DARWIN'S DILEMMA: TAPHONOMIC STUDIES HELP CONSTRAIN THE ORIGIN OF ANIMALS

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.