Newly uncovered fossils reveal in extraordinary clarity the strangeness of the Earth's earliest complex life.
The finds show that the organisms were assembled in fractal patterns from frond-like building blocks. They were unable to move and had no reproductive organs, perhaps reproducing by dropping off new fronds. The creatures, which were neither animals or plants, are called "rangeomorphs". They first appeared on the ocean floor 575 million years ago, after the last global glaciation, and were among the first of the soft-bodied creatures in the Ediacaran period. This biota survived until 542 million years ago, when modern animals diversified rapidly in the Cambrian explosion and most Ediacaran species vanished. Until now, almost all Ediacaran fossils were squashed flat, and the few that were not were poorly preserved. This led to debate over whether the poor preservation obscured links to later life, or if the Ediacaran organisms were in fact a failed experiment in evolution that simply became extinct. The newly unearthed fossils, from Newfoundland, Canada, were preserved three-dimensionally in fine-grained mud by a "one-in-a-million" streak of luck, says Guy Narbonne of Queen's University in Kingston, Ontario.
Just after a mud flow entombed the organisms, a nearby volcano erupted, covering it with a thick ash protective ash deposit. Later, the bed escaped the strain that altered most of the rock in the region. Now weathering is exposing them so "they basically pop out of the rock," Narbonne told New Scientist. "You are seeing what they looked like when they were alive." That exceptional preservation is cracking the mystery of Ediacara. In some spots the surface has eroded and "we see for the first time what was inside an Ediacaran fossil," Narbonne says. Each frond element, a few centimetres long, was made of many tubes held up by a semi-rigid organic skeleton. The frond elements had branches which themselves had branches, a classic fractal structure. Frondlets assembled themselves like building blocks to make larger living structures attached to the sea floor. Narbonne found rangeomorphs assembled in several different shapes, which he believes filtered food from different levels of the water column, as well as isolated free-living frondlets.
The fractal patterns look complex, but Narbonne says their self-similarity means that very simple genomes - expected in early organisms - would suffice both to assemble individual frondlets and to control their assembly into larger structures. That would explain why the rangeomorphs evolved first. They accounted for over 80% of fossils early in the Ediacara period, when there were no mobile animals or traces of burrows. But they declined as more mobile animals evolved, apparently unable to compete, or perhaps being eaten themselves. "This discovery is a real eye-opener," says Whitey Hagadorn of Amherst College, Massachusetts. But he adds that what will be "really exciting" are the follow-up papers that analyse the fossils in detail and try to explain the Ediacara organisms.
Journal reference: Science (DOI: 10.1126/science.1099727)