|Fig. 1. Dickinsonia (source: uonews.oregon.edu)|
My first impression upon hearing the hypothesis that Dickinsonia (Fig. 1) may be a lichen was that it does not look like any lichen I’ve ever seen. To be fair, the more normal interpretation that it is a marine animal, a segmented worm, sparks the same response…it does not look like any animal I’ve ever seen or heard of. What it actually resembles is a giant diatom, but that is pretty silly. This uncertainty is exactly what has been nagging paleontologists since the 1940’s….what are these things and what later life forms, if any, evolved from them? Dickinsonia and other Ediacaran Period (635-542 mya) fossils are enigmas. They left fossils in multiple locations world wide, then drop out of the fossil record before the Cambrian explosion in a mass extinction.
I’m no expert on this topic, so I had to do a little looking into the history of Earth. I read Gould’s “It’s a wonderful life” several years ago purely for fun, but that is the extent of my paleontological expertise. It is common in the popular press to equate the Ediacaran time as representative of the beginnings of life. This is not true. Rather it could be the beginnings of animal life or the beginning of clearly complex multicellular life, but the Earth was already quite alive at this point. The major events are the origin of single-celled organisms and their radiation into archaea and bacteria about 3500 mya. The conventional thought is that this all occurred in the oceans, though there are other opinions. Photosynthetic bacteria showed up around this time, and maybe even were on land. Then, came the evolution of the cyanobacteria about 3000 mya which was extremely significant because their brand of photosynthesis produced oxygen as a waste product, and drew down CO2 in the atmosphere. It is plausible that they colonized land, but because they do not often leave convincing fossils on land, this is hard to prove. I would love to have Hugo Beraldi-Campesi weigh in on this, as I am out of my depth on this topic. The oxygen produced by the cyanobacteria immediately began rusting all of the iron in the Earths rocks, until this sink was saturated, i.e., all the surface iron had oxidized. At this point the great oxidation event occurred wherein the atmospheric oxygen started rising (~2500 mya) causing a mass extinction of anaerobic organisms, and also crippling the Earth’s greenhouse effect and causing a “snowball earth” episode of global glaciation, and partially frozen seas. This phenomenon happened a couple more times, possibly linked to other phenomena like super volcanos. After the last snowball Earth episode, a group of fossils called the Ediacaran fauna turn up supposedly inhabiting microbial mats on seafloors....check out the video below.
They are known from multiple locations all over the place suggesting global ubiquity. “Shortly” after, the famously rapid radiation of complex lifeforms (most of which went extinct) occurred in the Cambrian explosion, exemplified by the fauna of the Burgess shale. The Ediacaran animals are not part of this fauna, suggesting an extinction event, and plausibly leaving a bottleneck from which the Cambrian radiation occurred.
It’s a great story, with holes, but all very reasonable given that the fossil record is always incomplete and many inferences must be made from incomplete data. What if the Ediacarans are not marine animals, and not animals at all? Then the animalian genetic lines that “exploded”, did not come form the Ediacaran…making the Cambrian explosion even more out of nowhere. The interpretation that these are lichens, marine ones (something that does actually exist today), was proposed by Retallack (1996). What if these were terrestrial organisms – including lichens and other fungi- inhabiting soils? That is what this new paper suggests (Retallack 2012). The new bit of information, that will be of great interest to people who read this blog, is the evidence that one key Ediacaran site in Australia (the type locality, actually) is composed of paleosols, not marine sediments (Fig. 2). The evidence is based on the patterns of oxidation, weathering into soil horizons, isotopic signatures, needle ice, carbonate and gypsum accumulations and cracking……..and “elephant skin” which is interpreted as a biocrust. Could there be biocrusts on land at this point, i.e. terrestrial community version 1.0? I think yes, we know cyanobacteria had been around for a very long time, and had all the tools they might need to grow on land (UV protection, desiccation tolerance, N-fixation, C-fixation). That is totally plausible, in fact they could have colonized after other photosynthetic bacteria actually making them terrestrial community version 2.0. As an ecologist, I find it hard to believe that the ocean would be teeming with microbial lifeforms in abundances great enough to overhaul the Earth's atmosphere, but the land surface would be sterile. I can easily believe that biocrusts are ancient. Retallack's (2012) interpretation is that this was a cold dry environment, so i do wonder why the putative fossil biocrusts don't have the high-relief pinnacled structure (up to 10 cm) associated with today's biocrusts in cold climates and sandy soils (e.g. the Colorado Plateau). Could the Ediacaran “fauna” be like the lichen components that colonize todays’ biocrusts? That’s the tougher one to believe. The first unambiguous fossil of a lichen is from the Devonian (Honegger et al. 2012), about the same time that plants radiated and came to dominate the continents.
|Retallack's reinterpretation of depositional environment and fossils of the Ediacara Hills, Australia (source: ounews.orgon.edu).|
It seems like Retallack has amassed multiple lines of evidence of the paleosol interpretation. As others have pointed out (Xiao & Nauth 2012), the weakness in the paleosol interpretation is that it is based on one of many localities. Even if these are paleosols, what about all of the other marine rocks which contain the same fossils, some of which are thought to be deep water formations. Some of them are clearly rippled, suggesting the action of water. Were these Ediacarans terrestrial AND marine organisms, then? It seems to me that for now the Ediacaran enigmas must remain enigmas. I do look forward to a reexamination of other Ediacaran sites, if nothing else this paper makes the Earth’s history just a little more fascinating than it already was (quite). Whether you believe him or not, Gregory Retallack seems like an intriguing scientist with a tendency towards provocative papers. This is how science works...once we get comfortable with a hypothesis, along comes a challenge. The scrutiny will likely lead us closer to the true nature of the Ediacarans, though we may have to wallow in deeper and deeper confusion for a while. As I’ve said multiple times here, this is not my expertise and many more knowledgeable people might come across this blog….. so please do light up the comments section with your impressions of this new paper.
Honegger, R., Edwards, D., Axe, L. 2012. The earliest records of internally stratified cyanobacterial and algal lichens from the lower Devonian of the Welsh borderlands. New Phytologist 197: 264-275
Rettalack, GJ. 1996. Were the Ediacaran fossils lichens? Paleobiology 20: 523-544.
Xiao S, Knauth, LP. 2012. Paleontology: fossils come in to land. Nature doi:10.1038/nature11765