NATURE|Vol 442| 13 July 2006 NEWS & VIEWS 147 new examples of Odontogriphus are among the fruits of that labour. Caron et al. 1 have now had a good look into the mouth of Odontogriphus. They see a set of chevron-shaped bars, set with sharp teeth. Each animal has at least two chevrons, which seem to stick tightly together even after death and decomposition. Sometimes the foremost chevron has flipped over, the two then produc- ing a pinched shape vaguely like that figure of eight. In addition, however, many specimens show additional chevrons, faintly expressed, behind the two distinct front ones. This, to Caron et al., is a giveaway for a radula, the tooth apparatus found in many living molluscs (Fig. 1). Witness a snail scraping microbial films from the inside of an aquarium glass. It does so with teeth positioned in rows on a ribbon, and as the front teeth wear out they get thrown out (or, not uncommonly, swallowed) and the ribbon advances to expose fresh sets of teeth. Other features of Odontogriphus also point towards a mollusc affinity. A structure like a muscular sole lies on what was presumably the lower side, and the transverse stripes observed on the first specimen now seem to be wrinkles on that sole. Furthermore, the sole is surrounded by serially repeated dark structures that Caron et al. interpret as serial gills similar to the ones that occupy the crease between body and sole in chitons, the eight- plated molluscs we see clinging to rocks on tidal shores. The great match, however, is not so much with known and accepted molluscs as with Wiwaxia, another Burgess Shale fossil of even greater repute than Odontogriphus. Wiwaxia is a scale-clad conundrum that in evolutionary reconstructions usually moves around among the early lineages of the lophotrochozoans (a group incorporating lophophorates, molluscs and annelids) 7,8 , but it has even been inter- preted as an annelid of modern type 9 . Its scaly armour resembles that shared by a disparate group of Cambrian animals known as coeloscleritophorans 10 , but its mouth appara- tus is nearly indistinguishable from that of Odontogriphus. Is there a lesson to be learned from this mosaic of resemblances? In terms of evolu- tionary time, Cambrian animals are all closely related to one another, and the first place to look for similarities should be in other organ- isms from the same time. Comparisons with animals living 500 million years later, although crucial for our understanding of how the living world came to be, cannot pick up the lost treasures of the fossil record, be they lineages, organisms or characters. A final kink to the new story is that Caron et al. 1 point to similarities between Odontogri- phus and an even older mollusc-like organism, Kimberella 11 , from the late Precambrian about 555 million years ago. Kimberella doesn’t show any teeth, but the fossil is commonly associ- ated with sweeping marks on the surrounding microbial mats 12 , much like those produced by that snail on the inside of the aquarium glass. The bite of the ghost may be deep indeed. ■ Stefan Bengtson is in the Department of Palaeozoology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden. e-mail: stefan.bengtson@nrm.se 1. Caron, J.-B., Scheltema, A., Schander, C. & Rudkin, D. Nature 442, 159–163 (2006). 2. Conway Morris, S. Palaeontology 19, 199–222 (1976). 3. Briggs, D. E. G., Clarkson, E. N. K. & Aldridge, R. J. Lethaia 16, 1–14 (1983). lattice sites, which acts to delocalize particles, and interactions between particles, which favour localized states. Two different types of interaction are present in extended Hubbard models: ‘on-site’, between particles occupying the same lattice site as a result of tunnelling; and ‘off-site’, in which particles are affected by others in nearby sites. Off-site interactions are crucial for the existence of a supersolid phase. It is relatively simple to write down quantum-mechanical wavefunctions of Hubbard models that corre- spond to substances displaying supersolidity. But determining whether these states can actually be attained experimentally in macro- scopic crystalline samples, and whether the states are thermodynamically stable, is much harder. Indeed, attempts to observe supersolidity in the laboratory, mostly using the helium iso- tope 4 He, were unsuccessful for 35 years until the first experimental evidence 3 was reported in 2004. In that experiment, a solid 4 He sample was rotated to and fro in a torsional oscillator. When the sample was cooled to below 175 millikelvin, a decrease in its rotational inertia was detected, indicating that about 1% of the sample stopped oscillating with the solid. The origin of this superfluid component, in partic- ular whether it is truly a state of matter in thermodynamic equilibrium — and therefore long-lasting — is still unclear. The dependence of superfluidity on added 3 He and ambient pressure, as well as the fact that the superfluid component is reduced by annealing (warming and then slowly re-cooling the sample), are also not well understood, and require further experimental and theoretical insights. So what can optical lattices contribute to the physics of supersolids? Such lattices are created by the interference of laser beams SOLID-STATE PHYSICS Supersolid simulations Dieter Jaksch Supersolids — substances that are crystalline but also behave as free-flowing superfluids — can exist, according to quantum theory. Models now suggest a route to the clinching experimental evidence. Quantum theory predicts the existence of several phases of matter that have counter- intuitive properties. Some of these phases — such as superconductors, through which elec- tricity flows without resistance, and super- fluids, which flow without friction — have been experimentally verified and have assumed major roles in science and technology. Others, such as atomic supersolids, are more elusive. Supersolids have seemingly mutually exclusive properties: they are rigid crystals with well- defined, long-range spatial order, but they can also behave as if they are superfluids. Writing in Physical Review A, Scarola and colleagues 1 propose a method for detecting supersolids by analysing the quantum inter- ference that occurs between atoms when released from a web of confining laser light known as an optical lattice. The great advan- tage of this technique is that it measures the defining signatures of superfluidity and spatial order directly, and could therefore answer long-standing questions on the existence and properties of supersolid matter. The history of possible physical mecha- nisms leading to supersolidity can be traced to 1969, when Andreev and Lifshitz 2 investigated the role of ‘vacancy sites’, at which an atom is absent in a crystal. They proposed that vacan- cies could delocalize over the crystal by form- ing a Bose–Einstein condensate (a shared quantum state) at very low temperatures. This phenomenon could give rise to superfluid cur- rents that would not spoil the regular ordering of the atoms. This effect was later shown also to be present in the phase diagram of so-called extended-lattice Hubbard models at a temper- ature of absolute zero. The phases in these models occur through competition between particle tunnelling between neighbouring 4. Müller, K. J. & Walossek, D. Trans. R. Soc. Edinb. Earth Sci. 76, 161–172 (1985). 5. Conway Morris, S., Peel, J. S., Higgins, A. K., Soper, N. J. & Davis, N. C. Science 326, 181–183 (1987). 6. Hou, X. & Sun, W. Acta Palaeontol. Sinica 27, 1–12 (1988). 7. Conway Morris, S. Phil. Trans. R. Soc. Lond. B 307, 507–586 (1985). 8. Eibye-Jacobsen, D. Lethaia 37, 317–335 (2004). 9. Butterfield, N. J. Paleobiology 16, 287–303 (1990). 10. Bengtson, S. in Evolving Form and Function: Fossils and Development (ed. Briggs, D. E. G.) 101–124 (Yale Peabody Mus., New Haven, CT, 2005). 11. Fedonkin, M. A. & Waggoner, B. M. Nature 388, 868–871 (1997). 12. Seilacher, A. Palaios 14, 86–93 (1999). Nature Publishing Group ©2006