgow G12 8QQ, UK. In 2003, the description of Microraptor gui as before has the effect been a natural conse- e-mail: [email protected] a ‘four-winged dinosaur’7 rocked the palaeon- quence of microscopic droplet structure. tological world. This creature, a basal member A surprising feature of Brasselet and col- 1. Poynting, J. H. Proc. R. Soc. Lond. A 82, 560–567 (1909). (that is, on an early-branching twig) of a dif- 2. Brasselet, E., Murazawa, N., Misawa, H. & Juodkazis, S. leagues’ microscopic converter is that it works Phys. Rev. Lett. 103, 103903 (2009). ferent theropod lineage, the dromaeosaurids, over a wide range of optical wavelengths — a 3. Allen, L. et al. Phys. Rev. A 45, 8185–8189 (1992). had elongate, bird-like feathers not just on its feat previously made possible only using com- 4. He, H., Friese, M. E. J., Heckenberg, N. R. & Rubinsztein- arms but also on its legs and feet. This find was 8 Dunlop, H. Phys. Rev. Lett. 75, 826–829 (1995). binations of optical components . In their 5. Franke-Arnold, S., Allen, L. & Padgett, M. Laser Photon. Rev. entirely unexpected in that, although feath- present form, however, the inherent structure 2, 299–313 (2008). ered forelimbs and tails had been reported in of the droplets2 means that the resulting beam 6. Knoner, G. et al. Opt. Express 15, 5521–5530 (2007). various non-avian theropods, there was little 7. Marrucci, L., Manzo, C. & Paparo, D. Phys. Rev. Lett. 96, contains only two intertwined wavefronts, 163905 (2006). reason to suspect that elongate pennaceous whereas traditional approaches can generate 8. Leach, J. & Padgett, M. J. New J. Phys. 5, 154 (2003). feathers (that is, with shaft and vanes, as in the flight feathers of living birds) occurred on the legs, let alone the feet. Later, elongate feathers were found on the legs and feet in Pedopenna, PALAEONTOLOGY a basal member of the avialans, the group that includes Archaeopteryx and other birds8. Anchiornis, a basal troodontid, also has long Feathered dinosaurs in a tangle feathers on its legs and feet to match those on Lawrence M. Witmer its arms and tail, so the family Troodontidae now joins the Dromaeosauridae and Avialae A dramatic feathered dinosaur fossil from the Jurassic of China resolves a ‘temporal paradox’. But it adds intriguing complications to the debates on the evolution of feathers and flight in birds. Other birds Late Birds are dinosaurs. That’s hardly the stuff of Cretaceous rocks. Even more significantly, Other troodontids headlines any more, as data have streamed Anchiornis is older (by 5 million to 10 million 100 Other dromaeosaurids in revealing anatomical similarities between years) than the iconic ‘first bird’ Archaeopteryx, birds and the theropod dinosaurs from the which comes from younger Jurassic rocks in tips of their noses to the tips of their feathered Germany. Cretaceous tails. More elusive have been the details of the One lingering problem with the hypothesis transition to birds and the evolution of flight. that birds descended from dinosaurs had been Microraptor On page 640 of this issue, Hu and colleagues1 that the most bird-like theropods occurred present a spectacular new specimen of the later in time than did Archaeopteryx. It has 146 feathered theropod Anchiornis huxleyi that been argued that this ‘temporal paradox’ (how Archaeopteryx solves some problems. But it simultaneously can a ‘descendant’ arise before an ‘ancestor’?) Pedopenna creates new ones, revealing what a gloriously both invalidates the theropod ancestry of Late 161 Anchiornis 3 messy business it is to tease apart the evolu- birds and, reversing the ancestor–descendant Dromaeosauridae Avialae tionary tangles that we retrospectively anoint relationship, suggests that some of the Creta- Jurassic Troodontidae Paraves as an ‘origin’. ceous bird-like theropods actually descended Middle Early Anchiornis is a small, crow-sized theropod, from Jurassic Archaeopteryx-like birds4,5. Figure 1 | Anchiornis huxleyi in context. The assigned to a group known as the troodon- In truth, the temporal paradox never seri- 1 fossil described by Hu et al. is assigned to the tids (Fig. 1), which in life was covered with ously challenged the theropod hypothesis, family Troodontidae, which together with the long bird-like feathers. The new fossil, like because it essentially assumed that fossils like closely related Dromaeosauridae and Avialae other, more poorly preserved specimens, Anchiornis wouldn’t be found — arguments comprise the Paraves (itself a subgroup of the was collected from the Tiaojishan Formation based on negative evidence are always dicey. theropod dinosaurs). One significant aspect of Liaoning, China. Liaoning Province has However, the notion of some Cretaceous thero- of Anchiornis is that it predates Archaeopteryx, yielded many specimens of feathered thero- pods being secondarily flightless descend- the iconic ‘first bird’, by some 5 million to pods and true birds2, and so it might seem that ants of early birds remains a valid hypothesis 10 million years. Another is that it shows that yet another feathered dinosaur shouldn’t merit given the common and repeated evolution of basal members of all three of the Paraves groups much attention. But what’s important about the flightlessness in birds6. — Anchiornis, Microraptor and Pedopenna — had long pennaceous feathers on their lower legs fossils of Anchiornis is their age — they are from Anchiornis resets that whole debate. By pre- and feet, as well as on their hands and tail. The the Jurassic period, and at about 155 million dating Archaeopteryx, Anchiornis shows that implication is that avian evolution conceivably years old are much older (by about 25 million bird-like feathered theropods were around went through a ‘four-wing’ stage. Numbers are to 35 million years) than the other feathered ‘early enough’ to serve as ancestors, although approximate ages of the geological divisions in Liaoning theropods, which come from Early no one is suggesting that Anchiornis itself millions of years ago. 601 © 2009 Macmillan Publishers Limited. All rights reserved 5599-60599-605 NewsNews & ViewsViews MH.inddMH.indd 601601 225/9/095/9/09 117:13:417:13:41 NEWS & VIEWS NATURE|Vol 461|1 October 2009 on the list of theropods with ‘hind wings’. Biomedical Sciences, Ohio University College of 4. Paul, G. S. Dinosaurs of the Air (Johns Hopkins Univ. Press, Among modern birds, only a few species have Osteopathic Medicine, Athens, Ohio 45701, USA. 2002). e-mail: [email protected] 5. Feduccia, A. et al. Auk 124, 373–380 (2007). long feathers on their legs (the tibiotarsal 6. Witmer, L. M. in Mesozoic Birds (eds Chiappe, L. M. & region), and none has long, aerodynamically Witmer, L. M.) 3–30 (Univ. California Press, 2002). relevant feathers on their feet. So palaeon- 1. Hu, D., Hou, L., Zhang, L. & Xu, X. Nature 461, 640–643 7. Xu, X. et al. Nature 421, 335–340 (2003). tologists have been scrambling to make sense (2009). 8. Xu, X. & Zhang, F. Naturwissenschaften 92, 173–177 2. Norell, M. A. & Xu, X. Annu. Rev. Earth Planet. Sci. 33, (2005). of what feathered legs and feet in basal birds 277–299 (2005). 9. Padian, K. BioScience 53, 450–452 (2003). and dromaeosaurids mean for the evolution 3. Feduccia, A. The Origin and Evolution of Birds (Yale Univ. 10. Chatterjee, S. & Templin, R. J. Proc. Natl Acad. Sci. USA 104, of flight. Press, 1996). 1576–1580 (2007). When we just had Microraptor, it was easier to dismiss the long foot feathers as potentially a mere early experiment in aerodynamics that was independent of the evolution of avian SUPRAMOLECULAR CHEMISTRY flight9,10. And indeed, it potentially biases the functional argument to refer to these elongate leg and foot feathers as ‘flight feathers’ that Molecular crystal balls formed a lift-generating ‘hind wing’, in that other Seth M. Cohen functions are conceivable9 (such as display). Still, these elongate feathers would have Sorcerers have long gazed into crystal balls to conjure up information. had aerodynamic effects even if they had Chemists are also getting in on the act, using porous crystals to trap not evolved originally as flight adaptations, and credible aerodynamic models have been unstable reaction intermediates and to reveal their structures. proposed10. But Anchiornis shows that Micro- raptor was no one-off, and that basal members The reactions of molecules with one another Kawamichi et al.
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