Developmental processes in Ediacara macrofossils

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.

Keywords: Ediacara biota; Ediacaran; Metazoa; development; evolution; regulatory genes.

Figure 1.

Holozoan phylogeny with inferred placement of representative White Sea taxa (dashed boxes) based on developmentally relevant characters (1–5, black box). Characters represent those that can be identified based on morphological expression in representative Ediacara fossils, and thus are not indicative of their earliest appearance. Arrow represents increased combinatorial complexity of transcription factor interactions in all three groups of bilaterians. Question marks represent uncertainty of placozoan placement. Ctenophores omitted to avoid uncertainty. CNS, central nervous system.

Figure 2.

Representative taxa of the White Sea assemblage from the Ediacara Member, South Australia, including: (a,b) Kimberella quadrata (K) with frill or muscular foot (MF), proboscis (P) and associated scratch marks (SM); (c,d) Ikaria wariootia with wider end indicated by white stars and with associated trace fossil Helminthoidichnites; (e) Dickinsonia costata with white arrow indicating the direction of movement; and (f) Tribrachidium heraldicum. Fossils are external moulds preserved in negative relief on the base of fossil beds (hyporelief). (a,b) and (d–f) are photographs of original fossils and (c) is a three-dimensional laser scan. (f) Photo courtesy Christine Hall. (a) LV-FUN 001; (b) P35660; (c) 1T-A 001; (d) P57686; (e) TB-ARB 001; (f) P12898. Scale bars total 1 cm. (Online version in colour.)