Unwin, D.M

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Nature © Macmillan Publishers Ltd 19988Ever since John Ostrom resuscitated theidea in the 1970s, palaeontologistshave been piling up the evidence infavour of theropod dinosaurs as the ances-tors of birds. Recently, the gap betweentheropods and birds has been narrowed evenfurther. Important avian characters, such asthe furcula (wishbone), have been discov-ered in theropods thought to be close tobirds1. And typical theropod characteristics,such as an enlarged claw on digit two of the foot, have been found in early birds2. Thefinal, clinching fact would be the discovery of evidence of feathers — a defining featureof birds — in theropods.Cue Sinosauropteryx prima, the so-called‘feathered’ dinosaur from China (Fig. 1).Reports of this sensational discovery firstappeared3 in late 1996. Although few scien-tists have yet seen the fossil material, someare already incorporating Sinosauropteryxinto models for the origin of feathers andbird flight4. Still others argue5 that the ‘feath-ers’ are merely an artefact of preservation.On page 147 of this issue6, Chen, Dong andZhen present the first detailed description ofSinosauropteryx, and show that the integu-ment (skin) bore discrete filamentous struc-tures. But are they feathers? Chen et al. describe two almost complete,near-adult individuals, with evidence of softtissues including the integument, the eyesand possibly some internal organs7. Theyare, without doubt, the best-preserveddinosaur remains yet found. These, alongwith a third specimen, were recovered byChinese farmers in Liaoning Province,China (P. J. Currie, personal communica-tion). They were found in beds of the YixianFormation, part of a thick sequence of lake sediments intercalated with volcanicdeposits. During the past six years, these sediments have yielded many superbly preserved fossils. The result is an almostcomplete Early Cretaceous (145–97.5 mil-lion years ago) continental biota, composedof plants, insects, fish, lizards, turtles,pterosaurs, dinosaurs, mammals andnumerous birds8 — the latter often pre-served with their plumage intact9.In life, Sinosauropteryx was about the sizeof a large chicken and distinguished by itsdeep, narrow body, remarkably long tail andrather short, stout forelimbs (Fig. 1). Like its close relative Compsognathus, from theLate Jurassic of Europe (163–145 millionyears ago), Sinosauropteryx has a very specialized hand. Its massive first digit bearsa large claw that might have served as akilling tool. Last meals provide further evidence of an active, predatory lifestyle —a lizard in the gut region of one individual6and a tiny mammal in the third specimen (P. J. Currie, personal communication). Thebody of the larger individual described byChen et al. also contains another surprise —two small, oval structures which, judging bytheir shape, size and position, are almostcertainly eggs. This is the first reasonablyconvincing record of this type of associationfor any dinosaur and, if correctly identified,provides incontrovertible evidence that thisindividual was female. Moreover, the rela-tively small size of the eggs and the possibili-ty of paired oviducts6 suggest that, unlikemodern birds (which have a single oviductand, in general, smaller clutches of relativelylarge eggs), theropods had a more reptile-like reproductive system with two oviductsthat produced larger numbers of relativelysmall eggs.The most striking features of the fossilsare the well-preserved remains of the integu-ment, which forms a dark halo above theskull, neck, back, hips and both sides of thetail. Small patches of integument also occuron the skull, and are associated with the fore-limbs, rib-cage and legs. The halo is com-posed of many coarse, sinuous filaments,which are possibly hollow and up to 40 mmlong6. The filaments seem to be discretestructures and, although often matted andtangled, are not encased in skin or remnantsof the decaying dermis, as some have suggested5. Branching of the filaments — aconstruction that is also typical of feathers —has been reported6,10, but the topography ofthis branching is not yet clear.This brings us to the critical question:are these structures some kind of ‘proto-feather’? Chen et al.6 and others4,10 clearlyfavour this idea. They draw comparisonswith the plumules of modern birds, whichare relatively simple structures without barbules or hooklets. But any argument for homology between the feathers of birds and the integumentary structures ofSinosauropteryx needs to be supported bymore than general similarities in structureand position. High-resolution microscopyand biogeochemical tests might providesome answers, but they will not solve all ofthe problems. Moreover, if Sinosauropteryxbears proto-feathers, we might expect similar (or perhaps even more feather-like)NATURE | VOL 391 | 8 JANUARY 1998 119news and viewsFeathers, filamentsand theropoddinosaursD. M. UnwinOne of the hottest debates in palaeontology is whether birds evolvedfrom dinosaurs. A study of two exceptionally well-preserved specimensof a theropod dinosaur from China — complete with skin, internal organsand eggs — provides new clues to the origin of feathers.DROMAEOSAURIDAESAUROPODOMORPHAManiraptoraORNITHISCHIAEoraptorCERATOSAURIACARNOSAURIAOVIRAPTOROSAURIATYRANNOSAURIDAEORNITHOMIMOSAURIATROODONTIDAEAVESCoelurosauriaTheropodaSaurischiaDinosauriaSinosauropteryxFigure 1 Reconstruction of Sinosauropteryx byMichael W. Skrepnick. Three exceptionally well-preserved specimens of this dinosaur have beenrecovered from the Early Cretaceous YixianFormation of China. Two of these are describedby Chen et al.6, who show that Sinosauropteryxbore discrete structures that could have been ‘proto-feathers’.Figure 2 The relationships of Sinosauropteryx toother dinosaurs. Sinosauropteryx is not asclosely related to birds as many other theropods,yet it is the first non-avian theropod that seemsto show evidence of feather-like structures.(Modified from ref. 10.)Nature © Macmillan Publishers Ltd 19988structures to have been present on at leastsome of the theropods that are more closelyrelated to birds than is Sinosauropteryx (Fig.2). Exceptionally well-preserved remains ofthe integument are now known for two ofthese dinosaurs — the ornithomimosaurPelecanimimus11, and a small, unnamedmaniraptoran theropod from Brazil12. Inboth cases, however, there is no evidence of the filamentous structures found inSinosauropteryx.So, it seems that we still do not haveabsolute proof that some dinosaurs werefeathered. Or do we? The Liaoning depositshave yielded three examples of another putative dino-bird intermediate, Protar-chaeopteryx. According to a preliminaryreport by Ji and Ji13, these individuals havewell-preserved evidence of true feathers.While we wait for further details of theseChinese fossils, we might consider the ironyof the present situation — nothing for hundreds of years and then, suddenly, awhole flock of evidence.D. M. Unwin is in the Department of Geology,University of Bristol, Wills Memorial Building,Queen’s Road, Bristol BS8 1RJ, UK.e-mail: dave.unwin@uk.ac.bristol 1. Norell, M. A., Mackovicky, P. & Clark, J. M. Nature 389, 447(1997).2. Forster, C. A., Chiappe, L. M., Krause, D. W. & Sampson, S. D.J. Vert. Paleontol. 16, (Suppl.) 34A (1996).3. Hecht, J. New Scientist No. 2052, 7 (1996).4. Padian, K. J. Vert. Paleontol. 17, (Suppl.) 68A (1997).5. Geist, N. R., Jones, T. D. & Ruben, J. A. J. Vert. Paleontol. 17,(Suppl.) 48A (1997).6. Chen, P., Dong, Z. & Zhen, S. Nature 391, 147–152 (1998).7. Ruben, J. A., Jones, T. D., Geist, N. R. & Hillenius, W. J. Science278, 1267–1270 (1997).8. Hecht, J. New Scientist No. 2094, 32–35 (1997).9. Hou, L., Martin, L. D., Zhou, Z. & Feduccia, A. Science 274,1164–1167 (1996).10.Currie, P. J. in Encyclopedia of Dinosaurs (eds Currie, P. J. &Padian, K.) 241 (Academic, San Diego, 1997).11. Briggs, D. E. G., Wilby, P. R., Pérez-Moreno, B. P., Sanz, J. L. &Fregenal-Martinez, M. J. Geol. Soc. (Lond.) 154, 587–588 (1997).12.Kellner, A. W. A. Nature 379, 32 (1996).13. Ji, Q. & Ji, S. A. Chin. Geol. 238, 38–41 (1997).ear structures (filaments). Sylos-Labini et al.argue that D = 2, which suggests a roughlyplanar (sheet-like) distribution of galaxies.Most cosmologists would accept that thedistribution of galaxies on relatively smallscales, up to perhaps a few tens of mega-parsecs (Mpc), can indeed be described interms of a fractal model. This small-scaleclustering is expected to be dominated bypurely gravitational physics, and gravity hasno particular length scale associated with it.But standard theory requires that the fractaldimension should approach the homo-geneous value D = 3 on large enough scales.According to standard models of cosmo-logical structure formation, this transitionshould occur on scales of a few hundredMpc. The main source of the controversy isthat most available three-dimensional mapsof galaxy positions are not large enough toencompass the expected transition to homo-geneity. Distances must be inferred fromredshifts (see box), and it is difficult to con-struct these maps from redshift surveys,which require spectroscopic studies of largenumbers of galaxies.Sylos-Labini et al. have analysed a num-ber of redshift surveys, including the largestso far available, the Las Campanas redshiftsurvey3. They find D = 2 for all the data theylook at, and argue that there is no transitionto homogeneity for scales up to 4,000 Mpc,way beyond the expected turnover. If thiswere true, it would indeed be bad news forthe orthodox among us. Their results are, however, at variancewith the visual appearance of the Las Cam-panas survey, for example, which certainlyseems to display large-scale homogeneity(Fig. 1). Objections to these claims havebeen lodged by Luigi Guzzo4, for instance,who has criticized their handling of the dataand has presented independent results thatappear to be consistent with a transition to homogeneity. It is also true that Sylos-Labini et al. have done their cause no goodby basing some conclusions on a hetero-geneous compilation of redshifts called theLEDA database5, which is not a controlledsample and so is completely unsuitable forthis kind of study. Finally, it seems clear thatthey have substantially overestimated theeffective depth of the catalogues they areusing. But although their claims remaincontroversial, the consistency of the resultsobtained by Sylos-Labini et al. is impressiveenough to raise doubts about the standardpicture. Mainstream cosmologists are not yet soworried as to abandon the CosmologicalPrinciple. Most are probably quite happy toadmit that there is no overwhelming directevidence in favour of global uniformity from current three-dimensional galaxy catalogues, which are in any case relativelyshallow. But this does not mean there is nonews and views120 NATURE | VOL 391 | 8 JANUARY 1998One of the central tenets of cosmologi-cal orthodoxy is the CosmologicalPrinciple, which states that, in abroad-brush sense, the Universe is the samein every place and in every direction. Thisassumption has enabled cosmologists toobtain relatively simple solutions of Ein-stein’s General Theory of Relativity thatdescribe the dynamical behaviour of theUniverse as a whole. These solutions, calledthe Friedmann models1, form the basis of theBig Bang theory. But is the CosmologicalPrinciple true? Not according to Sylos-Labi-ni et al.2, who argue, controversially, that theUniverse is not uniform at all, but has anever-ending hierarchical structure in whichgalaxies group together in clusters which, inturn, group together in superclusters, and soon.These claims are completely at odds withthe Cosmological Principle and thereforewith the Friedmann models and the entireBig Bang theory. The central thrust of thework of Sylos-Labini et al. is that the statisti-cal methods used by cosmologists to analysegalaxy clustering data are inappropriatebecause they assume the property of large-scale homogeneity at the outset. If one doesnot wish to assume this then one must usedifferent methods. What they do is to assume that the Uni-verse is better described in terms of a fractalset characterized by a fractal dimension D. Ina fractal set, the mean number of neighboursof a given galaxy within a volume of radius Ris proportional to RD. If galaxies are distrib-uted uniformly then D = 3, as the number ofneighbours simply depends on the volume ofthe sphere and the average number-densityof galaxies. A value of D < 3 indicates that thegalaxies do not fill space in a homogeneousfashion: D = 1, for example, would indicatethat galaxies were distributed in roughly lin-CosmologyAn unprincipled Universe?Peter ColesFigure 1 The big picture — the Las Campanasredshift survey, each dot marking a galaxy with awell-determined redshift. The survey maps theUniverse out to recession velocities of 60,000 km s−1, corresponding to distances of a fewhundred million parsecs. Although no fractalstructure on the largest scales is apparent (thereare no clear voids or concentrations on the samescale as the whole map), one statistical analysis2finds a fractal dimension of two in this and othersurveys, for all scales — conflicting with a basicprinciple of cosmology.

Feathers, filaments and theropod dinosaurs

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