Darwin’s Dilemma: The importance of fossilization to our reading of animal antiquity
- Ross P. Anderson
All Souls College, University of Oxford
Charles Darwin famously lamented the absence of animal fossils in Precambrian strata as a valid argument against his theory of evolution. Today, a version of Darwin’s Dilemma persists with molecular clocks placing the origin of animals as far back as ~800 Ma whereas animal fossils extend maximally ~574 Ma to the Ediacara Biota. The disconnect between these major lines of evidence for the timing of the origin of animals requires scientific explanation. Darwin argued that taphonomy was key: perhaps Neoproterozoic environments were unconducive to animal fossilization. In this talk, I compile data from recent taphonomic studies to address this issue. Cambrian Burgess Shale-type (BST) deposits are proven repositories for early animal fossils and capable of capturing small/fragile taxa composed of a variety of tissue biopolymers. The clay minerals kaolinite and berthierine suppress decay and interact directly with decaying organic matter in ways that favor its preservation. They are major constituents of Cambrian BST deposits and are commonly associated with their soft-bodied fossils. Yet Neoproterozoic mudstones rarely display the same clay mineralogy. Fossils of basal animals are consequently likely to be rare, but even where conditions favor BST preservation in Tonian strata they are absent. Thus, taphonomy places a maximum ~790 Ma constraint on animal antiquity.
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