États-Unis

Precious opal from Australia. (public display, Denver Museum of Nature & Science, Denver, Colorado, USA) A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates. The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4. Opal is hydrous silica (SiO2·nH2O). Technically, opal is not a mineral because it lacks a crystalline structure. Opal is supposed to be called a mineraloid. Opal is made up of extremely tiny spheres (colloids - <a href="https://www.uwgb.edu/dutchs/acstalks/acscolor/OPALSPHR.jpg" rel="nofollow">www.uwgb.edu/dutchs/acstalks/acscolor/OPALSPHR.jpg</a>) that can be seen with a scanning electron microscope (SEM). Gem-quality opal, or precious opal, has a wonderful rainbow play of colors (opalescence). This play of color is the result of light being diffracted by planes of voids between large areas of regularly packed, same-sized opal colloids. Different opalescent colors are produced by colloids of differing sizes. If individual colloids are larger than 140 x 10-6 mm in size, purple & blue & green colors are produced. Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979). Not all opals have the famous play of colors, however. Common opal has a wax-like luster & is often milky whitish with no visible color play at all. Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture. Several groups of organisms make skeletons of opaline silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates, and ebridians. Some organisms incorporate opal into their tissues, for example horsetails/scouring rushes and sawgrass. Sometimes, fossils are preserved in opal or precious opal. The precious opal shown above is surrounded by silicified claystone. The rock is from the Griman Creek Formation, a Cretaceous-aged succession of nonmarine, fine-grained and coarse-grained siliciclastic sedimentary rocks. Stratigraphy: Griman Creek Formation, Albian Stage, upper Lower Cretaceous Locality: Coocoran Opal Field, west-southwest of Coocoran Lake, northern New South Wales, eastern Australia Photo gallery of opal: <a href="http://www.mindat.org/gallery.php?min=3004" rel="nofollow">www.mindat.org/gallery.php?min=3004</a> References cited: Carr et al. (1979) - Andamooka opal fields: the geology of the precious stones field and the results of the subsidised mining program. Geological Survey of South Australia Department of Mines and Energy Report of Investigations 51. 68 pp.

Eubrontes dinosaur track from the Jurassic of Connecticut, USA. Trace fossils are any indirect evidence of ancient life. They refer to features in rocks that do not represent parts of the body of a once-living organism. Traces include footprints, tracks, trails, burrows, borings, and bitemarks. Body fossils provide information about the morphology of ancient organisms, while trace fossils provide information about the behavior of ancient life forms. Interpreting trace fossils and determination of the identity of a trace maker can be straightforward (for example, a dinosaur footprint represents walking behavior) or not. Sediments that have trace fossils are said to be bioturbated. Burrowed textures in sedimentary rocks are referred to as bioturbation. Trace fossils have scientific names assigned to them, in the same style & manner as living organisms or body fossils. This track was made by a theropod, a group of small to large, carnivorous, bipedal dinosaurs. The specimen comes from a Triassic to Jurassic terrestrial sedimentary succession that filled up a half graben, many of which occur along America's eastern seaboard. Such half-graben basins formed during the Triassic as the Pangaea supercontinent tried to rift apart, but failed. Pangaea successfully broke apart during the Jurassic. Stratigraphy: East Berlin Formation, Newark Supergroup, Lower Jurassic Locality: unrecorded / undisclosed site at or near the town of Rocky Hill, central Connecticut, USA Info. at: mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=CTJeb%3B0 and en.wikipedia.org/wiki/Eubrontes

Paleogeography and paleoclimate of the Late Jurassic - 150 Ma with dinosaur fossil localities: A = Tendaguru Formation, Tanzania C1 = Shishugou & Kalazha Formations, China C2 = Shangshaximiao (Upper Shaximiao) Formation, China E1 = Sables de Glos, Argiles d’Octeville, Marnes de Bléville, Kimmeridge Clay, Calcareous Grit, Corallian Oolite, Oxford Clay, Portland Stone, England & France E2 = Villar del Arzobispo, Alcobaça, Guimarota, Sobral, Amoreira-Porto Novo, Bombarral, Freixial, Lourinhã Formations, Spain & Portugal M1-6 = Morrison Formation, United States S1 = Toquí & Cañadón Calcáreo Formations, Chile & Argentina

Restored skeleton of Anzu wyliei (previously labelled as a specimen of Chirostenotes)

Restored skeleton of Anzu wyliei (previously labelled as a specimen of Chirostenotes)

Plotosaurus skeletal mount in Natural History Museum of Los Angeles County, California, United States.

Mounted skeleton of Plotosaurus from the Los Angeles County Natural History Museum; see the original image

Images documenting the 1947 Bikini Resurvey Project. Caption in scrapbook: Arlis McCartner from Canfield, Ohio, is shown with a seven foot shark, caught off the fantail of the CHILTON. ABCR 5032-1 Bikini Atoll Radiological Survey Scrapbook, p. 38. Subjects (LCTGM): Sharks--Marshall Islands Subjects (LCSH): McCartner, Arlis

Anhanguera skeleton, North American Museum of Ancient Life.

Drawings of Dryolestida left upper−molar, modified from the original descriptions. A–C, Kimmeridgian; D–F, Tithonian–Berriasian; G–H, early Barremian; I, late Barremian. A. Left M4 or M5 of Comotherium richi Prothero, 1981, from the Upper Jurassic Morrison Formation, Como Bluff locality, Wyoming, USA. B. Right M5, reversed, of Dryolestes leiriensis Martin, 1999, from Guimarota, Portugal.C. Left M6 of Krebsotherium lusitanicum Martin, 1999, from Guimarota, Portugal. D. Right M4 or M 6 (reversed) of Portopinheirodon asymmetricus Martin, 1999, Porto Pinheiro, Lourinha, Portugal. E. Right M5 (reversed) of Laolestes andresi Martin, 1999, from Porto Pinheiro, Lourinha, Portugal. F. Right upper molar of Donodon perscriptoris Sigogneau−Russell, 1991, Anoual, Morocco.G. Left M4 orM5 of Crusafontia amoae sp. nov., holotype, Cuesta Corrales 2, El Castellar Formation, Galve, Teruel, Spain. H. Right M6 orM7 (reversed) of Crusafontia amoae sp. nov., first described as upper molar of Crusafontia cuencana by Krebs (1993), P−2 H4 Pelejón 2, Galve, Teruel, Spain. I. Left M2 or M3, of Crusafontia cuencana Henkel and Krebs, 1969, Uña, La Huérguina Formation, Cuenca, Spain, (from Krebs 1993). Scale bars 1 mm.

Two Thescelosaurus skeletons, North American Museum of Ancient Life.

A Thescelosaurus at the Burpee Museum of Natural History in Rockford, Illinois, USA. Size:12 feet in length, 3 ft tall at the hip Weight: 670 lbs when alive.

Anhanguerid pterosaur Uktenadactylus wadleighi (Lee, 1994), SMU 73058, Tarrant County, Texas, USA, Albian. In anterior (A1), leftlateral (A2), palatal (A3) and right lateral (A4), and dorsal (A5) views. Photographs by BH

Khaan fossil holotype IGM 100/1127.

Falcarius skeleton reconstruction, Utah Museum of Natural History.

Achelousaurus horneri skull, collected in Glacier County, Montana, at the Museum of the Rockies in Bozeman, Montana. The Ceratopsidae are those dinosaurs with head frills. There are three large subgroups of Ceratopsidae: Centrosaurinae, Ceratopsinae, and Chasmosaurinae. The Triceratopsini are a "tribe" of the Chasmosaurinae -- a genus so vast that it gets the special name "tribe". The Pachyrhinosaurini are a "tribe" within the Centrosaurinae. Achelousaurus is a genus within the Pachyrhinosaurini. So far, only three skulls and some limited skeletal remains have been collected anywhere in the world -- and all of them in Montana. Their bony frills are quite similar to the Styracosaurus albertensis, although their other skull features (such as big bony bosses on the nose and behind the eyes) are not.