P ERSPECTIVES narcotic properties are recorded for chemi- because reactions involving platinum were References and Notes cally unreactive N gas, giving rise to how Bartlett initiated his first xenon chem- 1. C. Sanloup et al., Science 310, 1174 (2005). 2 2. G.Wilson, The Life of the Honourable Henry Cavendish “l’ivresse des grandes profondeurs” (the istry experiments (5). Perhaps the capsules (Cavendish Society, London, 1851). “rapture of the deep”—the intoxication expe- are not completely innocent in the high- 3. W. Ramsay, The Gases of the Atmosphere, the History of Their Discovery (Macmillan, London, 1915). rienced by divers) (8). An early explanation pressure and high-temperature experi- 4. G. N. Lewis, J. Am. Chem. Soc. 38, 762 (1916). for xenon anesthesia was given by Pauling, ments, because they might act as reaction 5. N. Bartlett, Proc. Chem. Soc. 1962, 218 (June 1962). who suggested that clathrate hydrate struc- sites for xenon in addition to providing the 6. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements (Pergamon, Oxford, 1984). tures encapsulating the rare gas atoms were host for platinum-silicon alloys formed dur- 7. N. P. Franks, R. Dickinson, S. L. de Sousa,A. C. Hall,W. R. formed near synapses, impeding interneu- ing the presumed silicate reduction reac- Lieb, Nature 396, 324 (1998). 8. J. Cousteau, The Silent World (Reprint Society, London, ronal transmission (9). Recent results suggest tions. However, even if that is the case, it 1953). a more interesting solution. The protein com- does not rule out the potential importance of 9. L. Pauling, Science 134, 15 (1961). plexes that form transmembrane ion pumps metal-silicate reactions involving xenon 10. P. Bennett, D. Elliott, The Physiology and Medicine of Diving (Saunders, New York, ed. 5, 2003). associated with neurotransmitters contain within Earth. These results could presage a 11. N. P. Franks,W. R. Lieb, Nature 300, 487 (1982). hydrophobic regions. It is thought that neutral new area in xenon solid-state chemistry 12. E.Anders,T. Owen, Science 198, 453 (1977). 13. A. P. Jephcoat, Nature 393, 355 (1998). species such as xenon and N2 might enter under high-pressure conditions, which 14. The author is supported by a Wolfson Royal Society these regions, especially at high pressure, and might be extended to other noble gases that Research Merit Award Fellowship. interfere with the neuronal process to result in have not yet been chemically awakened, if anesthesia and narcosis (10, 11). the conditions are made right. 10.1126/science.1121022 Highly oxidizing conditions are usually needed to activate xenon into true chemical PALEONTOLOGY reactivity and to form bonds with species such as oxygen. However, Sanloup et al. have used optical spectroscopy and synchrotron x- Dinosaurs Dined on Grass ray diffraction combined with chemical analysis to show that xenon can react with Dolores R. Piperno and Hans-Dieter Sues natural silicate materials, including SiO2, to form xenon oxide species under the high- rasses (family Poaceae or Gramineae), have long depicted dinosaurs as grazing on pressure, high-temperature conditions found with about 10,000 extant species, are conifers, cycads, and ferns in landscapes in Earth’s crust (1). This is an important Gamong the largest and most ecologi- without grasses. The work of Prasad et al. result, because the noble gases form useful cally dominant families of flowering plants, (1) is the first unambiguous evidence that geochemical tracers. Formed by radioactive and today provide staple foods for much of the Poaceae originated and had already decay processes or encapsulated within deep humankind. Dinosaurs, the dominant mega- diversified during the Cretaceous. The Earth materials since the formation of the herbivores during most of the Mesozoic Era research shows that phytoliths, which planet, the “inert” gaseous elements are (65 to 251 million years ago), are similarly have become a major topic of study in thought to diffuse out from the mantle, core, one of the largest and best known groups of Quaternary research over the last 20 years and crust at well-defined rates. Xenon is par- organisms. However, the possible coevolution (4–8), can provide a formidable means for ticularly important in this regard. If it does of grasses and dinosaurs has never been reconstructing vegetation and animal diets undergo redox reactions and enter into chem- studied. Now, Prasad et al. (1) report on page for much earlier time periods when early ical combination with silicates and other 1177 of this issue their analysis of phy- angiosperms were diversifying. These oxides, this could explain the apparent toliths—microscopic pieces of silica remarkable results will force reconsidera- “xenon deficit” in the atmospheres of Earth formed in plant cells—in coprolites that the tion of many long-standing assumptions and Mars, remarked upon by geochronolo- authors attribute to titanosaurid sauropods about grass evolution, dinosaurian ecology, gists and geophysicists (12). If this is true for that lived in central India about 65 to 71 mil- and early plant-herbivore interactions. xenon, then perhaps it also occurs for the lion years ago. Their data indicate that those Scientists have long known that grasses radioactive rare gas, radon, that is formed by dinosaurs ate grasses. make distinctive kinds of phytoliths in the radioactive decay processes in crustal rocks. Part of the difficulty in studying the epidermis of their leaves and leaflike cover- The physical properties of xenon in deep question of dinosaur-grass coevolution ings that surround their flowers (9). More Earth environments are as strange as its pos- results from the poor quality of the fossil recent work has examined in greater detail sible chemical behavior. Jephcoat (13) has record for early grasses. The earliest phytolith characteristics from a large set of shown that under lower mantle and core con- unequivocal grass fossils date to the grasses comprising taxa representing the ditions, the melting point of xenon exceeds Paleocene-Eocene boundary, about 56 mil- entire range of diversification within the that of iron, as does its density. This means lion years ago (2, 3), well after the demise family, showing that discriminations at the that if xenon did not react chemically with of nonavian dinosaurs at the end of the subfamily, tribe, and genus levels are often mantle or core materials, it would fall as Cretaceous Period. Pollen and macrofossils possible (1, 4–8, 10). In addition, publica- “hail” toward the center of Earth, through of Poaceae are uncommon in sedimentary tion of a well-resolved consensus phy- the molten outer core. However, the results strata until the middle Miocene, about 11 to logeny of the Poaceae by the Grass Phy- of Sanloup et al. suggest that it can also react 16 million years ago, when the family is logeny Working Group (GPWG) (11) con- with silicates, oxidizing them to metallic thought to have undergone considerable siderably advances our overall understand- alloys or replacing silicon in mineral struc- evolutionary diversification and ecological ing of the evolutionary history of grasses tures. There might be new geochemical par- expansion (2). Thus, dioramas in museums and leads to improved interpretations of the titioning equilibria to be considered within early grass fossil record. For example, by the deep crust, mantle, and core, involving mapping the phytolith characters that dis- The authors are at the National Museum of Natural xenon physics and chemistry. History, Washington, DC 20560, USA. D. R. Piperno is criminate clades and subfamilies of extant It is of interest that Sanloup et al. carried also at the Smithsonian Tropical Research Institute, taxa onto this phylogenetic tree, we can out their experiments in platinum capsules, Balboa, Panama. E-mail: [email protected] infer how phytolith morphology changed at 1126 18 NOVEMBER 2005 VOL 310 SCIENCE www.sciencemag.org Published by AAAS P ERSPECTIVES GPWG Poaceae phylogeny Phytolith shapes Flagellaria Anomochlooideae Pharoideae Elegia Baloskion Joinvillea Anomochloa Streptochaeta Anomochlooideae Pharus Pharoideae Guaduella Puelia Puelioideae Eremitis Bambusoideae Pariana Lithachne Olyra Bambusoideae Buergersiochloa Pseudosasa Chusquea Streptogyna Incertae sedis Ehrharta Oryza Ehrhartoideae Leersia Ehrhartoideae Phaenosperma Anisopogon Ampelodesmos Stipa Nassella Piptatherum Brachypodium Avena Bromus Pooideae Pooideae Triticum Diarrhena Melica Glyceria Lygeum Nardus Brachyelytrum Aristida Stipagrostis Aristidoideae Merxmuellera m. Aristidoideae Karroochloa Austrodanthonia Danthonioideae Danthonia Amphipogon Arundo Molinia Arundinoideae Phragmites Merxmuellera r. Centropodia Chloridoideae Eragrostis Uniola Pappophorum Chloridoideae Zoysia Spartina Sporobolus Distichlis Eriachne Incertae sedis Thysanolaena Zeugites Centothecoideae Panicoideae Chasmanthium Gynerium Incertae sedis Danthoniopsis Panicum Pennisetum Panicoideae Miscanthus Zea Micraira Incertae sedis Grass lineage. (Left) Phylogeny for grasses from GPWG (11). (Right) unlike those in the BEP clade. Phytoliths typical of the Aristidoideae, Examples of phytoliths from basalmost and later-diverging families of Panicoideae and Chloridoideae (PACCAD clade) are also shown. Phytolith grasses, showing that the earliest grasses probably contributed phytoliths images are from the work of the authors (4, 14). the origin of major clades and lineages (see (bamboos, rice relatives) in particular possess [see figure 2, a and b, of (1)] are confined to a the