共查询到20条相似文献，搜索用时 31 毫秒
1.
More than 40 base metal porphyry ore deposits in southwestern North America are associated with the Laramide orogeny (about 90 million to 50 million years ago). Rhenium-osmium dates on molybdenite, a rhenium-enriched sulfide common in many of the deposits, reveal that in individual deposits mineralization occurs near the final stages of magmatic activity irrespective of the time of inception, magnitude, or duration of magmatism. Deposits that differ widely in location and in the extent and timing of magmatism have nearly identical ages for mineralization. Rhenium-osmium-ages suggest that mineralization occurred during two distinct intervals from about 74 million to 70 million years ago and from 60 million to 55 million years ago. Most deposits that formed in the oldest interval are within the older Precambrian basement of northwestern Arizona, whereas the younger deposits are restricted to the younger Precambrian basement in southern Arizona and northern Mexico. Synchronous, widespread mineralization indicates that similar crust-mantle interaction occurred on a regional scale for ore deposits once thought to be the product of localized processes. 相似文献
2.
Hf-W Isotopic Evidence for Rapid Accretion and Differentiation in the Early Solar System 总被引：1，自引：0，他引：1
The time scales over which inner solar system objects accreted and differentiated are unclear because the isotopic systems of many meteorites are disturbed. 182Hf decays to 182W with a half-life of 9 million years and is a particularly useful chronometer because both elements are highly refractory and immobile. Tungsten isotopic data for IIA, IIIA, IVA, and anomalous iron meteorites and H, L, and LL chondrites indicate that their parent bodies accreted rapidly and segregated metal within just a few million years. 相似文献
3.
The Cambrian conundrum: early divergence and later ecological success in the early history of animals 总被引：1，自引：0，他引：1
Erwin DH Laflamme M Tweedt SM Sperling EA Pisani D Peterson KJ 《Science (New York, N.Y.)》2011,334(6059):1091-1097
Diverse bilaterian clades emerged apparently within a few million years during the early Cambrian, and various environmental, developmental, and ecological causes have been proposed to explain this abrupt appearance. A compilation of the patterns of fossil and molecular diversification, comparative developmental data, and information on ecological feeding strategies indicate that the major animal clades diverged many tens of millions of years before their first appearance in the fossil record, demonstrating a macroevolutionary lag between the establishment of their developmental toolkits during the Cryogenian [(850 to 635 million years ago (Ma)], and the later ecological success of metazoans during the Ediacaran (635 to 541 Ma) and Cambrian (541 to 488 Ma) periods. We argue that this diversification involved new forms of developmental regulation, as well as innovations in networks of ecological interaction within the context of permissive environmental circumstances. 相似文献
4.
Venkatesh B Kirkness EF Loh YH Halpern AL Lee AP Johnson J Dandona N Viswanathan LD Tay A Venter JC Strausberg RL Brenner S 《Science (New York, N.Y.)》2006,314(5807):1892
Cartilaginous fishes represent the living group of jawed vertebrates that diverged from the common ancestor of human and teleost fish lineages about 530 million years ago. We generated approximately 1.4x genome sequence coverage for a cartilaginous fish, the elephant shark (Callorhinchus milii), and compared this genome with the human genome to identify conserved noncoding elements (CNEs). The elephant shark sequence revealed twice as many CNEs as were identified by whole-genome comparisons between teleost fishes and human. The ancient vertebrate-specific CNEs in the elephant shark and human genomes are likely to play key regulatory roles in vertebrate gene expression. 相似文献
5.
Acarodomatia or "mite houses" are located on leaves of many present-day angiosperms and are inhabited by mites that may maintain leaf hygiene. Eocene deposits in southern Australia have yielded acarodomatia on fossil leaves of Elaeocarpaceae and Lauraceae and also contain oribatid mites with close affinities to those that inhabit the acarodomatia of the closest living relatives of the fossil plant taxa. The data indicate that mite-plant associations may have been widespread in southern Australia 40 million years ago. 相似文献
6.
Single-copy inverted repeats associated with regional genetic duplications in gamma fibrinogen and immunoglobulin genes 总被引：1，自引：0，他引：1
A J Fornace D E Cummings C M Comeau J A Kant G R Crabtree 《Science (New York, N.Y.)》1984,224(4645):161-164
We have found that a portion (150 base pairs) of the seventh exon of the human gamma fibrinogen gene is duplicated in the preceding intron. This duplicated sequence, termed a "pseudoexon," is flanked on each side by a single-copy inverted repeat sequence consisting of 102 base pairs. Frequencies of point substitutions indicate that both the pseudoexon and the inverted repeat sequence arose approximately 10 to 20 million years ago. The generality of this type of duplication is suggested by the occurrence of a similar duplication in the mouse immunoglobulin mu-delta region. As in the fibrinogen pseudoexon, the portion of the immunoglobulin mu-delta region containing the duplication and the inverted repeat was reported to be single-copy in the mouse genome. Since both of the first two single-copy inverted repeats to be sequenced are associated with regional duplications, it is likely that many of the single-copy inverted repeat sequences, which make up 1 to 2 percent of the genome, are also associated with regional duplications. 相似文献
7.
Marzoli A Renne PR Piccirillo EM Ernesto M Bellieni G De Min A 《Science (New York, N.Y.)》1999,284(5414):616-618
The Central Atlantic Magmatic Province (CAMP) is defined by tholeiitic basalts that crop out in once-contiguous parts of North America, Europe, Africa, and South America and is associated with the breakup of Pangea. 40Ar/39Ar and paleomagnetic data indicate that CAMP magmatism extended over an area of 2.5 million square kilometers in north and central Brazil, and the total aerial extent of the magmatism exceeded 7 million square kilometers in a few million years, with peak activity at 200 million years ago. The magmatism coincided closely in time with a major mass extinction at the Triassic-Jurassic boundary. 相似文献
8.
Evidence from Precambrian sediments appears to indicate that nucleated (eukaryotic) organisms had become well established and relatively diverse about 850 +/- 100 million years ago and that eukaryotes were probably extant, and may have first appeared, as early as 1400 +/- 100 million years ago. 相似文献
9.
Keigwin L 《Science (New York, N.Y.)》1982,217(4557):350-353
Comparisons of carbon isotopic data on benthic foraminifera from Deep Sea Drilling Project sites 502 (western Caribbean) and 503 (eastern Pacific) indicate that the difference between the Atlantic and the Pacific in the per mil enrichment in carbon-13 of total dissolved carbon dioxide increased about 6 million years ago and again 3 million years ago, when the difference reached the modern level (1 per mil). Comparisons of planktonic foraminiferal oxygen isotopic data for the Caribbean and the Pacific suggest that the salinity of Caribbean surface waters began increasing 4 million years ago, possibly in response to shoaling of the Panama isthmus. These results suggest that modern circulation patterns in the Caribbean and eastern Pacific developed by 3 million years ago in concert with changing tectonic, climatic, and biogeographic patterns. 相似文献
10.
U-Pb ages from the neoproterozoic Doushantuo Formation, China 总被引：4，自引：0，他引：4
U-Pb zircon dates from volcanic ash beds within the Doushantuo Formation (China) indicate that its deposition occurred between 635 and 551 million years ago. The base records termination of the global-scale Marinoan glaciation and is coeval with similar dated rocks from Namibia, indicating synchronous deglaciation. Carbon isotopic and sequence-stratigraphic data imply that the spectacular animal fossils of the Doushantuo Formation are for the most part younger than 580 million years old. The uppermost Doushantuo Formation contains a pronounced negative carbonate carbon isotopic excursion, which we interpret as a global event at circa 551 million years ago. 相似文献
11.
Recent paleomagnetic-radiometric data from six rhyolite domes in the Valles Caldera, New Mexico, indicate that the last change in polarity of the earth's magnetic field from reversed to normal (the Brunhes-Matuyama boundary) occurred at about 0.7 million years ago. A previously undiscovered geomagnetic polarity event, herein named the "Jaramillo normal event," occurred about 0.9 million years ago. 相似文献
12.
A nearly complete skeleton of the archaic pinniped Enaliarctos, found in late Oligocene or early Miocene rocks (approximately 23 million years old) of California, provides new evidence on the origin of pinnipeds. Enaliarctos retains many primitive features expected in the hypothesized common ancestor of pinnipeds. Skeletal modifications seen in Enaliarctos document swimming adaptations and indicate that pinnipeds primitively used the axial skeleton and both fore and hindflippers as sources of propulsion. Elongate hindlimbs with prominent bony processes (reflecting powerful musculature) suggest that Enaliarctos was more active on land than modern pinnipeds. 相似文献
13.
The evolution of the entire planetary system has been numerically integrated for a time span of nearly 100 million years. This calculation confirms that the evolution of the solar system as a whole is chaotic, with a time scale of exponential divergence of about 4 million years. Additional numerical experiments indicate that the Jovian planet subsystem is chaotic, although some small variations in the model can yield quasiperiodic motion. The motion of Pluto is independently and robustly chaotic. 相似文献
14.
Volcanic glasses collected on the rim of Shorty Crater in the Apollo 17 area were formed 3.63 x 10(9) years ago. The amounts of xenon-136 produced by neutron-induced fission of uranium-235 indicate that the glasses resided on the lunar surface for about 38 million years before they were deeply buried. The glass spherules were reexcavated by the impact that formed Shorty Crater 17 million years ago, and remained undisturbed until they were collected. 相似文献
15.
The age and evolution of the Grand Canyon have been subjects of great interest and debate since its discovery. We found that cave mammillaries (water table indicator speleothems) from nine sites in the Grand Canyon showed uranium-lead dating evidence for an old western Grand Canyon on the assumption that groundwater table decline rates are equivalent to incision rates. Samples in the western Grand Canyon yielded apparent water table decline rates of 55 to 123 meters per million years over the past 17 million years, in contrast to eastern Grand Canyon samples that yielded much faster rates (166 to 411 meters per million years). Chronology and inferred incision data indicate that the Grand Canyon evolved via headward erosion from west to east, together with late-stage ( approximately 3.7 million years ago) accelerated incision in the eastern block. 相似文献
16.
A deep-sea sediment core underlying the Benguela upwelling system off southwest Africa provides a continuous time series of sea surface temperature (SST) for the past 4.5 million years. Our results indicate that temperatures in the region have declined by about 10 degrees C since 3.2 million years ago. Records of paleoproductivity suggest that this cooling was associated with an increase in wind-driven upwelling tied to a shift from relatively stable global warmth during the mid-Pliocene to the high-amplitude glacial-interglacial cycles of the late Quaternary. These observations imply that Atlantic Ocean surface water circulation was radically different during the mid-Pliocene. 相似文献
17.
Kerr RA 《Science (New York, N.Y.)》2000,287(5459):1734-1736
The startling claim that Earth has frozen over from pole to pole for millions of years at a time, a hypothesis widely known as the "snowball Earth" scenario, has intrigued many earth scientists but as yet convinced few. But the theory, which would explain some previously inexplicable observations from the Neoproterozoic era 600 million to 700 million years ago, is gaining adherents. Last year, for example, most researchers agreed that one part of the sweeping hypothesis--the claim that glaciers once flowed into ice-covered tropical seas--is correct, even though this idea had been rebuffed for more than 30 years. 相似文献
18.
Carbon and water flow between plants and the atmosphere is regulated by the opening and closing of minute stomatal pores in surfaces of leaves. By changing the aperture of stomata, plants regulate water loss and photosynthetic carbon gain in response to many environmental stimuli, but stomatal movements cannot yet be reliably predicted. We found that the complexity that characterizes stomatal control in seed plants is absent in early-diverging vascular plant lineages. Lycophyte and fern stomata are shown to lack key responses to abscisic acid and epidermal cell turgor, making their behavior highly predictable. These results indicate that a fundamental transition from passive to active metabolic control of plant water balance occurred after the divergence of ferns about 360 million years ago. 相似文献
19.
A whiff of oxygen before the great oxidation event? 总被引：1，自引：0，他引：1
Anbar AD Duan Y Lyons TW Arnold GL Kendall B Creaser RA Kaufman AJ Gordon GW Scott C Garvin J Buick R 《Science (New York, N.Y.)》2007,317(5846):1903-1906
High-resolution chemostratigraphy reveals an episode of enrichment of the redox-sensitive transition metals molybdenum and rhenium in the late Archean Mount McRae Shale in Western Australia. Correlations with organic carbon indicate that these metals were derived from contemporaneous seawater. Rhenium/osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 +/- 8 million years ago (Ma). Molybdenum and rhenium were probably supplied to Archean oceans by oxidative weathering of crustal sulfide minerals. These findings point to the presence of small amounts of O2 in the environment more than 50 million years before the start of the Great Oxidation Event. 相似文献
20.
Aparicio S Chapman J Stupka E Putnam N Chia JM Dehal P Christoffels A Rash S Hoon S Smit A Gelpke MD Roach J Oh T Ho IY Wong M Detter C Verhoef F Predki P Tay A Lucas S Richardson P Smith SF Clark MS Edwards YJ Doggett N Zharkikh A Tavtigian SV Pruss D Barnstead M Evans C Baden H Powell J Glusman G Rowen L Hood L Tan YH Elgar G Hawkins T Venkatesh B Rokhsar D Brenner S 《Science (New York, N.Y.)》2002,297(5585):1301-1310
The compact genome of Fugu rubripes has been sequenced to over 95% coverage, and more than 80% of the assembly is in multigene-sized scaffolds. In this 365-megabase vertebrate genome, repetitive DNA accounts for less than one-sixth of the sequence, and gene loci occupy about one-third of the genome. As with the human genome, gene loci are not evenly distributed, but are clustered into sparse and dense regions. Some "giant" genes were observed that had average coding sequence sizes but were spread over genomic lengths significantly larger than those of their human orthologs. Although three-quarters of predicted human proteins have a strong match to Fugu, approximately a quarter of the human proteins had highly diverged from or had no pufferfish homologs, highlighting the extent of protein evolution in the 450 million years since teleosts and mammals diverged. Conserved linkages between Fugu and human genes indicate the preservation of chromosomal segments from the common vertebrate ancestor, but with considerable scrambling of gene order. 相似文献