<i>Nature</i> Paper Removes Enigmatic Ediacaran Fossils from the Ancestry of Cambrian Explosion Animals - Evolution News & Views

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Nature Paper Removes Enigmatic Ediacaran Fossils from the Ancestry of Cambrian Explosion Animals

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It's been a rough season for Precambrian animal fossils. Last week I reported on the demise of Vernanimalcula, which no longer stands as the ancestor of Cambrian animals -- many of them quite familiar looking -- which appear abruptly and seemingly without predecessors in the Cambrian explosion. Now, a new research paper in Nature has posed a strong challenge to the Ediacaran fauna, another group of enigmatic fossils that some have also claimed were the hallowed Precambrian ancestors to the Cambrian fauna.

For example, a 2000 article by J. William Schopf in Proceedings of the National Academy of Sciences argued that the Ediacaran fauna helped provide the "solution to Darwin's dilemma" -- namely the lack of Precambrian fossils. But the new paper in Nature by Gregory J. Retallack of the University of Oregon notes that the geological outcrops that bear the Ediacaran fossils were probably deposited on land, not underwater. As the Nature paper states:

These distinctive cracked and pustulose surfaces have a variety of features that are more like the biological soil crusts of desert and tundra than the parallel wrinkled, and undulose hydrated microbial mats of intertidal flats and shallow seas.

(Gregory J. Retallack, "Ediacaran life on land, " Nature (2012).)

So what does this mean? It means that the Ediacaran fauna are not marine animals, and thus could not have served as ancestors to the diverse marine animals that appear abruptly in the Cambrian explosion. This is made clear by Retallack who states elsewhere: "This discovery has implications for the tree of life, because it removes Ediacaran fossils from the ancestry of animals" (emphasis added).

Indeed, claims that the Ediacaran fossils were ancestral to the animals that appear in the Cambrian explosion have long been controversial. As evolutionary paleontologist Peter Ward notes:
[L]ater study cast doubt on the affinity between these ancient remains preserved in sandstones and living creatures of today; the great German paleontologist A. Seilacher, of Tübingen University, has even gone so far as to suggest that the Ediacaran fauna has no relationship whatsoever with any currently living creatures. In this view, the Ediacaran fauna was completely annihilated before the start of the Cambrian fauna.

(Peter Douglas Ward, On Methuselah's Trail: Living Fossils and the Great Extinctions (New York, NY: W. H. Freeman, 1992), 36.)

A number of modern evolutionary scientists today share Seilacher's skepticism, as Richard Fortey explains:
The beginning of the Cambrian period, some 545 million years ago, saw the sudden appearance in the fossil record of almost all the main types of animals (phyla) that still dominate the biota today. To be sure, there are fossils in older strata, but they are either very small (such as bacteria and algae), or their relationships to the living fauna are highly contentious, as is the case with the famous soft-bodied fossils from the late Precambrian Pound Quartzite, Ediacara, South Australia.

(Richard Fortey, "The Cambrian Explosion Exploded?," Science, 293 (July 20, 2001): 438-439.)

Likewise, Andrew Knoll and Sean B. Carroll observe in Science that "It is genuinely difficult to map the characters of Ediacaran fossils onto the body plans of living invertebrates" and thus evidence of these fossils being precursors to Cambrian fauna "remains equivocal." A Blackwell Scientific invertebrate biology textbook concurs that the Ediacaran fauna do not solve Darwin's dilemma:
Whether [the Ediacaran fossils] were, in fact, early members of any phyla still living today and possible ancestral forms, or were members of phyla long since extinct, is a question of considerable current debate. At any rate, they shed little light on the question of which phyla were ancestral to other phyla, or if indeed, animals have a common ancestry.

(Vicki Pearse, John Pearse, Mildred Buchsbaum, and Ralph Buchsbaum, Living Invertebrates (Palo Alto, CA: Blackwell Scientific Publications, 1987), 764.)

Finally, the prominent paleontologists Valentine, Erwin, and Jablonski are hesitant to claim these Ediacaran fossils bear any ancestral relation to Cambrian fauna, writing in the journal Development that "the relations of any of these fossils to Cambrian bilaterians remains uncertain and awaits further collecting and critical analysis."

Retallack's Nature paper itself affirms that "Most Ediacaran fossils have no clear relationship with modern animals." Indeed, an editorial in Nature on Retallack's paper likewise states: "If they [the Ediacaran fossils] were animals, they bore little or no resemblance to any other creatures, either fossil or extant."

So what were the Ediacaran fossils, exactly? Retallack suggests that many specimens either aren't fossils of animals, or aren't fossils at all. As his new paper in Nature states:

  • "Putative permineralized metazoans may instead have been crystal-lined vughs, and other permineralized Ediacaran fossils were red algae or glomeromycotan lichens."

  • "Precambrian shallow trails may have been made by slime moulds in their slug aggregation phase rather than worms."

  • "Multisegmented fossils from palaeosols of the Ediacara Member such as Dickinsonia, Charnia, Praecambridium and Spriggina are more likely to have been lichens or other microbial consortia than marine invertebrates or giant protists."

  • "Discoid Ediacaran fossils such as Cyclomedusa, Medusinites and Rugosoconites would not be jellyfish in such dry soils, but could have been microbial colonies."

  • "Small fossils such as Parvancorina or Tribrachidium could not have been pre-trilobites or proto-seat-stars, respectively, if they lived on land, but may have been fungal-fruiting bodies."

  • "Trace fossils such as Archaeonassa could have been created by metazoan slugs or worms after rainstorms on land, but terrestrial habitats also open the possibility that these trails were created by slug-aggregating phases of slime moulds."

  • "'Radulichnus' impressions from the Ediacara Member are too straight and sharp to be molluscan radular scratches, and in cool temperate soils may instead have been casts of needle ice."

No doubt many will find controversial Retallack's thesis that the Ediacaran fossils were deposited on land. The fact that it was published in Nature doesn't mean everyone will accept it. What can't be denied is that it's a compelling hypothesis that needs to be taken seriously. The bottom line?

Darwin-defenders often uncritically cite the Ediacaran fossils as supposedly showing that there are known Precambrian ancestors to the animals that appear abruptly in the Cambrian explosion. Sometimes they cite the Ediacaran fossils to claim that the Cambrian explosion wasn't an "explosion" at all. But Retallack's paper adds to the large body of evidence showing that the Ediacaran fossils were not ancestral to the Cambrian fauna, meaning they don't solve Darwin's dilemma after all.

Image: Yorgia waggoneri, Wikipedia.