The Mysterious Disappearance of the Ediacaran Biota

 What were the Ediacaran biota?

Almost six hundred million years before humans walked the Earth, the ocean floor was dominated by some of the earliest complex multicellular organisms, known as the “Ediacaran biota”.  They first arose 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, with a wide variety of forms, ranging from simple disc-shaped organisms to more intricate frond-like structures. 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, Ediacaran biotas are often found in the form of impressions or molds in fine-grained sedimentary rocks. It is still a mystery as to what kind of life style these organisms maintained, but current research suggests that they could have been filter feeders, photosynthesizes, or possibly engaged in symbiotic relationships.  

Illustration of Ediacaran animals and fossils (Source: Australia Post / Artist: Peter Trusler)

What happened to the Ediacaran biota?

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 organisms. 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 biotas are found. Carbonate rocks that contain Ediacaran biotas, preserved as nodules and compressions, have been studied to reconstruct the amount of oxygen present during the time that these organisms lived. A recent study that did this has found that the Ediacaran biotas may have survived under low oxygen conditions. If they maintained a lifestyle that did not require oxygen, then it is possible that an increase of oxygen could be responsible for their death. Carbonate rocks during this time record a negative carbon isotope shift, which is consistent with this idea. However, recent work reconstructing the redox 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 negative carbon isotope shift occurring at the Ediacaran-Cambrian boundary is preserved in carbonate rocks that represent 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 are relatively simple organisms, their bodies were made up mostly of water, so the falling of sea level would 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 biota. As the Ediacaran Period came to an end, new forms of life began to emerge. These new organisms may have engineered the ecosystem to better suit themselves, as the cost of the preexisting organisms. The Ediacaran biota lived and fed on microbial mats, essentially operating as grazers. The new organisms burrowed into the ground, disturbing microbial mats, destroying the food of the Ediacaran organisms. The inability to complete with bioturbation associated with new Cambrian organisms may have led to the extinction of the Ediacaran biota.

Diagram of the before (left) and after (right) the rise of bioturbation 

It is possible that environmental changes, biologic changes, or a combination of both caused the extinction of the Ediacaran biota. 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 the Ediacaran biota to disappear.

References 

Cherry, L.B., Gilleaudeau, G.J., Grazhdankin, D.V., Romaniello, S.J., Martin, A.J., and Kaufman, A.J., 2022, A diverse Ediacara assemblage survived under low-oxygen conditions: Nature Communications, v. 13, p. 7306, doi:10.1038/s41467-022-35012-y.

Darroch, S.A.F., Smith, E.F., Nelson, L.L., Craffey, M., Schiffbauer, J.D., and Laflamme, M., 2023, Causes and consequences of end-Ediacaran extinction – an update: Cambridge Prisms: Extinction, p. 1–30, doi:10.1017/ext.2023.12.