Abstract
The Ediacara Biota represents the oldest fossil evidence for the appearance of animals but linking these taxa to specific clades has proved challenging. Dickinsonia is an abundant, apparently bilaterally symmetrical Ediacara fossil with uncertain affinities. We identified and measured key morphological features of over 900 specimens of Dickinsonia costata from the Ediacara Member, South Australia to characterize patterns in growth and morphology. Here we show that development in Dickinsonia costata was surprisingly highly regulated to maintain an ovoid shape via terminal addition and the predictable expansion of modules. This result, along with other characters found in Dickinsonia suggests that it does not belong within known animal groups, but that it utilized some of the developmental gene networks of bilaterians, a result predicted by gene sequencing of basal metazoans but previously unidentified in the fossil record. Dickinsonia thus represents an extinct clade located between sponges and the last common ancestor of Protostomes and Deuterostomes, and likely belongs within the Eumetazoa.
Highlights
The Ediacara Biota is generally accepted as the first occurrence of macroscopic, complex, animals in the fossil record [1,2]
This investigation of over 900 specimens of D. costata represents the largest dataset of this taxon analyzed to date, illuminating which features are representative of organismal biology and those that represent taphonomic artifacts
The shape of D. costata is consistently ovoid in all specimens investigated except in rare examples that have obviously been altered by taphonomic processes
Summary
The Ediacara Biota is generally accepted as the first occurrence of macroscopic, complex, animals in the fossil record [1,2]. Close examination demonstrates that any apparent evidence for modules being offset at the midline is the result of alteration due to the soft-bodied nature of Dickinsonia and that modules are consistently symmetric about the long axis in all specimens.
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