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through a fully autonomous photochemicalcycle; but can these molecules repetitively dowork as long as sunlight is available? Theauthors did not use sunlight in the experi-ments they report, but instead treated therotaxanes with a single 10-nanosecond laserpulse at a visible-light wavelength. If continu-ously irradiated with sunlight, the distributionof the rings between the two stations wouldreach a steady state (the exact distributiondepending on the intensity of the light) withina few milliseconds. To generate net flows ofrings between the stations after that, it appearsone would have to switch the light sourcerapidly on and off.

This is in contrast to the performance ofanother family of machine molecules that havecomponents that rotate directionally, ratherthan shuttle linearly12,17. When these rotarymolecules reach a steady state under continu-ous irradiation, as with the rotaxanes the bulkdistribution of the machine components nolonger changes. But even at the steady state, ata sufficiently high temperature, net fluxes ofthe rotor components still occur through dif-ferent pathways between four different rotaryisomers that are present. Under constant irra-diation, the molecules thus operate continu-ously as directionally rotating motors17.

Synthetic molecular motors and machinesare very much in their infancy, and chemistsare still learning the most basic rules for theirdesign and operation. It is a field that can use-fully draw on input from physicists, biologists,engineers and materials scientists. For adeeper understanding of molecular machinesystems to evolve, therefore, it would be highlybeneficial if the terminology used to describe

Feathers are thought to be characteristic of theCoelurosauria, a group of theropod dinosaursthat includes Aves and several other groups.Because feathers are unique and thereforeinnovative in developmental and evolutionaryterms3, we can infer their presence in extinctforms using a method called phylogeneticbracketing. Based on the presence of feathersin some extinct coelurosaurs and all livingbirds, this approach suggests that all coeluro-saurs, with the possible exception of gigan-tic species such as Tyrannosaurus rex, are feathered4.

The small coelurosaur described by Göhlichand Chiappe now enters the picture. This new

the sub-microsecond timescale, even given theconsiderable distance between the rutheniumcomplex and the stations (which also slowsdown the rate of the forward electron-transferreaction). In fact, the charge-separated state hasan average lifetime of around 10 microseconds.

Reduced bipyridinium is a very muchpoorer binding site for the ring, and thecharge-separated state survives long enoughfor about 10% of the rings to undergo signifi-cant brownian motion. Detailed balance isbroken and a net flux of rings occurs as theyshift their allegiance to the unreduced station2. After 10 microseconds, however, as backelectron transfer finally takes place, station 1regains its stickiness. A net flux of rings occursback from station 2, the system returns to itsoriginal equilibrium, and a machine cycle hastaken place. The process can be repeated for atleast a thousand cycles.

This is a fascinating system, working (as domost other light-driven shuttles2,5–8) withoutthe consumption of chemical fuels or the for-mation of waste products. It can be properlycalled a machine because component dis-placements occur in response to an externalstimulus. But is it really best described as a‘nanomotor’? Chemists are still pondering themost useful way to understand the behaviourof the contemporary, early generations of synthetic molecular machines, but physics andbiology offer many useful phenomenon-basedideas in this regard. Brownian motors, forexample, use mechanisms9 that harness ran-dom molecular-level motion like that of thering motion in rotaxanes.

Such brownian ratchet mechanisms, whichare believed to account for the behaviour ofsome motor proteins10, all require detailed balance to be broken to allow a net, directedflux of particles. Crucially, however, the‘ratchet’ part of the mechanism ensures thatthe resulting change in particle distribution isnot undone when the motor is reset. Thisallows the machine to be able to pump the particle distribution further and further awayfrom equilibrium (as with the enzymes thatsynthesize the currency of intracellular energytransfer, ATP) or move itself progressivelydown a track (as with the cell’s internal pack-horse, kinesin).

This feature is missing from Balzani andcolleagues’ rotaxane1, and other simple mol-ecular shuttles2,5–8: the work done in breakingdetailed balance is undone by the reset step.For such a system to be understood to be amotor by a statistical physicist or biologist,therefore, the rings would have to be divertedalong a different track during the reset phase(making a rotary motor), or remain wherethey are while the machine is reset (making alinear motor or pump). The former is the basisof several wholly synthetic molecularmotors11–15; the latter has thus far beenachieved only with artificial structures madefrom DNA16.

Balzani and colleagues’ shuttle operates

PALAEONTOLOGY

Scales, feathers and dinosaurs Xing Xu

A fossil dinosaur that ‘nests’ with feathered relations in the dinosaurphylogenetic tree did not, it seems, have feathers. The discovery willencourage a re-evaluation of feather evolution.

Only birds have feathers — or so we thoughtuntil the discovery of fossils of feathereddinosaurs in China and elsewhere1. Since then,palaeontologists and biologists have togetherbeen painting a simple picture of feather evo-lution based on evidence from fossils and fromdevelopmental biology. A new example of apredatory dinosaur, described by Göhlich andChiappe on page 329 of this issue2, makes thatpicture a little bit more complicated.

The context is provided by Figure 1 (over-leaf), which shows the generally understoodrelationships among the main groups ofdinosaur (Aves — the birds — is the only oneof these groups to have extant members).

them were to become consistent across all thecontributing disciplines. Balzani and col-leagues’ latest photochemical experiments1

represent a fascinating advance in our under-standing of how a charge-separated state canbe used to bring about a nuclear displacementin a unimolecular machine. It will doubtlessprove an important stage on the route towardsautonomous artificial nanomotors powered by sunlight. ■

Euan R. Kay and David A. Leigh are in the School of Chemistry, University of Edinburgh,King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, UK.e-mail: david.leigh@ed.ac.uk

1. Balzani, V. et al. Proc. Natl Acad. Sci. USA 103, 1178–1183(2006).

2. Brouwer, A. M. et al. Science 291, 2124–2128 (2001).3. Ashton, P. R. et al. Chem. Eur. J. 6, 3558–3574 (2000).4. Onsager, L. Phys. Rev. 37, 405–426 (1931).5. Murakami, H., Kawabuchi, A., Kotoo, K., Kunitake, M. &

Nakashima, N. J. Am. Chem. Soc. 119, 7605–7606 (1997).6. Wurpel, G. W. H., Brouwer, A. M., van Stokkum, I. H. M.,

Farran, A. & Leigh, D. A. J. Am. Chem. Soc. 123, 11327–11328(2001).

7. Altieri, A. et al. Angew. Chem. Int. Edn 42, 2296–2300(2003).

8. Wang, Q.-C., Qu, D.-H., Ren, J., Chen, K. & Tian, H. Angew.Chem. Int. Edn 43, 2661–2665 (2004).

9. Reimann, P. Phys. Rep. 361, 57–265 (2002).10. Astumian, R. D. Science 276, 917–922 (1997).11. Kelly, T. R., De Silva, H. & Silva, R. A. Nature 401, 150–152

(1999).12. Koumura, N., Zijlstra, R. W. J., van Delden, R. A., Harada, N.

& Feringa, B. L. Nature 401, 152–155 (1999).13. Leigh, D. A., Wong, J. K. Y., Dehez, F. & Zerbetto, F. Nature

424, 174–179 (2003).14. Hernández, J. V., Kay, E. R. & Leigh, D. A. Science 306,

1532–1537 (2004).15. Fletcher, S. P., Dumur, F., Pollard, M. M. & Feringa, B. L.

Science 310, 80–82 (2005).16. Kelly, T. R. Angew. Chem. Int. Edn 44, 4124–4127 (2005).17. Feringa, B. L., Koumura, N., van Delden, R. A.

& ter Wiel, M. K. J. Appl. Phys. A 75, 301–308 (2002).

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at least as early as the divergenceof the Compsognathidae fromthe main trunk of the theropodtree, but we don’t know yet when they first evolved, owing to the poor fossil record. Thescaled Juravenator would then be a starting point for featherevolution.

In my opinion, this possibilitycannot yet be ruled out. First, theevidence2 supporting a compso-gnathid affinity for Juravenator is not very strong. Second, thedescription of Juravenator isbased on a juvenile specimen,some features of which are likelyto have pulled the species up thetree if Juravenator followed thedevelopmental patterns revealedby certain other theropod speci-mens12. Finally, although theyagree in most respects, the pro-posed relationships6–9 betweenthe theropods themselves are notentirely stable.

Whatever the explanation, ourknowledge of early feather evolu-tion has been enriched by the discovery of Juravenator. More-over, the discovery emphasizeswhere future research might

fruitfully be concentrated. The Middle-to-LateJurassic, between about 176 million and 146million years ago, was a critical time for theorigins and early evolution of the coeluro-saurian lineages, including birds. But the fossilrecord is poor, and any new coelurosaur fromthis period adds considerably to our under-standing of the group. Juravenator may com-plicate the picture, but it makes it morecomplete and realistic. ■

Xing Xu is at the Institute of VertebratePaleontology and Paleoanthropology, Chinese Academy of Sciences, 142 Xiwai Street, Beijing 100044, China. e-mail: xu.xing@ivpp.ac.cn

1. Norell, M. & Xu, X. Annu. Rev. Earth Planet. Sci. 33, 277–299(2005).

2. Göhlich, U. B. & Chiappe, L. M. Nature 440, 329–332(2006).

3. Prum, R. O. & Brush, A. H. Q. Rev. Biol. 77, 261–295 (2002).4. Xu, X. Integrative Zool. (in the press).5. Chen, P. J., Dong, Z. M. & Zhen, S. N. Nature 391, 147–152

(1998).6. Sereno, P. C. Science 284, 2137–2147 (1999).7. Norell, M. A., Clark, J. M. & Makovicky, P. J. in

New Perspectives on the Origin and Evolution of Birds (eds Gauthier, J. & Gall, L. F.) 49–67 (Yale Univ. Press,2001).

8. Holtz, T. R. Jr Gaia 15, 5–61 (2000).9. Rauhut, O. W. M. The Interrelationships and Evolution of

Basal Theropod Dinosaurs Spec. Pap. Palaeontol. 69(Palaeont. Ass., London, 2003).

10. Yu, M.-K., Wu, P., Widelitz, R. B. & Chuong, C.-M. Nature420, 308–312 (2002).

11. Wellnhofer, P. in Feathered Dragons — Studies on theTransition from Dinosaurs to Birds (eds Currie, P. J.,Koppelhus, E. B., Shugar, M. A. & Wright, J. L.) 282–300(Indiana Univ. Press, 2004).

12. Xu, X. et al. Nature 439, 715–718 (2006).

dinosaur, a carnivore namedJuravenator, is represented by abeautifully preserved skeleton col-lected from Late Jurassic depositsfrom Solnhofen, Germany — thesource of all specimens of the earliest-known bird Archaeop-teryx. At 151 million to 152 mil-lion years old, the deposits inwhich Juravenator was found are only 2 million to 3 millionyears older than those containingArchaeopteryx.

Given the worldwide rarity ofcomplete specimens of smalltheropods from the Jurassic, the exceptionally well-preservedskeleton of Juravenator is in itself anotable find. Most significantly,however, the specimen preservesscaled skin around the tail andhindlimbs. This is a big surpriseconsidering that Juravenator is a small coelurosaur and a mem-ber of a small group called the Compsognathidae — whichincludes the first known feathereddinosaur, Sinosauropteryx, the tail and hindlimbs of which werefeathered5.

The evolution of biologicalstructures must be studied withinan evolutionary framework. In the case offeathers, a robust theropod phylogeny is the basis for reconstructing the sequence in which feathers evolved. The distribution ofvarious feather morphologies on the currentlyaccepted phylogeny6–9 suggests that simple, fil-amentous feathers first evolved no later thanthe earliest stage of coelurosaurian evolution.More complex feathers with a thick centralshaft and rigid symmetrical vanes on eitherside appeared early in the evolution of thecoelurosaurian group Maniraptora; and feath-ers with aerodynamic features, such as a curvedshaft and asymmetrical vanes, appeared withinthe maniraptors but before the origin of birds4.This inferred sequence of events is supportedindependently by developmental data3,10. Göhlich and Chiappe2 place Juravenator withinthe Compsognathidae, a group that is ‘basal’ inthe coelurosaurian tree (Fig. 1). So Juravenatorshould bear filamentous feathers. But it seemsto be a scaled animal, at least on the tail andhind legs.

Why, then, does a member of a feathereddinosaur family bear scales? The authors’answer2 is straightforward: feather evolution,they say, is more complex than we thought.This could well be so. The early evolution ofsome major structures — such as the incorpo-ration of jaw suspensory bones into the middleear of mammals — is often flexible and exper-imental. It would not be surprising if featherswere lost and scaly skin re-evolved in somebasal coelurosaurian species, or if feathersevolved several times independently early in

coelurosaurian evolution. Or perhaps, suggestGöhlich and Chiappe, the absence of featherson body parts that are feathered in othercoelurosaurs is due to variability in the extentof a feathery covering among coelurosaurs.Although modern birds are extensively feath-ered, this may not have been true of theirremote extinct relatives.

Feather preservation is extremely rare andbiased. For example, contour feathers — thosefeathers forming the general covering of a bird— are not seen in some otherwise beautifullypreserved specimens of Archaeopteryx, but areevident in other examples11. Even after the discoveries of so many fossils of feathereddinosaurs, we still have only a patchy picture offeather distribution in basal coelurosaurs. Andmany details are missing — for example, wedon’t know whether the hooked structuresknown as barbules are present in some types offeathers, and how exactly feathers covereddinosaurian bodies. Göhlich and Chiappepropose that Juravenator could be evidencethat the bodies of some basal coelurosaurswere more scaly than those of living birds,which have scales only on their lower legs.This explanation is the most likely, given that the authors have carefully examined and analysed the specimen. Yet other possibil-ities exist.

For example, if Juravenator in fact lies morebasally than the Compsognathidae in thetheropod tree (Fig. 1), we would have a clearerpicture of feather evolution instead of this com-plicated story. We know that feathers appeared

Ornithischia

Maniraptora

Advent of more complexfeathers and, later in thegroup, of feathers withaerodynamic features.

Sauropodomorpha

Ceratosauria

Carnosauria

Juravenator

Other Compsognathidae

Tyrannosauroidea

Ornithomimosauria

Alvarezsauridae

Therizinosauroidea

Oviraptorosauria

Troodontidae

Dromaeosauridae

Aves

Theropoda

Coelurosauria

Advent of simple,filamentous feathers.

Figure 1 | Dinosaurs and feather evolution. This evolutionary tree ofdinosaur groups is compiled from several phylogenetic analyses6–9, andshows the possible occurrence of the main events in feather evolution. Thetree shows alternative positions for Juravenator, the new species describedby Göhlich and Chiappe2, among the theropods. A compsognathid affinityfor Juravenator suggests that feathers evolved independently or were lost insome species early in coelurosaurian evolution, or that some coelurosaurshad limited feathery covering. If Juravenator turns out to be more ‘basal’ inthe tree than the known feathered dinosaurs, it would suggest that featherevolution started during the early history of the Coelurosauria.

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