The mysterious disappearance of the Ediacaran organisms

What were the Ediacaran organisms? 

Almost six hundred million years before humans walked the Earth, the ocean floor was dominated by some of the earliest experiments in animals, known as the “Ediacaran biota”. They roamed the ocean floor during the Ediacaran Period up until the beginning of the Cambrian Period, from ~600 million years ago to 538.8 million years ago. These creatures represent a diverse array of species, without any resemblance to life that we see today. The wide variety of forms range from simple disc-shaped organisms (Dickinsonia and Kimberella) to more intricate frond-like structures (Charnia). They are generally characterized by their soft bodies and lack of hard parts (such as shells or skeletons), making it difficult to preserve complete specimens in the rock record. Today, these animals are often found in the form of impressions or molds in fine-grained sedimentary rocks, which allows us to study them in great detail. While their forms were relatively simple, their bodies contain detailed symmetrical patterns, which may have benefitted them in life. It is still a mystery as to what kind of lifestyle these organisms maintained, but current research suggests that they could have been filter feeders, photosynthesizes, or possibly engaged in symbiotic relationships.

Illustration of the Ediacaran organisms (above) and fossils (below) (Art by Peter Trusler).

What happened to the Ediacaran organisms?

The transition between the Ediacaran and Cambrian periods is marked by the sudden disappearance of the Ediacaran biota. It is still unknown just what exactly caused the extinction of these creatures. Current hypotheses point towards environmental as well as biological causes. Clues about their lifestyle (and death) may be found by studying the rocks in which the animals are found. Carbonate rocks that contain fossils of Ediacaran organisms have been studied to reconstruct the amount of oxygen present during the time that they lived. A recent study that did this has found that the Ediacarans may have survived under low oxygen conditions. If they maintained a lifestyle that did not require much oxygen, then it is possible that an increase of oxygen could be responsible for their death. Consistent with this idea, carbonate rocks during this time record a disturbance in the carbon cycle. However, recent work reconstructing the oxidation state during this transition has found there to be no change in the oxygen level, leading the door open for other mechanisms to cause the extinction of the Ediacaran biota.

In some locations the disturbance in the carbon cycle is interpreted as the falling of sea level. Additionally, higher rates of evaporation are supported by higher oxygen isotope values during this time. Sea level drop may have had negative impacts on these organisms. As the Ediacaran biota were simple creatures, their bodies were made up mostly of water, so the falling sea level could increase the salt concentration of the oceans. This would cause the fresher water contained within their bodies to diffuse out of them, essentially dehydrating and killing them. 

Alternatively, studies have been done that point towards biological changes that may have resulted in the extinction of the Ediacaran animals. As the Ediacaran Period came to an end, and the Cambrian Period was beginning, new forms of life began to emerge. These new creatures may have engineered the ecosystem to better suit themselves. By burrowing into the ground, the new Cambrian animals disturbed the algal mats that the Ediacarans fed on, eliminating their primary food source. The inability to compete with bioturbation associated with new Cambrian organisms may have led to the extinction of the Ediacaran biota.

Illustration of the transition from the Ediacaran (left) to the Cambrian (right) Ediacaran ecosystems existed at or near the surface, while Cambrian ecosystems extended deeper into the ground. (Art by Peter Trusler).

It is possible that environmental changes, biologic changes, or a combination of both caused the extinction of the Ediacaran organisms. Scientists around the world are working to answer this question, but without a time machine it is impossible to know with complete certainty what caused these mysterious creatures to suddenly disappear. Despite these animals bearing little to no resemblance to modern organisms, understanding how they went extinct may help us better understand how the creatures of today may go extinct as well.