There has been life on earth for over three billion years, but for most of this time it consisted of microbes with limited diversity. Most modern phyla suddenly appeared in the fossil record 542 million years ago. For many years the so-called Cambrian explosion was thought to be the start of complex macroscopic life on earth. However, we now know that complex macroscopic life appeared before that, during the Ediacaran (635–542 Ma).
Ediacaran ecosystems were very different to those of the Cambrian. Microbial mats formed the foundation of the ecosystem, providing a stabilizing base on which the Ediacaran species could live. These microbial mats also helped preserve fossils of the soft-bodied organisms: bacterial precipitation of iron sulphide cemented sediments around dead and decaying organisms, forming a ‘death mask’ which resulted in exceptional preservation. Furthermore, because movement was limited during the Ediacaran, there are hundreds of bedding planes where almost entire ecosystems were preserved. Some of the oldest Ediacaran macrofossil bedding planes preserved thousands of specimens under volcanic ash, often to sub-millimetre detail, providing a unique window into these ancient ecosystems, which is unprecedented anywhere else in the fossil record.
On top of the microbial mats grew many exotic organisms, some up to 2 m long, with body plans so unique they are difficult to place on the tree of life. Some species such as the mollusc-like Kimberella may have been one of the first animals, others such as Thectardis might have been a sponge. Others, such as Charnia, had a fractal branching structure in which each branch has many sub-branches, a body plan unique to the Ediacaran.
During the Ediacaran we see a number of key innovations, including movement, burrowing, and the consumption of other organisms. Most of the oldest Ediacaran macrofossils were incapable of movement, but some later bedding planes show trace fossils similar to those made by cnidiarians, suggesting some species were capable of movement. Younger beds contain strong evidence of undermat burrowing. Yorgia has several instances where there are body impressions in the microbial mat, which then end in a body fossil, as though the organism
has flopped forward. It is thought the organisms may have been feeding off the
microbial mat, absorbing nutrients through their undersides. Other body fossils, such as Kimberella have been found near concentric scratch marks in the microbial mat, suggesting they were eating at the mat. Closer to the Cambrian we see species such as Cloudinia, a reef-builder which is thought to have bio-mineralized as a defence against predation, as evidenced by the presence of boreholes in some shells. Despite the fact that the Ediacaran macrofossils occur during such a key transition of life on earth, there are still many fundamental unanswered questions: While the phylogenetic affinity of some species is fairly well resolved (e.g. Kimberella), many — including the oldest fractally branching rangeomorphs — are still unknown. Furthermore, there are only two possible Ediacaran species that survived into the Cambrian, but it is not known yet whether the lack of Ediacaran survivors is due to an absence of preservation conditions, a mass extinction, or a slow dying-off as the Ediacaran macrobiota were replaced by their Cambrian counterparts. Ediacaran macrofossils and their ecosystems represent the start of complex life on earth. Because of this, understanding the Ediacaran, and attempting to answer these key questions, is key to understanding animal evolution and the development of life on our planet.