Fossilisation processes and our reading of animal antiquity
Ross P. Anderson, Christina R. Woltz, Nicholas J. Tosca, Susannah M. Porter, Derek E.G. Briggs
Published:June 27, 2023 DOI: https://doi.org/10.1016/j.tree.2023.05.014
Highlights
The last common ancestor of animals is thought to have been small and soft-bodied and therefore would have required special conditions for its preservation.
Limited availability of these conditions in the Neoproterozoic could explain the discrepancy between molecular clock predictions for the timing of animal origins and the fossil record of animals.
We assess the availability of these conditions, particularly those of Burgess Shale-type, which are known to preserve animals with tissues of varied composition.
Burgess Shale-type conditions are rarely associated with Neoproterozoic fossil biotas, but in the few assemblages with these conditions, dated to 789 million years ago or older, no animals have been identified, suggesting they had not evolved by this time.
This provides a soft maximum age constraint on crown group animals of 789 million years ago.
Abstract
Estimates for animal antiquity exhibit a significant disconnect between those from molecular clocks, which indicate crown animals evolved ∼800 million years ago (Ma), and those from the fossil record, which extends only ∼574 Ma. Taphonomy is often held culpable: early animals were too small/soft/fragile to fossilise, or the circumstances that preserve them were uncommon in the early Neoproterozoic. We assess this idea by comparing Neoproterozoic fossilisation processes with those of the Cambrian and its abundant animal fossils. Cambrian Burgess Shale-type (BST) preservation captures animals in mudstones showing a narrow range of mineralogies; yet, fossiliferous Neoproterozoic mudstones rarely share the same mineralogy. Animal fossils are absent where BST preservation occurs in deposits ≥789 Ma, suggesting a soft maximum constraint on animal antiquity.
Keywords Burgess Shale-type clays molecular clocks Neoproterozoic Era origin of animals
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