The first mass extinction of complex life

In September Proceedings of the Royal Society published an article titled ‘Biotic replacement and mass extinction of the Ediacara biota’. We asked one of authors, Dr Simon Darroch, from Vanderbilt University, to comment on his work.

The Study

The events that occurred at the Ediacaran-Cambrian boundary, immediately prior to the ‘Cambrian explosion’ of animals, are obscure. Principally, there has been intense debate surrounding what factors were responsible for the extinction of the enigmatic ‘Ediacara biota’ (a mysterious bunch of multicellular eukaryotes that may not be related to modern animals). We looked at latest Ediacaran fossil assemblages to test the hypothesis that the evolution of modern animal groups, which are capable of altering the physical and chemical parameters of their environments (and are consequently termed ‘ecosystem engineers’) caused the extinction of the ‘Ediacaran biota’. We found that, in latest Ediacaran fossil sections from Namibia, communities of Ediacaran organisms are noticeably depauperate (i.e. have low species richness, and appear ecologically ‘stressed’) compared to older Ediacaran communities. We also find that these same sections preserve evidence for a greater diversity and intensity of animal ecosystem engineering, particularly in terms of complex deposit feeding, making it plausible that the evolution of animals drove the extinction of the Ediacara biota. We suggest that the appearance of modern-looking animals introduced new behaviors into Ediacaran ecosystems, which fundamentally began to alter the environment (including oxygenating the water column, mixing the sediment, and aerating the oceans), and drove the extinction of the Ediacarans. Finally, we used a variety of geochemical techniques to test whether the environment was somehow restricted (i.e., hypersaline, low oxygen, or extremely shallow) – factors which could also produce low diversity ecologies. We found that the environment was in all likelihood normal marine (i.e., not unusual or stressful to life), and therefore environmental factors were not responsible for the low diversity communities. This supports our general idea that the appearance of modern animals was likely responsible.

Background

A flourishing biosphere with atmospheric nitrogen 3.2 billion years ago New Nature article debate dating of nitrogen fixation emergence

This work was performed on the back of an important study published in a journal called Gondwana Research by Dr. Marc Laflamme (University of Toronto) who was a coauthor on the recent paper. In this earlier study, Marc laid out 3 hypotheses that potentially explain the disappearance of the Ediacara biota. We decided that Namibia was the best place to actually start testing these hypotheses (what we needed was a well-exposed fossil site where we could study the bedding surfaces), and so we started going there in 2013 to look for potential sites. In the end, we settled for a site around Farm Swartpunt, which had been originally discovered in the ’90’s by John Grotzinger (Caltech), and described by Guy Narbonne (Queens University) in 1997.

Future Direction

The field is probably moving more towards new quantitative methods for studying the ecologies and ecosystems constructed by the Ediacara biota. For the past 100 years we have been trying to work out exactly what they are (i.e. how they’re related to modern life forms), but with a few exceptions we hardly become closer to answering these questions by purely studying their body plans. Instead, if we study their ecology and behavior (i.e., the way they’re distributed, how their populations are constructed, and how they likely fed and/or reproduced), we can gain new insights into their biology. In this, we’re borrowing methods that modern ecologists and marine biologists use to study extant organisms and adapting them to be more suitable for fossil material.

In terms of studying this particular extinction event, we really need more fossil sites of the same age, because Ediacaran fossils are pretty rare. Although I personally think our study provides good evidence in support of a biologically-driven extinction, in order to really make it convincing we need more datasets to test our model. Consequently, the field is also moving back towards more traditional fieldwork exploring ever more remote and harder-to-access parts of the world, because finding new fossil sites that show the same is ultimately the only way to really test and back-up our conclusions.