The creatures of the Ediacaran period are among the most ancient complex multi-cellular lifeforms yet discovered. The distinctive and varied biota of this period represent something of a mystery to palaeontology as it is still not clear what kinds of lifeform they actually were.
It is now recognised that a number of key evolutionary features emerged during this period including bilateral symmetry, rudimentary heads and tails, hydrostatic skeletons, digestive tracts and locomotion. However no clear lineages have as yet been identified between the Ediacaran and the Cambrian biota.
During this period most anatomy was hydrostatic in nature, meaning that bodies were made up of tissue segments filled with pressurised fluid, similar to modern worms. The morphology of creatures followed basic patterning which varied from fractal branching to radial segmentation to bilateral symmetry. All anatomies were simple and most were comprised of sequenced cellular units (metamerism). Repetition of these simple forms, within a changing environment and in conjunction with genetic variation is thought to have led to ever more complex life.
No clear evidence of developed gut systems has appeared in the fossil record so it is presumed that most of the creatures absorbed nutrients directly through membrane wall contact with the surrounding enriched waters (osmosis). The general makeup of these membranes or skins is also largely unclear, but it is thought that some amount of hardening had taken place (though not full mineralisation) which allowed for the subsequent fossil preservation.
These creatures lived within complex ecosystems right across the globe (Droser, Gehling, 2015). Some lifeforms flourished at great depths away from any sunlight whilst others ranged across wide shallow seas. Similar species have been found as far apart as Australia and Russia. This all indicates that complex life existed in vast numbers throughout the oceans of the world during the Ediacaran period and not just in isolated pockets. This is a pattern that appears to repeat throughout evolutionary history. Once life regains a foothold within an empty environment - possibly created by an earlier mass extinction event - it tends to progress rapidly, filling every available niche as part of a vigorous adaptive radiation.
Unlike the creatures of the Cambrian period that were to follow, very few of the Ediacaran biota can as yet be associated with any modern phylum. Many are thought to have become extinct during a period of global mass extinction that occurred just before the Cambrian period. The cause of this mass extinction remains unknown but it resulted in radically different environmental conditions that may have included elevated oxygen levels and a change in the nutrient content of the seas. With those changed conditions whole new phylums emerged that constitute the base morphology of most lifeforms we know today.
Currently most creatures from the Ediacaran period remain only partially classified and the search for affinities with Cambrian biota continues.