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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 theropods might be flightless entation of the optic axis of the liquid crystal. construct a robust, miniature converter that descendants of early 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 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 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 of flight. converters based on liquid crystals7, but never Glasgow G12 8QQ, UK. In 2003, the description of gui as before has the effect been a natural conse- e-mail: [email protected] a ‘four-winged ’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, -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 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 , PALAEONTOLOGY a basal member of the avialans, the group that includes and other birds8. Anchiornis, a basal troodontid, also has long Feathered in a tangle feathers on its legs and feet to match those on Lawrence M. Witmer its arms and , so the family now joins the and A dramatic from the of 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 ) 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 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 - 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 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 based on negative evidence are always dicey. theropod dinosaurs). One significant aspect of , 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.

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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 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 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 (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. examined a reaction familiar of three separate theropod groups had long often proceed through intermediates before the to every student of organic chemistry: the com- foot feathers. final products are formed. Such intermediates bination of an amine and an aldehyde to form This association of Troodontidae, Dromaeo- are frequently short-lived and unstable, which a Schiff base (Fig. 1a). Although the mecha- sauridae and Avialae is no chance occurrence. restricts our ability to characterize them. Typi- nism of this fundamental reaction has been These groups together form a branch of the cally, the identification of such fleeting species extensively studied, direct observations of the theropod evolutionary tree known as Paraves is limited to fast, time-resolved spectroscopic ephemeral intermediate — a hemiaminal — — they’re each other’s closest relatives (Fig. 1). measurements. But in this issue (page 633), are rare. The crystal structure of a hemiami- The fact that basal members of all three groups Kawamichi et al.1 report that they have trapped nal trapped in the active site of an enzyme has had long pennaceous feathers on their lower an unstable chemical intermediate in a porous been reported2, but structure determination in legs and feet strongly suggests that the para- crystalline material, and were thereby able to protein crystals is not a general approach for vian common ancestor also had feathered characterize the structure of the intermediate characterizing reaction intermediates. feet. It’s not at all certain yet whether these unambiguously by X-ray crystallography. The The authors1 use a ‘coordination network’ feathers comprised an aero dynamically com- authors suggest that such porous crystals can of organic ligand molecules and metal ions petent flight surface that provided lift and/ act as protective matrices within which chemi- to trap the elusive hemiaminal. Coordination or thrust. But it’s hard to imagine that they cal reactions can be performed, allowing us to networks — also known as porous coordina- wouldn’t have had some aerodynamic effects peer into the details of reaction mechanisms tion polymers or metal–organic frameworks (drag, for instance). in an unprecedented way. — are solid, crystalline materials that generally More to the point, it now looks as if we’ll

have to accept that avian evolution indeed a HO H H CH CH3 went through — at the risk of overstatement NH2 3 O NH –H O N — a four-wing stage, only to eventually lose + 2 the long foot feathers. What this means for the H CH3 evolution of the avian flight stroke9 is now an open question. Likewise, we’ll need to seri- ously consider how these otherwise seemingly Amine Aldehyde Unstable hemiaminal Schiff base intermediate very adept and agile runners (Anchiornis has extremely long and slender hindlimbs) could b manage with long feathers on their feet. N Even apart from feathers and aerodynam- 1 NH Self- ics, Hu and colleagues’ analysis of Anchiornis 2 assembly + Zinc ions + reveals just how similar these early paravians N N were to each other: the basal members of each N of the three groups seem to mix-and-match N N their anatomical similarities, so that their Linker unique attributes are becoming more and more

subtle. It’s getting hard to tell members of one 1 group from another. On the bright side, in this Figure 1 | Caught in a trap. a, Kawamichi et al. have stabilized and observed the crystal structure of Darwin, that fact provides a comfort- of an elusive hemiaminal intermediate that is transiently formed during the reaction of an amine and an aldehyde to form a Schiff base. b, They did this by performing the reaction within the ing affirmation of the evolutionary prediction restrictive and orderly confines of a crystalline coordination network (a cage-like molecular structure) that species in different groups will become at low temperatures. The authors trapped amine reactants as guest molecules in the network as it increasingly similar as we approach their self-assembled from its constituent parts — organic ‘linker’ molecules and zinc ions that act as nodes common origin. ■ between the linkers. In the network structure, the amines are shown in green, and one is highlighted Lawrence M. Witmer is in the Department of in the red box.

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