GSA Connects 2021 in Portland, Oregon

Paper No. 178-9
Presentation Time: 4:00 PM


ANDERSON, Ross1, WOLTZ, Christina2, TOSCA, Nicholas J.3, PORTER, Susannah2 and BRIGGS, Derek E.G.4, (1)Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, United Kingdom; All Souls College, University of Oxford, High Street, Oxford, OX1 4AL, United Kingdom, (2)Department of Earth Science, University of California, Santa Barbara, 1006 Webb Hall, Santa Barbara, CA 93106, (3)Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom, (4)Peabody Museum of Natural History, Yale University, 170 Whitney Avenue, New Haven, CT 06511; Department of Earth and Planetary Sciences, Yale University, 210 Whitney Avenue, New Haven, CT 06511

Charles Darwin famously lamented the lack of animal fossils in Precambrian strata as a valid argument against his theory of evolution. Today, a version of Darwin’s Dilemma remains with a disconnect between the two main sources of evidence for animal antiquity. Molecular clocks commonly place the origin of animals as far back as ~800 million years ago (Ma). In contrast, unambiguous animal fossils extend maximally ~574 Ma. If molecular clock estimates are accurate, the lack of animal fossils in older Neoproterozoic strata requires explanation. Darwin’s answer was that early entirely soft-bodied animals were either too small or fragile to be fossilized, or environments were unconducive to their preservation. Yet few studies have directly compared Neoproterozoic fossilization processes to those of the Cambrian Period. We compile recent taphonomic studies to address this issue. Burgess Shale-type fossilization in Cambrian mudstones is a proven repository for early animal evolution, capable of capturing small and fragile taxa with a variety of tissue biopolymers across animal phylogeny. The clay minerals kaolinite and berthierine suppress decay and can also interact directly with decaying organic matter in ways that favor its fossilization. They are commonly associated with Burgess Shale-type fossils and are a major constituent of their host mudstones. Yet Neoproterozoic mudstones rarely display the same clay mineralogy. Fossils of basal animals are thus likely to be rare, but even where conditions favor Burgess Shale-type fossilization in Tonian strata they are absent. Thus, taphonomy places a maximum ~800 Ma constraint on animal antiquity.