Stephen A. Leslie

The Chengjiang Biota: Record of the Early Cambrian Diversification of Life and Clues to Exceptional Preservation of Fossils

The Chengjiang Biota, from Yunnan, China, is the most diverse assemblage of Early Cambrian marine fossils known. Just like the celebrated Burgess Shale (Middle Cambrian) of British Columbia, Canada, Chengjiang preserves not only fossils having hard skeletal parts (which is typical of most sedimentary deposits), but it also preserves in exquisite detail nonmineralized skeletal parts and internal soft parts of organisms (which is much more unusual in sedimentary deposits). The Chengjiang deposit, and the somewhat younger Burgess Shale, both provide important guides to diversity and evolutionary rates during the early Phanerozoic diversification event known as the Cambrian “explosion.” The Chengjiang Biota bridges a critical time between decline of the Late Neoproterozoic (latest Precambrian) Ediacaran biota and the terminal Early Cambrian extinction, and provides further evidence that the Cambrian explosion is part of an evolutionary transition that began in the Late Neoproterozoic. As we seek to understand the circumstances surrounding exceptional preservation generally, not just during the Cambrian, Chengjiang provides an important perspective on depositional conditions. Interpretation of the preservation of Burgess Shale–type organisms has been long dominated by the Burgess Shale model, in which organisms were washed from an oxic environment, where they were living, into an anoxic environment, where they were quickly buried. Anoxia inhibited the destructive activity of biodegraders (scavengers, bacteria, and burrowers) and probably played a role in early diagenesis. Other deposits of exceptional preservation indicate that biodegraders were limited long enough for the early stages of fossilization to occur under at least two other circumstances. First, immobile benthic creatures could be smothered in place by rapidly deposited mud, and then preserved through early diagenetic activity mediated by anoxic conditions developed within the sediment. Second, in Chengjiang muds, exceptional preservation is inferred to have occurred in a restricted-shelf, shallow sea. Here, as in some Carboniferous deposits, factors related to tidally influenced shelf conditions limited the activity of biodegraders, and high sedimentation rates provided for quick burial. With the addition of new models for exceptional preservation in the Cambrian, this phenomenon should be viewed less as a result of extraordinary, one-time-only, depositional conditions, and more the result of minor or short-lived perturbations in depositional circumstances common to epeiric seas. Similar perturbations led to exceptional preservation in similar environments, but at different times, during the Phanerozoic. INTRODUCTION The Neoproterozoic-Cambrian transition was a time of fundamental change in the history of life. Between the Late Neoproterozoic (ca. 570 Ma) and the late Early Cambrian (ca. 510 Ma), representatives of most important, multicellular marine animals and plants had evolved sufficiently to leave a fossil record, and had undergone early experimentation with body plans and habitats. During the transition interval, predation had emerged as a significant factor in evolution (McMenamin, 1986; Babcock, 1993; Bengtson, 1994), and probably had a causal link, along with geochemical factors, to the appearance and later development of mineralized skeletons. By the Early Cambrian, a transition from a microbial matdominated sediment-water interface to a more blurry, burrowed interface in shallowmarine settings was well under way (Seilacher and Pfluger, 1994; Bottjer et al., 2000). This time of rapid diversification among marine animals, experimentation with new body plans, and shifting ecological setting, is referred to as the Cambrian “explosion.” In terms of macroscopic organisms, this rather protracted “event” is represented in the rock record by a transition (Bengtson, 1994; Grotzinger et al., 1995) from Neoproterozoic strata yielding nonmineralized Ediacaran-type organisms (Seilacher, 1989; Fedonkin, 1994; Narbonne, 1998), a few small, hard part–secreting organisms (Grant, 1990; Bengtson, 1994; Gehling and Rigby, 1996; Grotzinger et al., 2000), and a limited number of trace fossils (e.g., Corsetti and Hagadorn, 2000) to Lower Cambrian strata having an increasing array of fossils (Figs. 1, 2). Lowermost Cambrian strata (Nemakit-Daldynian to Tommotian stages) yield small, isolated plates (small shelly fossils, which are disarticulated multielement skeletons) and few trace fossils, but overlying strata of the Atdabanian and Botomian stages have more abundant and diverse fossils comprising shelly skeletons of invertebrates (mollusks, brachiopods, echinoderms, hyoliths, and reef-forming archaeocyathid sponges) and an increasing number of trace fossils (Fig. 2). In this interval, the rich fossil record of the Phanerozoic begins. By about 518 Ma, and in the midst of this dramatic biological change, siliciclastic muds in present-day Chengjiang County and surrounding areas of Yunnan Province, China (Figs. 3, 4), buried and preserved in great anatomical detail the remains of animals, plants, and macroscopic bacterial colonies that comprise the Chengjiang Biota (Figs. 1, 5, 6; Table 1). Biological changes that occurred during the Neoproterozoic-Cambrian transition closely followed major physical and chemical changes of global scale. The Late Neoproterozoic witnessed the breakup of Rodinia, and collisional events that resulted in partial assembly of Gondwana (Hoffman, 1992; Unrug, 1997; Karlstrom et al., 1999). The Chengjiang Biota:

Isotopic and elemental systematics of Sr and Nd in 454 Ma biogenic apatites: implications for paleoseawater studies

Pristine conodonts (CA1 4 IS), inarticulate brachiopods, and conulariids, all from a single hand sample of Ordovician limestone, define a co-varying trend of *‘Sr/ “Sr and Sr concentration. Most of the apatitic fossils have *‘Sr/ *6Sr ratios that are more radiogenic than the enclosing whole-rock limestone, indicating a general susceptibility of biogenic apatites to post-depositional Sr exchange. ‘Ihe largest isotopic shifts were measured in inarticulate brachiopods and conulariids, and deduced for conodont basal body material. Conodont crown material exhibits the smallest effects. The Sr exchange effects are strongly dependent on differences in apatite composition, as revealed by contrasting Ca/P ratios. Although conodont crown material (with low Ca/P ratios) is less prone to isotopic disturbance relative to other types of coexisting apatite fossils, high resolution X-ray mapping reveals that even conodont crowns exchange Sr, as is shown by a gradient of decreasing Sr concentration from crown rim to core. In contrast to Sr, all coexisting fossil apatites have identical initial 143Nd/ l”Nd ratios over a wide range of Nd concentration. No relationship between *‘Sr/ 86Sr and ‘43Nd/ l”Nd was observed despite a pronounced antithetic pattern of Sr and Nd distribution both between the fossil types, and within individual conodonts containing preserved basal body material. In agreement with earlier studies, it is concluded that the bulk of the Nd in fossil apatites is from seawater that originally overlay the depositional site.

Three new Ordovician global stage names

Extensive work by the International Subcommission on OrdovicianStratigraphy (ISOS) has led to considerable progress in the establish-ment of a new chronostratigraphic classification of the OrdovicianSystem which now includes seven stages (Webby 1998; Finney 2005).However, until recently, only three of these global stages havebeen formally named and ratified by the International Commissionon Stratigraphy (ICS). These are the lower stage of the LowerOrdovician (Tremadocian or 1

MOHAWKIAN (UPPER ORDOVICIAN) CONODONTS OF EASTERN NORTH AMERICA AND BALTOSCANDIA

Abstract A collection of 57,877 conodont elements that includes 39 species representing 30 genera was made from the upper part of the Phragmodus undatus Zone and the lower part of the Plectodina tenuis Zone (late Turinian–early Chatfieldian). Conodont samples were collected from 30 sections in eastern North America and Baltoscandia, where the P. undatus–P. tenuis Zone boundary projects into B. alobatus Subzone based on K-bentonite bed correlation. Elements previously assigned to form-species of Oistodus are shown to be apparatus associates in coniform apparatuses of Besselodus. Pseudobelodina manitoulinensis new species is described and the apparatus-based taxonomy of Scyphiodus primus and Staufferella polonica are proposed.

DID A VOLCANIC MEGA-ERUPTION CAUSE GLOBAL COOLING DURING THE LATE ORDOVICIAN?

Abstract The Late Ordovician Taconic orogeny was associated with volcanic eruptions along the subduction zones of the Iapetus Ocean. One of these eruptions, which led to the deposition of the Deicke K-bentonite Bed, is believed to have been larger than the largest recent and subrecent volcanic eruptions (e.g., Toba, Pinatubo). The Deicke eruption has been proposed to have led to a cooling event and associated faunal turnover during the Sandbian–Katian of Laurentia based in part on the observed lowering of global surface temperature after recent mega-eruptions. We tested for a geologically resolvable climatic perturbation associated with the Deicke eruption by estimating changes in ocean temperatures from the oxygen isotope ratios of single-species separates of conodont apatite from a section of the Carimona Member of the Platteville Formation in southeastern Minnesota, United States, that includes the Deicke K-bentonite. In contrast to predictions of models invoking more or less direct volcanic forcing for Ordovician climate trends, we found no obvious or consistent change in temperature at or above the bentonite, but did see evidence of cooling (∼4 °C) among presumed nekto-benthic taxa in the 0.7 meters of the section below the bentonite. Thus, at least for the study area, there is no evidence that the Deicke eruption induced a significant cooling event.

Precise timing of the Late Ordovician (Sandbian) super-eruptions and associated environmental, biological, and climatological events

Two of the largest known eruptions in the Phanerozoic produced the Ordovician Millbrig K-bentonite of North America and the Kinnekulle K-bentonite of Scandinavia, which have been previously suggested to be coeval. The Millbrig K-bentonite from Kentucky, USA and the Kinnekulle K-bentonite from Bornholm, Denmark yielded chemical abrasion thermal ionization mass spectrometry U–Pb zircon dates of 452.86 ± 0.29 and 454.41 ± 0.17 Ma (2σ analytical uncertainty), respectively, thus showing significant age differences contrary to what is generally held. These data and four additional newly dated K-bentonites directly establish the first radioisotopic age constraints for the Ordovician Katian–Sandbian global stage boundary, refine global stratigraphic correlations, date associated chemostratigraphic events, and suggest an alternative volcanic–climate hypothesis for the Late Ordovician. Supplementary material: U–Pb radioisotopic data table and analytical methods are available at www.geolsoc.org.uk/SUP18636.

Calibration of a conodont apatite-based Ordovician 87Sr/86Sr curve to biostratigraphy and geochronology: Implications for stratigraphic resolution

The Ordovician 87 Sr/ 86 Sr isotope seawater curve is well established and shows a decreasing trend until the mid-Katian. However, uncertainties in calibration of this curve to biostratigraphy and geochronology have made it diffi cult to determine how the rates of 87 Sr/ 86 Sr decrease may have varied, which has implications for both the stratigraphic resolution possible using Sr isotope stratigraphy and efforts to model the effects of Ordovician geologic events. We measured 87 Sr/ 86 Sr in conodont apatite in North American Ordovician sections that are well studied for conodont biostratigraphy, primarily in Nevada, Oklahoma, the Appalachian region, and Ohio Valley. Our results indicate that conodont apatite may provide an accurate medium for Sr isotope stratigraphy and strengthen previous reports that point toward a signifi cant increase in the rate of fall in seawater 87 Sr/ 86 Sr during the Middle Ordovician Darriwilian Stage. Our 87 Sr/ 86 Sr results suggest that Sr isotope stratigraphy will be most useful as a high-resolution tool for global correlation in the mid-Darriwilian to mid-Sandbian, when the maximum rate of fall in Sr/

Taphonomy of lacustrine interbeds in the Kirkpatrick Basalt (Jurassic), Antarctica

Abstract The Kirkpatrick Basalt (Jurassic) of South Victoria Land and the Central Transantarctic Mountains, Antarctica, includes sedimentary interbeds representing shallow lakes and ephemeral ponds (some with microbial mat accumulations), deep permanent lakes, and lake-margin areas, especially vegetated wetlands. Fossil assemblages in these sedimentary interbeds are dominated by spinicaudatans (conchostracans), but ostracodes, insect nymphs, actinopterygian fish, and plants are locally abundant. Similar biotas in contrasting contemporaneous deposits allow the taphonomy of these organisms to be compared across lacustrine depositional settings. Spinicaudatan carapaces and fish remains are preserved primarily in calcium phosphate, whereas ostracode carapaces are preserved in calcium carbonate, reflecting the original skeletal composition of the animals. Where microbial mats are present, silica replacement of spinicaudatan carapaces occurs more extensively than in other deposits; microbial processes may have enhanced silicification. This study is the first well-documented example of microbial mat influence on preservation in high-latitude lacustrine systems.

Depositional history, tectonics, and provenance of the Cambrian-Ordovician boundary interval in the western margin of the North China block

Cambrian–Ordovician strata of the North China block, one of China’s main tectonic provinces, are a thick (up to 1800 m) succession of mixed carbonate and siliciclastic sedimentary rocks. Sedimentological, biostratigraphic, and chemostratigraphic analysis of strata that straddle the Cambrian-Ordovician boundary at the Subaiyingou section in the present-day western part of Inner Mongolia (northwest China) indicate the presence of a significant unconformity between mixed carbonate–fine-siliciclastic strata of the Cambrian Series 3 Abuqiehai Formation, and dominantly carbonate strata of the early Middle Ordovician Sandaokan Formation. The latter is a transgressive systems tract with retrogradationally stacked parasequences that include lowstand shoreline quartz sandstone deposits. The Abuqiehai strata have similar sedimentological characteristics to those of the Cambrian Laurentian inner detrital belt, including slightly bioturbated lime mudstone and marlstone/shale, grainstone, flat-pebble conglomerate, and microbialite. The lower part of the Sandaokan Formation records the rising limb of the middle Darriwilian positive isotopic excursion, recognized herein for the first time in the western North China block. A Cambrian-Ordovician unconformity is developed in many successions globally, and our section in Inner Mongolia records a hiatus of similar timing and duration to a regionally extensive unconformity recorded along the ancient northern Indian continental margin. Other parts of the North China block record a hiatus of much shorter duration but show a similar record of input of siliciclastic sediment above the unconformity. We interpret the western margin of the North China block to have been affected by a regionally significant tectonic event that occurred on the northern margin of east Gondwana, the Kurgiakh or Bhimphedian orogeny. The Inner Mongolian region was, therefore, likely an along-strike continuation of the northern Indian margin, in contrast to most recent paleogeographic reconstructions.

Strontium isotope (87Sr/86Sr) stratigraphy of Ordovician bulk carbonate: Implications for preservation of primary seawater values

The present study on bulk carbonate 87 Sr/ 86 Sr stratigraphy represents a companion work to earlier research that presented a conodont apatite-based Ordovician seawater 87 Sr/ 86 Sr curve for the Tremadocian–Katian Stages (485–445 Ma). Here, we directly compare the curve based on conodont apatite (including some new data not published in earlier work) with a new curve based on 87 Sr/ 86 Sr results from bulk carbonate from the Tremadocian–Sandbian Stages. We sampled eight Lower to Upper Ordovician carbonate successions in North America to assess the reliability of bulk carbonate to preserve seawater 87 Sr/ 86 Sr and its utility for 87 Sr/ 86 Sr chemostratigraphy. A high-resolution 87 Sr/ 86 Sr curve based on 137 measurements of bulk conodont apatite is used as a proxy for seawater 87 Sr/ 86 Sr ( 87 Sr/ 86 Sr seawater ). In total, 230 bulk carbonate samples that are paired to conodont samples were measured for 87 Sr/ 86 Sr in order to determine the conditions under which 87 Sr/ 86 Sr seawater is preserved in bulk carbonate. Results indicate that well-preserved bulk carbonate can faithfully record the 87 Sr/ 86 Sr seawater trend, but that its 87 Sr/ 86 Sr values are commonly more variable than those of conodont apatite. On average, bulk carbonate samples of the same age vary by 10–20 × 10 −5 , compared to 5–10 × 10 −5 for conodont apatite. The amount of isotopic alteration of bulk carbonate from seawater 87 Sr/ 86 Sr (Δ 87 Sr/ 86 Sr) was determined by taking the difference between 87 Sr/ 86 Sr values of bulk carbonate and the approximated seawater trend based on the least radiogenic conodont 87 Sr/ 86 Sr values. Cross plots comparing Δ 87 Sr/ 86 Sr values to bulk carbonate Sr concentration ([Sr]) and conodont color alteration indices (CAI; an estimate of the thermal history of a rock body) indicate that bulk carbonate is most likely to preserve 87 Sr/ 86 Sr seawater (minimally altered) when either: (1) bulk carbonate [Sr] is greater than 300 ppm, or (2) carbonate rocks experienced minimal thermal alteration, with burial temperatures less than ~150 °C. Carbonates with intermediate [Sr] (e.g., between 130 and 300 ppm) can also yield 87 Sr/ 86 Sr seawater values, but results are less predictable, and local diagenetic conditions may play a greater role. Modeling results support the argument that seawater 87 Sr/ 86 Sr can be preserved in bulk carbonates with low [Sr] if pore water:rock ratios are low ( 87 Sr/ 86 Sr is similar to the seawater 87 Sr/ 86 Sr value preserved in limestone. Bulk carbonate samples that meet these criteria can be useful for high-resolution measurements of 87 Sr/ 86 Sr seawater , with a sample variation on par with fossil materials ( −5 ), particularly for successions where well-preserved fossil material (i.e., conodonts or brachiopods) is not available, such as Precambrian strata, sequences recording mass extinction events, or otherwise fossil-barren facies. These criteria and model predictions based on bulk carbonate [Sr] must be considered in the context of whether a limestone accumulated under calcite seas (e.g., Ordovician), with relatively high seawater Sr/Ca, or aragonite seas, in which case the diagenetic transformation of aragonite to calcite may result in incorporation of non-seawater Sr.