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A new look at evolutionary rates in deep time: Uniting paleontology and high-precision geochronology

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A new look at evolutionary rates in deep time: Uniting paleontology and high-precision geochronology
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  Vol. 8, No. 9September 1998 GSATODAY A Publication of the Geological Society of America ABSTRACT It is now possible to routinelydetermine the age of 200–600-m.y.-oldvolcanic rocks interlayered with fossil-bearing deposits to uncertainties of lessthan 1 m.y. with uranium-lead zircongeochronology. This level of precision,coupled with the recognition that vol-canic ash beds are much more commonin fossiliferous rocks than previouslyrealized, opens new opportunities forthe study of evolutionary rates in deeptime. It is now possible to constrainrates of evolutionary radiations, massextinctions, and other evolutionaryevents as well as evaluate potentiallydiachronous biostratigraphic bound-aries. For example, a combination of detailed biostratigraphic and chemo-stratigraphic data with new U-Pb zircondates for the late Neoproterozoic andEarly Cambrian has demonstrated thatthe soft-bodied Ediacaran fossils imme-diately underlie the Cambrian, that thebase of the Cambrian is much youngerthan previously recognized, and thatthe Cambrian explosion lasted 10 m.y.or less. Other recent studies have shownthe Middle and Late Cambrian eachlasted only about 10 m.y., suggestingthat the duration of the included trilo-bite zones was similar to those of Juras-sic ammonites. Recent data from theLate Permian and earliest Triassic of south China now constrain the durationof the most profound mass extinctionin the history of life to less than 1 m.y.Collaboration between paleontologistsand geochronologists offers the prospectof accurately assessing the rates of evo-lutionary processes, from speciationtoevolutionary radiations and mass ex-tinctions, throughout the Phanerozoic. “How fast, as a matter of fact, do animals evolve in nature? That is the fundamental observational problem that the geneticist asks the paleontologist” (Simpson, 1944). INTRODUCTION Answers to Simpson’s question aboutevolutionary rates have generally lackedprecision, particularly for the pre-Ceno-zoic. Although many paleontological Evolutionary Rates continued on p. 2 A New Look at Evolutionary RatesinDeep Time: Uniting PaleontologyandHigh-Precision Geochronology Samuel A. Bowring,  Department of Earth, Atmospheric and Planetary Sciences,  Massachusetts Institute of Technology, 54-1124, 77Massachusetts Avenue, Cambridge, MA 02139, sbowring@mit.edu.  Douglas H. Erwin,  Department of Paleobiology, MRC-121, National Museum of Natural History, Washington, DC 20560,erwin.doug@nmnh.si.edu Figure 1. The Permian-Triassic boundary at Meishan,China, showing dates fortheash beds described inBowring et al. (1998). Theend Permian mass extinctionis recorded within the last1m below the lowest ashbed (left). This ash bed liesabout 10 cm below thePermian-Triassic boundaryasdefined on the basis ofconodonts, and just belowthe major isotopic excursion.The upper two ash beds arein Lower Triassic strata. Seetitles/ authorsdatabase — http://www.geosociety.org Technical ProgramSchedule — page 19 Preregistration Deadline — September 18 1998AnnualMeeting TORONTOTORONTO  2GSA TODAY, September 1998 A New Look at Evolutionary Rates inDeep Time: Uniting Paleontology andHigh-Precision Geochronology ......1In Memoriam..........................2Correction.............................2About People..........................3New IEE Director.......................3GSA Thanked for Books................8Notice of Open Council Meeting.......8Washington Report....................10Earth Science Week....................11GSAF Update..........................12Energy for the 21st Century............12Rock Stars: J. W. Dawson...............14New Congressional Science Fellow......161998 Annual Meeting—Toronto Late-Breaking Research Sessions........ 17 Program Calendar ...................... 19 Short Courses .......................... 24 Grad School Information Forum ......... 25 Student Breakfast ...................... 25Call for Proposals—Denver 1999........251999 Section Meetings South-Central ...................... 27 Northeastern ....................... 28 Southeastern ....................... 31GSA Awards Research Grants...........33 Bulletin and Geology Contents..........35Future GSA Meetings..................36Classifieds.............................37Calendar..............................39 IN THIS ISSUE GSA TODAY September Vol. 8, No. 91998  GSA TODAY (ISSN 1052-5173) is published monthlyby The Geological Society of America, Inc., with offices at 3300Penrose Place, Boulder, Colorado. Mailing address: P.O. Box9140, Boulder, CO 80301-9140, U.S.A. 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Contact Membership Services at (800) 472-1988,(303) 447-2020 or member@geosociety.org for member-ship information. Nonmembers & Institutions: Freewith paid subscription to both GSA Bulletin and Geology, otherwise $50 for U.S., Canada, and Mexico; $60 else-where. Contact Subscription Services. Single copies maybe requested from Publication Sales. Also available on anannual CD-ROM, (together with GSA Bulletin, Geology, GSAData Repository, and an Electronic Retrospective Index to journal articles from 1972); $89 to GSA Members, otherscall GSA Subscription Services for prices and details. Claims:For nonreceipt or for damaged copies, members contactMembership Services; all others contact Subscription Ser-vices. Claims are honored for one year; please allow suffi-cient delivery time for overseas copies, up to six months. STAFF: Prepared from contributions from the GSAstaffand membership. Executive Director: Donald M. Davidson, Jr. Science Editors: Suzanne M. Kay, Department of Geological Sciences, Cornell University, Ithaca, NY 14853; Molly F. Miller, Department of Geology, Box 117-B, Vanderbilt University, Nashville, TN 37235. Forum Editor: Bruce F. Molnia, U.S. Geological Survey,MS917, National Center, Reston, VA 22092  Director of Publications: Peggy S. Lehr  Managing Editor: Faith Rogers  Assistant Editor: Vanessa Carney  Production Manager: Jon Olsen  Production Editor and Coordinator: Joan E. Manly  Graphics Production: Joan E. Manly, Leatha L. Flowers  ADVERTISING: Classifieds and display: contact AnnCrawford, (303) 447-2020; fax 303-447-1133; acrawfor@geosociety.org.Issues of this publication are available as electronic Acrobatfiles for free download from GSA’s Web Site, http://www.geosociety.org. They can be viewed and printed on variouspersonal computer operating systems: MSDOS, MSWin-dows, Macintosh, and Unix, using the appropriate Acrobatreader. Readers are available, free, from Adobe Corporation:http://www.adobe.com/acrobat/readstep.html.This publication is included on GSA’s annual CD-ROM, GSA Journals on Compact Disc. Call GSA Publication Sales for details.Printed in U.S.A. using pure soy inks. 50% Total Recoverd Fiber10% Postconsumer issues have revolved around evolutionaryrates, the inadequacies of the geologictime scale have generally precluded theirresolution. Yet, accurate determination of rates is critical to answering importantquestions: How does the rate of speciationvary in different environments? How longdo stable community assemblages persist?How much time is involved in evolution-ary radiations or postextinction recover-ies? How rapidly do species and communi-ties respond to climatic and otherenvironmental changes? And perhapsmost significant, have these rates changedthrough the Phanerozoic? Without gooddata on the amount of time involved inthese events, any determination of ratemust be suspect.Rates of some evolutionary and geo-logical processes can be determined for theCenozoic with some precision by meansofa variety of techniques (e.g., climaticcyclicity, paleomagnetic reversals; Hilgenet al., 1997). However, farther back in thegeologic record, the precision with whichwe can resolve events decreases, the accu-racy of the geologic time scale degrades,and the reliability of information used toassess evolutionary rates falls dramatically.Consequently, much of our currentknowledge of rates of change is based oninterpolation of absolute time betweenafew well-constrained tie points usedtoconstruct relatively imprecise geologictime scales (e.g., Harland et al., 1990),often with the additional assumptionthatequal thicknesses of rock representequal amounts of time. In Memoriam Saul Aronow Beaumont, TexasMay 15, 1998 Robert E. Barnett Washington Court House,Ohio Bruce B. Hanshaw McLean, VirginiaJuly 18, 1998 Laurence B. James Folsom, CaliforniaJune 8, 1998 Louis Pavlides College Park, MarylandApril 8, 1998 Grover Reinbold Reno, NevadaApril 24, 1998 Mark Springett Boulder, ColoradoJuly 16, 1998 Leonard R. Wilson Norman, OkalahomaJuly 15, 1998 Evolutionary Rates continued from p. 1 C ORRECTION : JulyScience Article The illustration in the center of the July 1998 issue (p. 16 and 17) is Figure 2 (notFigure3) of the paper “Probing the Archean and Proterozoic Lithosphere of WesternNorth America” by Deep Probe Working Group. The Figure 2 caption on p. 4 should beon p.16, and the Figure 3 caption on p. 16 should be on p. 4. On p. 3, leftmost column, the fourth line under the head Seismic Observations: ThreeProvince-Related Seismic Signatures should be: Cheyenne belt (Fig. 2; see p. 16–17),and the footnote at the bottom of the leftmost column on p. 4 should be: Figure 2 is onp.16–17.  GSA TODAY, September 19983 For Paleozoic and Mesozoic rocks,high-precision U-Pb geochronology of zir-cons can be exploited to yield uncertain-ties of 1 m.y. or less. The continued refine-ment of chemical separations of U and Pbfrom zircon and the improvement of massspectrometry allow high-precision analy-ses of single grains of zircon containing aslittle as 10 picograms (10 -12 g) of radio-genic Pb (Mundil et al., 1996; Bowring etal., 1998). The ultimate test of the resolv-ing power of the technique occurs whenmultiple volcanic horizons are interlay-ered with fossil-bearing rocks and the cal-culated ages do not violate stratigraphicorder.The integration of geochronology,paleontology, and chemostratigraphy hasrevolutionized our knowledge of severalimportant episodes in geologic history,including the sudden major increase of animals during Neoproterozoic to Cam-brian time and, most recently, the mostextensive mass extinction in the historyoflife, the end-Permian event (Fig. 1). EVOLUTIONARY RATES Although Simpson (1944) identifiedand focused on the importance of deter-mining fine-scale evolutionary rates, hewas handicapped by lack of data on abso-lute dates. Kurten (1959) found that ratesof morphological change in Pleistocenemammals exceeded mammalian evolu-tionary rates during the Tertiary. Subse-quent work has established that thesehigher rates were an artifact: Evolutionaryrates are inversely related to time scale.The shorter the time period studied, thefaster the observed rates of change,whether the object of study is morphology(Gingerich, 1983, 1993), or sedimentation(Sadler, 1981). This is simply because slowlong-term rates are difficult to measureover the short term and because fast long-term rates are generally unsustainable overthe long term. Since evolutionary rates arestrongly dependent upon the interval of time over which they are measured, theycan only be meaningfully compared whenthe same time scale is used (Gingerich,1993; Foote, 1994).More recently, the determination of precise evolutionary rates has faded fromcurrent paleobiological research in favorofidentification of large-scale macroevolu-tionary patterns. This is in part becausetemporal resolution has been too coarsetoallow evaluation of finer-scale processes(Campbell and Marshall, 1987). Recentdevelopments in geochronology suggestthat it is time for a new examination of the issue of evolutionary rates, at thescaleenvisioned by Simpson (1944). GEOCHRONOLOGY Since stratigraphic thickness cannotbe simply extrapolated to geologic time,alarge number of high-precision agesfrom interstratified volcanic ash beds arerequired in order to evaluate rates of geo-logical change. In the past 10 years therehas been a dramatic increase in the num-ber of studies concerned with calibrationof the time scale (e.g., Tucker et al., 1990;Tucker and McKerrow, 1995; Claoue-Longet al., 1991; Mundil et. al., 1996; Tucker etal., 1998). The U-Pb method applied to zir-cons separated from stratabound volcaniclayers is a powerful method for dating sed-imentary rocks because it exploits twoindependent decay schemes ( 238 U → 206 Pband 235 U → 207 Pb) within each zircon sam-ple. This method provides independentage information and a test for the degreeto which the systems were closed follow-ing crystallization. If a closed system hasbeen maintained, the two U-Pb dates( 238 U/  206 Pb and 235 U/  207 Pb) and the Pb-Pbdate ( 207 Pb/  206 Pb) for a zircon analysisshould be the same within uncertaintiesand are referred to as concordant.The past 15 years have seen markedimprovement in high-precision isotopedilution–thermal ionization mass spec-trometry (IDTIMS) U-Pb dating of zircons(Krogh, 1982; Parrish and Krogh, 1987).This is largely the result of being able toanalyze small amounts of zircon that maycontain as little as 10–25 × 10 –12 g of radiogenic Pb. This capability is the resultof low analytical blanks (0.5–2.0 × 10 –12 gof common Pb ) and improvement inmass spectrometry, especially ion-count-ing techniques. Although these methodshave revolutionized our understanding of how geological time is distributed in therock record, only recently has the fullpotential of this technique begun to berealized. High precision for its own sake isoften not an efficient strategy. We feel thatthe calibration of evolutionary rates is anexample of a problem that requires themaximum resolving power of the U-Pbtechnique. For example, Tucker et al.(1990) showed through high-precisionzircon geochronology that the meanduration of Ordovician graptolite zonesis1–2m.y. and recognized that with thisapproach, evolutionary rates of Paleozoicfauna could be evaluated. Hughes (1995)used available U-Pb geochronology toconstrain durations of Silurian graptolitezones; the range was 0.44–1.43 m.y. Calibration of the Time Scale Geochronometric calibration of arelative, chronostratigraphic time scale isstraightforward. Ideally, a volcanic rock isfound very close to the point in a strat-igraphic section chosen as the globalstratotype for the boundary between twogeologic intervals, and the volcanic rock contains a mineral, such as zircon or mon-azite, for which a precise crystallizationage can be determined. Such ideal situa- Evolutionary Rates continued on p. 4 About People GSA Fellow Daniel JeanStanley , National Museum of Natural History, Washington,D.C., was chosen by Italian uni-versities to receive the GoldenTrident Medal, a first for a NorthAmerican scientist. He was alsoinducted as a member of Italy’sAccademia Internazionale diScienze e Techniche.GSA Fellow Julia AnnTullis , Brown University,Providence, Rhode Island, willreceive the 1998 OutstandingEducator Award from the Associ-ation for Women GeoscientistsFoundation. NEW DIRECTOR FOR IEE Cathleen May has joined the headquarters staff at GSA as theDirector of the Institute for Environmental Education. Cathleenpreviously directed the national paleontological resources man-agement program for the U.S. Forest Service. More recently,shehas consulted to government and professional societies onresource management and legislative issues, and to the enter-tainment industry on issues of scientific accuracy and literacy.May earned undergraduate degrees in geology and insecondary science education at the University of Wyoming andher doctorate in integrative biology at the University of Califor-nia at Berkeley. Her primary research has focused on macro-evolutionary patterns in the terrestrial Triassic, particularly asrecorded in northwest Argentina, and on geoecological contingency in modern ecosystems.May brings to the IEE her long-standing commitment to the crucial role of the geosciencesinbuilding scientific literacy, environmental problem-solving, and Earth-system science.Contact May regarding the IEE and its programs at cmay@geosociety.org, or IEE admini-strative support person Stacey Ginsburg at sginsbur@geosociety.org. Cathleen May  4GSA TODAY, September 1998 tions are uncommon, and so calibrationrequires dating rocks elsewhere in sectionsthat can be correlated to the stratotypebymeans of bio-, chemo-, and magne-tostratigraphy. The late Neoproterozoic–Cambrian boundary, for example, isdefined as a point in rock in a sectionlocated in southeastern Newfoundland(Landing, 1992), but no volcanic rocksarepresent at or in close proximity to theboundary (Myrow and Hiscott, 1993;Landing, 1992). Consequently, the ageofthe boundary can be calibrated onlythrough correlation with other sectionsthat contain datable volcanic rocks inclose proximity to the boundary. It is a useful exercise to consult yourfavorite time scale to see how the age of aparticular boundary was determined. Thechronostratigraphic time scales that we alldepend on (Harland et al., 1990; Shergold,1995; Gradstein et al., 1995) typicallyassign an absolute age for a biostrati-graphic boundary that reflects averagingof several, often imprecise age determina-tions and estimates; in many cases theuncertainty is several million years. As thegeochronological resolution and the num-ber of calibration studies has increaseddramatically in the past ten years, existingtime scales have been rendered inade-quate, especially for the Paleozoic (Fig. 2). Methods Pb analyzed from zircon samples isamixture of radiogenic and common Pb.Radiogenic Pb is produced by the decay of U in the zircon crystal. A small amountofcommon Pb is sometimes incorporatedinto the zircon when it crystallizes, andcommon Pb is added to the sample viasample processing (analytical blank).When calculating a date for a zircon, onemust subtract the common Pb from thetotal Pb, and in doing so, one mustassume a composition of both the blank and any indigenous common Pb. In gen-eral, the uncertainties associated withmaking blank and common Pb correctionscan be minimized with large radiogenicPb/ common Pb ratios, which generallyscale with sample size. Sources of system-atic error may include error in spike cali-bration and uncertainty in the decay constants for uranium. These later uncer-tainties would apply to all analyses donein a particular lab; although they mightaffect absolute age determinations, the rel-ative age differences between beds are notaffected. Systematic errors can be a prob-lem when comparing dates obtained bydifferent methods.Resolution of time with uncertaintiesof 1 m.y. or less in volcanic rocks providesa special set of problems. The most signifi-cant is the ability to distinguish smallamounts of Pb loss and/or inheritance.Itis common in airfall ash deposits to findzircon grains, probably incorporated intothe eruption column, that are identical inmorphology to the indigenous popula-tion, but which can be <1 to >10 m.y.older (Landing et al., 1998). This problemcan be minimized by analyzing singlegrains of zircon. A zircon crystal’s size andits concentration of radiogenic Pb ulti-mately determine whether or not single-grain analysis is feasible. One seeks as higha ratio of radiogenic to common Pb as pos-sible for each analysis. In this way, theuncertainty on all three dates ( 206 Pb/  238 U, 207 Pb/  235 U, and 207 Pb/ 206 Pb) is low(0.1%–0.5%), and the difference betweenthe 206 Pb/  238 U and 207 Pb/  235 U dates can beevaluated for inheritance of slightly oldergrains and/or Pb-loss. In the case of com-plex zircons, it is often necessary to relaxprecision requirements so as to be able toanalyze a single grain or grain fragment.Itis this trade-off that requires the super-high-resolution ion microprobe (SHRIMP)to rely on the 206 Pb/  238 U date when deter-mining the age of Paleozoic zircons (e.g.,Claoue-Long et al., 1995).In the best-case scenario, a statisti-cally significant cluster of concordantanalyses is obtained for each sample, andweighted mean 206 Pb/  238 U, 207 Pb/  235 U and 207 Pb/  206 Pb dates are calculated. Morecommonly, a suite of zircons is discordantand defines a linear array that intersectsconcordia. In these cases, uncertainty inthe age of the zircons can be calculated forthe intersection of the discordant arraywith the concordia curve (Ludwig, 1980),or, more often, the weighted mean of the 207 Pb/  206 Pb dates can be used (e.g., Tuckeret al., 1998; Bowring et al., 1993). Whenthis approach is used, the minimumuncertainty in age is generally 1–2 m.y.There isno question that the best resultsare obtained from concordant zircons, andinolder rocks they become increasinglydifficult to find. Our technique could betermed the “brute force” approach. Wetypically attempt to analyze a minimumof 5–10 fractions of zircon for each ashbed to assess our reproducibility and toreduce errors in the age (this does notinclude the analyses that show evidencefor inheritance, severe Pb loss, high com-mon Pb, etc.). The test of our approach isto analyze multiple samples of the samehorizon, as well as different beds in strati-graphic order (Grotzinger et al., 1995;Bowring et al., 1998). RESOLVING THE CAMBRIAN RADIATION The explosive diversification of higher marine invertebrates in the EarlyCambrian is the single most dramaticevent documented in the fossil record.Rocks that are late Neoproterozoic in agecontain the soft-bodied remains of Edi- Figure 2. Changing views oflate Neoproterozoic–Early Ordovician time. The estimated boundary datesshown are from Harland et al. (1982), the Decade of North American Geology (DNAG) in 1983, Harlandet al. (1990), the International Union of Geological Sciences (IUGS; Cowie and Bassett, 1989), and thelatest evidence (1998) discussed in this paper. The open circles represent poorly constrained geochrono-logic tie-points and the black circles better-constrained tie-points. Error bars are shown for the Harlandetal. time scales. Note that the Manykaian stage was added to the Cambrian in 1992, and that thesubdivisions of the late Neoproterozoic have not been firmly established. 476493510517536570590610470490510523570650483490509500543565605 478478488505523540570488505523540590630670670650630610590570 550530510490470Harland1982DNAGHarland1989IUGS1998      L    o    w    e    r     O    r     d    o    v     i    c     i    a    n     C    a    m     b    r     i    a    n     V    e    n     d     i    a    n ArenigTremadocianLateMiddleEarly EdiacaranVarangian Evolutionary Rates continued from p. 3  GSA TODAY, September 19985 acaran fossils and a small assemblage of skeletonized tubes, as well as recentlydiscovered fossil embryos and spongespicules (ca. 570 Ma; Xiao et al., 1998;Liet al., 1998). The first Cambrian shellyfossils occur in carbonates near the base of the Manykaian Stage, currently the basalstage of the Cambrian. Trace fossils, skele-tal fossils, and spiny organic microfossilsdiversified rapidly during the ensuingTommotian and Adtabanian stages, sothatby the end of the Adtabanian, mostdurably skeletonized phyla and classes of marine invertebrates are recognized. Con-troversy continues over the rapidity of thisradiation and the possibility that consider-able diversity existed long before the baseof the Cambrian, but has not been recog-nized in the fossil record because of smallsize or low preservation potential.Since 1990, U-Pb geochronologicalstudies have constrained the age of thelate Neoproterozoic–Cambrian boundary,the duration of diverse Ediacaran fossils,the burst of innovations during the Tom-motian-Atdabanian, the Lower-MiddleCambrian boundary, and the Cambrian-Ordovician boundary (Compston et al.,1992, 1995; Bowring et al., 1993; Isachsenet al., 1994; Grotzinger et al., 1995; Land-ing et al., 1997, 1998; Davidek et al.,1998). Figure 3 is a revised time scale forthe Cambrian Period showing the controlprovided by U-Pb zircon ages on biostrati-graphic boundaries.Although volcanic rocks are uncom-mon interbedded with Ediacaran fossils,the fossils generally postdate Varanger-aged glaciogenic rocks (ca. 600 Ma) ineastern North America. In Newfoundland,Benus (1988) reported an age for volcanicrocks immediately overlying Ediacaranfossils at 565 ± 3 Ma. Compston et al.(1995) reported for volcanic rocks fromthe subsurface of Poland which are corre-lated with Ediacaran-bearing strata inUkraine an age of 551 ± 4 Ma. Grotzingeret al. (1995) and Narbonne et al. (1997)have documented Ediacaran fossils includ-ing the new genus Swartpuntia immedi-ately below the basal Cambrian inNamibia; this deposit is younger than543.3 ± 1 Ma. Grotzinger et al. (1995) alsoshowed that diverse small shelly fossilsoverlap with the Ediacaran fossils inNamibia. Although the Cambrian is oftenviewed as lacking Ediacaran fossils, severalexceptions have appeared recently (Crimeset al., 1995; Conway Morris, 1993; Jensenet al., 1998). The lack of any obvious gapbetween the last Ediacaran fossils and theonset of Cambrian fossils leads to the sim-ple conclusion that the Cambrian explo-sion is part of a continuous evolutionaryradiation that started in the late Neopro-terozoic (Grotzinger et al., 1995). The bios-tratigraphically defined boundary doesnot mark a sudden event or explosion inthe diversification of life, but insteadserves as an important reference point inan increasingly rich evolutionary record.Carbon isotope stratigraphy is anessential tool for correlating latest Neo-proterozoic rocks. Globally, many strati-graphic sections have yielded very similarfluctuations in carbon isotopes (Kaufmanet al., 1997; Narbonne and Knoll, 1994).This pattern of isotopic variation providesan independent framework for correlationbetween sections and allows, in somecases, calibration of the isotopic shiftsbydating volcanic layers. In Namibia,Grotzinger et al. (1995) showed that anisotopic interval known as the +2 plateauhas a duration of about 5–6 m.y. and coin-cides with occurrence of the most diverseEdiacaran assemblages.Temporal calibration of past globalevents, correlated using bio-, chemo-, andmagnetostratigraphic data sets, is possibleonly with the precise absolute age controloffered by U-Pb zircon dating of volcanicsinterlayered within sedimentarysequences. This temporal framework hasimportant implications for our under-standing of biological diversification andits possible links to contemporaneoustectonic, biogeochemical, and climaticchanges. Exciting problems remain unre-solved. What is the lower boundary of theEdiacaran faunas? Can we resolve timesufficiently during the late Neoproterozoicto identify distinct assemblages of fossilsor migration between different biogeo-graphic regions? Will additional data onthe Manykaian Stage allow better tempo-ral constraints on the gradual expansionof the small shelly fossils? Is the distribu-tion of Ediacaran organisms diachronous? NEW RESULTS FOR THE MIDDLECAMBRIAN–EARLY ORDOVICIAN Trilobites dominate Middle and LateCambrian marine assemblages in bothspecies diversity and numbers of speci-mens. Rapid speciation in trilobites hasallowed biostratigraphers to divide theMiddle Cambrian of Laurentia into sixbiostratigraphic zones, and the UpperCambrian into seven (but see Geyer andPalmer, 1995). A detailed examination of the evolutionary patterns underlying trilo-bite history during this time reveals amore interesting pattern, however. In1965 A. R. Palmer recognized a series of five larger biostratigraphic units, eachbeginning with a small number of trilobitefamilies unrelated to those in underlyingrocks. He traced the rapid diversificationof these families across several biostrati-graphic zones; the resulting diverse assem-blage was finally eliminated by a massextinction, and the cycle was repeated. Figure 3. Summarydiagram of biostrati-graphically andgeochronologicallywell constrained sam-ples that define thelate Vendian to LateCambrian time scale(after Bowring et al.,1993; Grotzinger etal., 1995). Newer dataare from: Compstonet al. (1995), Landinget al. (1998), andDavidek et al. (1998).MIT IDTIMS is Mas-sachussetts InstituteofTechnology, isotopedilution–thermalionization massspectrometry; ANUSHRIMP is AustralianNational University,super-high-resolutionion microprobe;ROMis RoyalOntarioMuseum. 580570560540530510500490520    E   d   i  a  c  a  r  a  n   F  a  u  n  a  s HighestDiversityLowDiversity      V    e    n     d     i    a    n Middle    E  a  r   l  y Nemakit-DaldynianAtdabanianBotomianTommotian LateMillions of YearsBefore Present      C    a    m     b    r     i    a    n 04080OrdersClasses04080? 531.0 ±  1.0 Ma 1 522.0 ±  1.0 Ma 1 550PolandAvalonNamibiaSiberiaWalesMoroccoU-Pb Zircon Age Control (1-MIT IDTIMS; 2-ANU SHRIMP; 3-ROM IDTIMS)   491.0 ±  1.0 Ma 1 510.0 ±  1.0 Ma 1 519.0 ±  1.0 Ma 1 543.3 ±  1.0 Ma 1 545.1 ±  1.0 Ma 1 548.8 ±  1.0 Ma 1 551.0 ±  4.0 Ma 2 565.0 ±  3.0 Ma 3 539.4 ±  1.0 Ma 1   543.9 ±  1.0 Ma 1 Evolutionary Rates continued on p. 6 
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