by Matthew Cobb
Paleontologists sometimes use terms that are rather different from the rest of the scientific community. One of my favourites is ‘enigmatic’. This term is often and appropriately applied to the soft-bodied multicellular organisms found in the Ediacaran strata (635-542 million years ago), which immediately predate the Cambrian rocks that saw the astonishing ‘explosion’ of multicellular animal life.
Indeed, so enigmatic are the fossils found in the Ediacara that many palaeontologists refer to the organisms that left these remains as ‘the Ediacaran biota’, maintaining a strictly agnostic position as to whether the organisms were animals, plants or something else entirely.
Here’s an example of how weird these things can be: Parvancorina. These fossils are 1-2 cm in length. They could be a hold-fast (the remnants of some organism that was attached to the sea-bed), although there are things about the fossils that suggest this is not the case. Other people have suggested they may be some primitive arthropod (the lack of any sign of legs suggests to me that this is a superficial identification).
In an article published online in Nature last week, geologist Gregory Retallack of the University of Oregon put forward what he accepts is an ‘unconventional’ hypothesis. Not only does he suggest that Parvancorina, which is often identified as an animal, was in fact a fungal fruiting body, above all he suggests that these fossils are not, as is universally accepted, from marine layers, but were in fact overwhelmingly terrestrial. This would mean that life first colonised the land at least 100 million years before the currently accepted dates (mid-Ordovician).
Using a close reading of the geological evidence, he applies this categorisation to most of the iconic Ediacaran fossils and reinterprets them as organisms that lived on dry soils. (Retallack has been arguing this for some time. Here’s a link to a 1994 paper of his arguing essentially the same thing, though with less geological detail. – the key thing in the new paper is his confident assertion that the rocks were terrestrial.)
So, for example, Retallack argues that Dickinsonia is ‘more likely to have been lichens or other microbial consortia’ rather than an early invertebrate. But he is arguing here simply on the basis of the geological identification of the rocks as being terrestrial – which he accepts is cotnroversial – not on the basis of the anatomy of the fossil.
Here is Dickinsonia. I appreciate that taphonomy – the way that things decay and become fossils – can have weird effects, but I find it hard to see this as anything other than an early invertebrate.
Unusually, the appearance of Retallack’s research article was accompanied by the publication of four ‘magazine’ articles, the first of which is an Editorial, justifying the publication of the article partly on the basis that previous whacky theories (eg Jane Gray’s advocacy of an Ordovician terrestrialisation date – around 460 million years ago) have become accepted. But as we know, most of the people who argue against a widely-accepted scientific view are not like Galileo, they are simply wrong.
That’s certainly the view of two of the three other pieces that accompany Retallack’s article. Shuhai Xiao dismisses Retallack’s terrestrial interpretation and puts forward counter-arguments for Retallack’s view. One of his most telling points is made by this picture. It shows Dickinsonia fossils on a bed of rock. And if those aren’t undersea ripples, I’ll eat my geologist’s hammer (I don’t have one).
In an online article at Nature News, Brian Switek has hunted out quotes from other geologists:
“I and my colleagues are quite weary with being asked to review his material over the last ten years,” says James Gehling, a palaeontologist at the South Australian Museum in Adelaide.
Guy Narbonne, a palaeobiologist at Queen’s University in Kingston, Ontario, says that the new paper is little more than a summary of Retallack’s “long-standing views” on Ediacaran life.
He adds: “Most of us appreciated that Retallack’s lichen hypothesis was innovative thinking and tested his ideas critically, but it quickly became clear that there are simpler explanations for the features Retallack had validly noted, and most of us moved on to more promising explanations.”
Gehling is unconvinced by the new paper: “Retallack has presented not a single piece of evidence that would contradict the interpretation of the sedimentary layers involved as anything other than marine.”
He and Narbonne argue the red coloration of the rock and its weathering patterns, which Retallack presents as new evidence, could be just as easily accounted for by a marine origin.
Narbonne says that “multiple sedimentary and geochemical approaches by multiple independent laboratories worldwide have nearly universally converged on a marine origin for the Ediacara biota”.
Traces of animal behaviour in the wave-rippled Ediacaran sediments also contradict the terrestrial hypothesis, in Gehling’s view. Dickinsonia, an animal possibly related to today’s blob-like placozoans (the simplest multicellular organisms), left tracks after “spending time on one site, decaying the organic matter below, and then creeping across the mats to the next site,” he says, and the “mollusc-like” Kimberella created scratch marks on the sea floor as it grazed.
“If 60 years of published interpretations of the Ediacara biota have shown anything,” Gehling says, “it is that the Ediacara biota were a diverse array of organisms with remarkably consistent body plans found in distinct associations and most often preserved in place on fossil sea floors.”
Retallack remains unfazed. “I am expecting controversy,” he says, adding that he anticipates “the usual trajectory of grief, beginning with denial, then proceeding to mourning and acceptance” of his idea.
I’m no geologist, and as Paul Knauth, who remains open to Retallack’s suggestion rightly says in his commentary ‘We were not there when all this happened’. However, as a biologist with an interest in paleontology (I teach this stuff to second year students, and will include Retallack’s ideas in my lectures next year) I think that ‘mourning and acceptance’ might be the future for Retallack, not for the rest of us. As to what exactly the Ediacaran biota were, that remains one of the most fascinating enigmas in science.