Abstract
The Ediacara Biota preserves the oldest fossil evidence of abundant, complex metazoans. Despite their significance, assigning individual taxa to specific phylogenetic groups has proved problematic. To better understand these forms, we identify developmentally controlled characters in representative taxa from the Ediacaran White Sea assemblage and compare them with the regulatory tools underlying similar traits in modern organisms. This analysis demonstrates that the genetic pathways for multicellularity, axial polarity, musculature, and a nervous system were likely present in some of these early animals. Equally meaningful is the absence of evidence for major differentiation of macroscopic body units, including distinct organs, localized sensory machinery or appendages. Together these traits help to better constrain the phylogenetic position of several key Ediacara taxa and inform our views of early metazoan evolution. An apparent lack of heads with concentrated sensory machinery or ventral nerve cords in such taxa supports the hypothesis that these evolved independently in disparate bilaterian clades.
Highlights
The fossil record of complex, macroscopic community-forming organisms, including animals, begins with the Ediacara Biota (570–539 Ma)
The absence of a central nervous system (CNS), segmentation and appendages in the Ediacara Biota suggests that Notch/Delta signalling was likely restricted to germ cell differentiation and/or establishment of the nervous system in Ediacara taxa such as Kimberella and Ikaria
We have evaluated the developmental capacity of representative White Sea taxa, identified several metazoan-specific morphogenetic processes and the likely regulatory elements responsible for their expression
Summary
The fossil record of complex, macroscopic community-forming organisms, including animals, begins with the Ediacara Biota (570–539 Ma). Molecular clock estimates suggest that major metazoan phyla and their constituent clades evolved prior to this period [1,2,3,4]; phylogenetic affinities for most of the Ediacara Biota remain enigmatic [5]. Potential explanations for this phylogenetic uncertainty include the simplicity of early animal forms, preservational biases, and lags between character acquisition and ecological success We identify characters of these organisms controlled by conserved developmental processes and suggest genetic elements likely responsible for their expression.
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