December 13, 2012 | 4
Does these look like lichens to you? According to Gregory Retallack, they should.
Yesterday, Nature published an article by Retallack that makes a radical claim: the Ediacaran Biota (635-542 mya) of bizarre creatures that preceded the Cambrian Explosion were not pneumatic semi-mobile marine animals, but instead sessile land-dwelling lichens and protists living high and very much dry on land.
For those of us raised on pictures and dioramas of puffy Ediacaran animals ensconced happily on the seafloor, this is a bit of a shocker. Although scientists in the past have suggested that they *might* be giant marine protists, Retallack seems to be alone in carrying the lichen torch.
His analysis of the rock surrounding the fossils suggests to him the soil of dry land, and his analysis of cross sections of these fossils to him suggests lichen rootlets and biological soil crusts. Lichens today are symbiotic associations of fungi and algae. You’ve probably seen a few in your time plastered to rocks, tombstones, or tree bark. Biological soil crusts are loose associations of cyanobacteria, algae, fungi, lichens, mosses familiar to those of us who live in the west*. They may represent some sort of proto-lichens or lichen incubators.
Retallack envisions the Ediacaran fossils (here linked to photos for your viewing pleasure — please go have a look at each) Dickinsonia, Charnia, Praecambridium and Spriggina as lichens or similar “microbial consortia”. He sees disc-like Cyclomedusa, Medusinites and Rugosoconites not as jellyfish, but as microbial colonies. He see in small fossils such as Parvancorina or Tribrachidium fungal fruiting bodies (like mushrooms or cup fungi) rather than pre-trilobites or proto-sea stars.
A trace fossil like Archaeonassa he sees not the burrows of marine worms, but of the slug phase of cellular slime molds (in my experience with the modern versions, they leave a trail of slime but not an impression) or even land-based slugs or worms.
From my almost completely uninformed position, Ediacarans just don’t say “lichen” to me — at least as we know them today. I’ve spent a lot of time (relatively speaking) looking at lichens, and most of them seem to have fairly basic and highly repetitive body plans without any sort of higher-level “design”. Ediacarans, to me, lack that microbial look that lichens often have of a central colony with growing margins often surrounded by younger satellite colonies. Others tend to be bunched and irregular. The Ediacarans’ complex body designs and defined margins look much more animal- or plant-like to me.
And I’m apparently not the only one to think this hypothesis is a bit … out there. Critical reception as reported in the press so far might be conservatively described as scathing, with at least one scientist going so far as to question Nature‘s choice to publish the article.
An accompanying critical review by Shuhai Xiao in Nature points out that whatever the merits of his soil-based evidence, it’s outweighed by the “compelling evidence” for the action of waves or currents on Ediacaran fauna: preservation of Cyclomedusa and Dickinsonia on rippled bedding surfaces, holdfasts attached to some Ediacaran fossils that were dragged in the same direction as the alignment of attached stalks, “current lineations” in the sediments. Some show evidence of periodic movement — a very un-lichen-like trait. Furthermore, many Ediacaran fossils are found in rocks that are “unambiguously” marine like black shales and limestone. And slime mold slugs, he points out, do not burrow.
Xiao does point out that lichens as we know them may well have existed during the Ediacaran and made a major contribution to the course of evolution. He cited a Science paper I was delighted to discover presenting evidence of marine lichens — which are almost wholly unknown today (although there are many lichens that live in sea spray) — 600 million years ago, prior to the evolution of simple vascular plants like mosses. I looked at that paper, and to me those fossils do look very much like modern crustose lichens (sorry for small image — best I could do for free).
It is always worth remembering in these cases that scientists with cockeyed ideas who are maligned by the scientific community for years — Alfred Wegener, Represent — *occasionally* turn out to be right. As L. Paul Knauth, the author of the Nature review more friendly to Retallack’s ideas, wrote, unless you are a hard-core paleosoil expert, it will be difficult to mount a convincing rebuttal. Perhaps some Ediacaran creatures — whose identities and certainly biologies we are still ignorant of — were capable of living on land as well as in the sea.
And in a much larger sense, as Knauth further points out, none of us were there during the time these creatures, lived, and much as we might wish it so (and some of us wish it a lot), none of us will ever gaze over the vistas or plunge into the ocean depths of early Earth to see what it truly looked like. In the absence of a definitive answer, is it so wrong to indulge a provocative alternative hypothesis now and then?
*If you’ve been out on western U.S. public land, you’ve likely heard the injunction not to “Bust the Crust” since they take a very long time to regenerate, but this can be tough for kids to resist since BSCs are (from their perspective) the desert version of bubble wrap.