Collaborative Research: Estimating the Tempo of the Cambrian Explosion

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Title: Collaborative Research: Estimating the Tempo of the Cambrian ExplosionPrinciple Investigators: Steve Wang, Susannah Porter, Adam MaloofSwarthmore College, University of California Santa Barbara, and Princeton UniversityTechnical description: Due to recent advances in geochronology, stratigraphy, and paleontology, the broad pattern of the Cambrian explosion is now known, but details of this event remain unclear, including the rate of diversification, the order of appearances of major clades, and the number and timing of pulses of origination. The goal of the work proposed here is to generate a statistically rigorous timeline for the appearances of skeletal animals during the first ~25 million years of the Cambrian(Nemakit-Daldynian and Tommotian stages). It will build on earlier work by PIs Maloof and Porter and their collaborators that used radiometrically calibrated carbon isotope chemostratigraphy to reconstruct the pattern of appearances through this interval at two million year resolution. That work suggested that the diversification of skeletal animals began early and extended throughout this interval, with pulses of appearances ca. 540-538 Ma, 534-530 Ma, and 524-522 Ma. However, it is not clear to what extent preservational biases and uncertainties in correlation and dating have influenced these patterns. The research proposed here will use recently developed statistical methods to address these concerns. In particular, it will (1) use a new method developed by PI Wang and colleagues to estimate a confidenceinterval for the duration of the diversification (and thus provide an estimate of its rate), and (2) use both a randomization procedure and a new method developed by Wang and an undergraduate student to identify the most likely number of pulses of origination and their timing. As a side benefit, this work will also provide estimates of the time of origination for the skeletal genera in the database, as well as estimates of recovery potential and diversity curves. Finally, this work will fund ongoing efforts by PIs Maloof and Porter and graduate student Moore to augment the dataset, currently composed of 150 skeletal genera from 24 sections in China, Mongolia, and Siberia, with the eventual goal of extending coverage to later Cambrian time.Non-technical explanation: The Cambrian Explosion has long been regarded as one of the most important but most perplexing events in Earth history. It puzzled Darwin, who thought it a serious challenge to his ideas. We now know that there is a Precambrian fossil record, of which Darwin was unaware, but the significance of the Cambrian Explosion is still unparalleled: after approximately 3 billion years of mostly unicellular existence, multicellular life suddenly (on geological timescales) diversified, as nearly all of the major 'types' of animals (phyla or body plans) appeared during the early Cambrian. Furthermore, after approximately 3 billion years of soft-bodied existence, life suddenly evolved mineralized skeletons in many different lineages independently. We still do not know what drove this event: ecology (predator-prey interactions), biology (changes in the developmental tool kit), environment (changes in oxygen levels) or some combination of all three. In this project, we attempt for the first time to combine all available stratigraphic data (isotopic, biostratigraphic, geochronologic) to piece together a detailed record of biological and environmental change in the Cambrian.