600 million years ago something remarkable appeared on Earth. Since the dawn of evolution simple clusters of cells with little definition or structure existed. Then the Ediacara biota evolved - creatures with complex body plans, foreshadowing organisms yet to evolve.
Displaying limbs, heads and sensory organs, members of the group must also have had basic nervous systems to control such features. The anatomy and physiology of the Ediacara biota laid the framework for the modern groups of organisms, yet they themselves are extremely enigmatic, displaying few distinct relationships to other, younger creatures in the fossil record. Now another bizarre member has been added to this class of oddballs.
|The weird world of the Ediacara biota|
Named in 2013 by researchers from the University of California Riverside, led by Dr Mary Droser - famous for her work on the Ediacara biota - Plexus ricei is odd when compared to other Ediacarans. At 575 million years old it is of a similar age to creatures such as Dickinsonia or Spriggina, yet unlike these organisms it did not possess a fleshy, quilted body. Instead it was very long, much longer than most animals at the time reaching a massive 80 centimetres in length, and flat. 'It was bilaterally symmetrical at a time when bilaterians - all animals other than corals and sponges - were just appearing on this planet,' said Droser.
|Fossils of the 575 million year old bilaterian Plexus ricei|
The size range of the species is substantial, 5 to 80 centimetres long and 5 to 20 millimetres wide. What is truly incredible, however, is the degree of preservation seen in this soft bodied and therefore structurally delicate organism. Not only were the general dimensions of the creature displayed, some of its internal features were present also. A reconstruction of Plexus ricei showed that it had a central tube running along the length of the body - almost certainly an alimentary canal - although how it lived is something of a mystery. It was most likely a bottom feeder, but whether it fed on detritus from the sea bed or underneath algal mats is unclear.
'In the Ediacaran we really need to know the difference between the fossils of actual tubular organisms and trace fossils because if the fossil we are looking at is a trace fossil, then that has huge implications for the earliest origins of bilaterian animals,' said Lucas V. Joel, an undergraduate student at UC Riverside. 'Plexus ricei is not a trace fossil. What our research shows is that the structure we see looks very much like a trace fossil, but is in fact a new Ediacaran tubular organism.'
As our knowledge of the group and their relationships to other organisms improves, we will be able to reconstruct the anatomies of these biological enigmas with greater confidence. The Ediacarans were the first true animals on the planet. to understand their origins and evolution is to understand life on Earth as it has been for the past 600 million years.