"GEOLOGY : The Calibration of Ediacaran Time Alan J. Kaufman * More than a century ago , the last great geological period was formally ratified by an international committee . This was the final rocky step in the subdivision of deep time based on the evolutionary progression of animal fossils . However , recent years have seen the identification of an older and tumultuous new interval , the Ediacaran Period , during which Earth 's earliest soft-bodied organisms emerged in the oceans . This interval was recently ratified ( 1 ) , underscoring advances in the absolute dating ( 2-6 ) and worldwide correlation of geological strata that were deposited in isolated basins before true animals exploded onto the scene in the succeeding Cambrian Period . On page 95 of this issue , Condon et al . ( 7 ) present precise age constraints for the Ediacaran Period . The authors have analyzed volcanic dust in two key depositional layers in the Doushantuo Formation of southern China . Their radiometric dates provide important insights into the rates of geological and evolutionary processes . The first layer , with an age of about 635 million years , is at the base of the new interval , whereas the second , at about 550 million years , may constrain the age of an environmental disaster ( 8-10 ) that is closely associated with the rapid diversification of the Ediacara biota ( see the figure ) that lend their name to the new Period . http : //www.sciencemag.org/content/vo ... m/59-1-med.gif Figure 1 Early animals ? A pile of three-dimensionally preserved casts of the soft-bodied Ediacaran organism Ernietta from ~545 million-year-old sediments in the Nama Group of southern Namibia . The scale bar corresponds to 15 cm . Convention previously focused on the evolutionary first appearance of a specific fossil or assemblage to define the beginning of new geological periods . In contrast , the beginning of the Ediacaran period is defined by the base of a marine carbonate rock , which formed in southern Australia in the aftermath of a distinctive and potentially global ice age ( 11 , 12 ) . Equivalent glacial rocks occur immediately beneath similar carbonates at the base of the Doushantuo Formation . In the area studied by Condon et al. , the new ages constrain the Doushantuo Formation , which represents most of the Ediacaran Period , to some 85 million years -- a remarkably long interval for only about 100 m of rock . This observation begs the question : How much time may be missing in Ediacaran strata from southern China ? In the absence of dates between the two radiometric tie points , one must consider two possibilities : Either the sediments accumulated continuously , albeit slowly ( some two orders of magnitude more slowly than in similar environments of the same age ) , or there are breaks in time ( hiatuses or unconformities ) hidden within the poorly exposed layers . On the basis of limited physical data , Condon et al . suggest the presence of two such unconformities in their study area near the Yangtze Gorges . The duration of these stratigraphic breaks with respect to the environmental anomaly -- reflecting a dramatic change in the cycling of carbon on Earth 's surface -- and subsequent biological innovations form the cornerstone of their conclusions , and deserve closer examination . Spectacular fossil animal embryos ( 13 , 14 ) of great evolutionary importance are found at Weng'an ( some 375 km to the southwest of the authors ' study area ) , where the Doushantuo Formation accumulated closer to the shoreline and records only a single surface of unconformity . In such nearshore environments , oceanic sediments are often exposed above sea level , resulting in the removal of underlying sediments and hence the erasure of some fraction of geological time . Which of the two hiatal surfaces from the deeper-water sections at the Yangtze Gorges correlates to the single unconformity at Weng'an ? The data are more equivocal than presented by Condon et al. , and the choice carries important evolutionary consequences . At the Yangtze Gorges , the extraordinary carbon cycle anomaly recorded in marine carbonates near the top of the Doushanto Formation is truncated by the uppermost unconformity . Condon et al . suggest that there is little time missing across the surface , thereby preserving a causal relation between the environmental perturbation and the rapid diversification of Ediacara organisms and associated faunas around 550 million years ago . However , correlation of this surface and the intervening sediments back to Weng'an tells another story . At Weng'an , both the carbon cycle anomaly and the sediments typical of the uppermost Doushantuo Formation at the Yangtze Gorges are missing , implying a substantial hiatus . Comparison of broadly equivalent strata in southern Australia and the western United States suggests a stratigraphic architecture similar to that in southern China , where similar carbon cycle anomalies are truncated by unconformities ( 15 ) . In the western United States , the post-anomaly unconformity removes a minimum of 130 m of section , more than the entire thickness of the Doushantuo Formation . These observations suggest that the unconformity separating the Ediacaran faunas and the carbon cycle anomaly in southern China may , in fact , hide a lot of time , thereby decoupling the environmental and biological events that the authors wish to connect . This is not meant to detract from the important radiometric calibration that Condon et al . provide , but rather to note that the stratigraphic relations between these dates , and therefore their connection to evolutionary events , are far from straightforward . According to Condon et al. , the rapid diversification of complex multicellular organisms in Ediacaran oceans forced the carbon cycle anomaly seen worldwide , but this seems possible only if there is no real time missing across the upper Doushantuo unconformity . If this is not the case , then this interpretation may be placing the cart before the horse . Alternative models suggest that environmental changes may have driven evolutionary transformations . In particular , atmospheric oxygen -- long believed to be an external forcing factor to evolution -- appears to have built up rapidly during the Ediacaran Period , not because of a discrete biological event but as a result of the tectonic forces that lift and erode mountain ranges ( 16 ) . Through precise radiometric clocks and clever stratigraphic connections , geoscientists can increasingly correlate Ediacaran sediments that are separated widely in space and time . These tools allow us to piece together a complex puzzle of unforgettable biological events against a background of repetitive climatic and environmental perturbations . However , even with exact dates , the cyclicity of these events and the specter of a fragmentary rock record add uncertainty to our picture of Ediacaran Earth history . References 1 . A. H. Knoll et al. , Science 305 , [ 621 ] ( 2004 ) . 2 . J. P. Grotzinger et al. , Science 270 , [ 598 ] ( 1995 ) . 3 . M. W. Martin et al. , Science 288 , [ 841 ] ( 2000 ) . 4 . J. E. Amthor et al. , Geology 31 , 431 ( 2003 ) [ Abstract ] . 5 . K.-H. Hoffmann et al. , Geology 32 , 817 ( 2004 ) [ Abstract ] . 6 . C. R. Zhou et al. , Geology 32 , 437 ( 2004 ) [ Abstract ] . 7 . D. Condon et al. , Science 308 , 95 ( 2005 ) ; published online 24 February 2005 ( 10.1126/science.1107765 ) . 8 . S. J. Burns , A . Matter , Eclogae Geol . Helv . 86 , 595 ( 1993 ) . 9 . C. R. Calver , Precambrian Res . 100 , 121 ( 2000 ) [ Abstract ] . 10 . F. A. Corsetti , A. J. Kaufman , Geol . Soc . Am . Bull . 115 , 916 ( 2003 ) [ Abstract ] . 11 . P. F. Hoffman et al. , Science 281 , [ 1342 ] ( 1998 ) . 12 . P. F. Hoffman , D. P. Schrag , Terra Nova 14 , 129 ( 2002 ) [ Abstract ] . 13 . S. H. Xiao et al. , Nature 391 , 553 ( 1998 ) [ Abstract ] . 14 . J. Y. Chen et al. , Science 305 , [ 218 ] ( 2004 ) . 15 . N. Christie-Blick et al. , Am . J. Sci . 290A , 295 ( 1990 ) . 16 . A. J. Kaufman , F. A. Corsetti , EOS Trans . AGU 85 , Fall Meet . Suppl . Abstr . PP41A-0575 ( 2004 ) ."
"So , any creationist want to tell us why these people are so wrong ? Is the international committee a sign of a conspiracy ? Are they trying to hide the fact that radiometric dating techniques do n't work ? Is there really a strong causal relationship between the environmental changes recorded in the rocks and the fossils found there ? Is there really `` missing time `` or was the 85 million years of the Ediacaran Period really contained in 100m of rock ? Is atmospheric oxygen a result of tectonic forces ? Well , what part of this article do creationist object to ?"
"Early animals ? A pile of three-dimensionally preserved Comparison of broadly equivalent strata in southern Australia and the western United States suggests a stratigraphic architecture similar to that in southern China"
"So , any creationist want to tell us why these people are so wrong ? Is the international committee a sign of a conspiracy ?"