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Are the Ediacarans Transitional Forms for the Cambrian Explosion?


Another paper in PNAS tackles the Cambrian explosion with the usual confident-sounding headline: “The advent of animals: The view from the Ediacaran.” This entry by Mary L. Droser of UC Riverside and James G. Gehling of the University of Adelaide (edited, incidentally, by Neil “Your Inner Fish” Shubin) promises much but delivers little. And, like the others published papers in scientific journals, it completely ignores the challenge to evolution presented by Stephen Meyer in Darwin’s Doubt.

Here’s the hype:

Patterns of evolution, origination, and extinction of early animal life on this planet are largely interpreted from fossils of the soft-bodied Ediacara Biota, Earth’s earliest multicellular communities preserved globally. The record of these organisms predates the well-known Cambrian Explosion by nearly 40 million years and provides critical information concerning early experimentation with complex life-forms on Earth. Here we show that, although in appearance, these organisms look very strange and unfamiliar, many of them may have had a biology and/or ecology similar to animals today, and some were most certainly bilaterians, cnidarians, and poriferans. [Emphasis added.]

DebatingDDsmall.jpegSince Meyer already pre-empted the Ediacaran argument in Chapter 4 of Darwin’s Doubt, we were curious to see if they have anything new. Droser and Gehling do an admirable job of describing the Ediacarans and the prime localities where they can be found. But as far as making them ancestors of Cambrian phyla, the authors provide mostly guesswork, interpretation, and inference.

Cnidarians include the jellyfish, and poriferans the sponges. Right off the bat, you might ask what this has to do with solving the explosion. Meyer already listed these two phyla as present in the Precambrian (fig. 2.5, p. 32), with a question mark after Cnidaria. Bilateria simply refers to all the animals with left-right symmetry. This, too, is not that controversial. It’s all the other phyla with complex limbs, digestive tracts, eyes, and active motility that need explaining.

Since this paper is classified by PNAS under “Evolution,” and since the authors seek to find the Ediacarans’ “overall place in animal evolution on this planet,” we’d hope they would address the real problems more directly. Their approach, however, is to take “the view from the Ediacaran” with a longing look forward, hoping to find glimpses of innovation that will explode onto the scene 40 million years later.

They divide the Ediacaran into three phases, the Avalon (mostly sessile, frond-like creatures), the White Sea phase (with more diversity), and the Nama. Here’s what they’re promising to show:

Elements of the Avalon Assemblage persisted, but there are major increases in biologic and ecologic complexity following in the wake of the appearance of the older Avalon Assemblage that more strongly relate to animals as we understand them. The younger, “second wave” of the Ediacara Biota comprising the White Sea and Nama assemblages includes a wide range of new taxa (Fig. 1) that exhibit pronounced biologic and ecologic innovation, as evidenced by dramatic increases in body plans and ecospace use; it is a radiation in its own right. Notable aspects of this radiation include dramatic increases in mobility; the appearance of undisputed bilaterians, such as burrowing organisms and stem-group mollusks; the advent of sexual reproduction; the appearance of the first biomineralizers; and the advent of active heterotrophy by multicellular organisms.

All this drama will require some convincing evidence. What’s needed are (a) new body plans, (b) ecospace use, (c) motility, (d) undisputed bilateral symmetry, (e) sexual reproduction, (f) biomineralization (e.g., skeletons), and (g) new ways of feeding (heterotrophy).

Body plans: Funisia dorothea is Exhibit A here. Large assemblages of these tube-like body casts suggest something new on the Darwinian stage. Alas, “The phylogenetic affinity of F. dorothea is problematic. The lack of evidence for polypoid openings or pores in the body wall limits our understanding of its taxonomic affinities.” With the power of suggestion, though, we can imagine glimpses of great things to come.

However, although it is difficult to place these fossils within Metazoa, the morphology and ecology are suggestive of cnidarian or poriferan grade animals. The branching patterns and rarity of branching of Funisia is consistent with metazoan asexual budding. The consistency of tube widths on individual bedding surfaces, the densely packed nature of the attachment structures, and the clustering pattern of developmental stages of attachment structures on individual bedding planes suggests the juveniles settled as aggregates in a series of limited cohorts.

Ecospace use: The evidence here consists of sandstone textures they interpret as microbial mats. Microbes have little to do with the Cambrian explosion. They also try to make something of the communities that are found together on various horizons. Again, this is not demonstrating evolutionary ancestry to Cambrian phyla.

Motility: The evidence for movement of Kimberella and a couple of other creatures is flimsy. There are no articulated limbs or appendages; just correlations and speculations.

External molds of Kimberella (Fig. 1D) have been documented in close association with casts of arrays of fanned sets of bifid scratch traces (Kimberichnus teruzzii; Fig. 1F) from the Ediacara Member of the Rawnsley Quartzite, South Australia, and the Verkhovka Formation, Zimnie Gory Formation, and in the basal part of the Erga Formation of the White Sea region of northwestern Russia. From their orientation and proximity, these body and trace fossils are interpreted as evidence of mat grazing activities….

Alone among the Ediacara biota, dickinsoniids feature preserved external body molds with apparent contraction marks and variable patterns of body divisions that might be explained by muscular peristaltic contraction.

Bilateral symmetry: Even flimsier. The evidence is indirect, based on interpreted motions like “avoidance behavior” from tracks of a tube-like creature, Helminthoidichnites. They say, “It is unlikely these complicated but definitive trace fossils could have been made by anything other than a bilaterian.” Is this clear evidence? “It is clear that bilaterians, cnidarians, and poriferans are represented among the Ediacara Biota.” Let the reader judge.

Sexual reproduction: Back to Funisia. See if you find this convincing:

These solitary organisms thus exhibit growth by the addition of serial units to tubes and by the division of tubes, and dispersed propagation via the production of spats [i.e., things resemble oyster larvae]. Among living organisms, spat production is almost ubiquitously the result of sexual reproduction but is known to occur rarely in association with asexual reproduction. Hence, despite its morphologic simplicity, the F. dorothea provides evidence of a variety of growth modes and a complex arrangement for the propagation of new individuals.

Biomineralization: The one example they point to as an incipient “skeleton” is a funnel-shaped creature named Coronacollina that has about four straight spicules radiating outward from its top. They don’t know what these spicules are made of, or were used for, but interpret them as structural supports. Would sticks coming out of your scalp do much to support you?

Heterotrophy: This “innovation,” like the others, is inferred, not demonstrated. “Many of the new body and trace fossil taxa characteristic of second wave assemblages occur in intimate association with TOS [textured organic surfaces] and appear to represent matground-based, heterotrophic lifestyles.” Certainly nothing like the complex digestive tracts of Cambrian animals, with mouth, gut, and anus, is demonstrated.

That’s all that Droser and Gehling have to offer as incipient “innovations” that “might” be “interpreted” as “characteristics of modern taxa.” It’s pretty underwhelming. Surprisingly, Droser did not even mention the fossil that was all the rage in her paper last year, Plexus ricei. If that organism illustrated the innovations any better, it seems she would have mentioned it.

But now, let’s quote them on the problem of the Cambrian explosion:

  • “With rare exceptions, fossils of the Ediacara biota are not found in Cambrian strata, and those that are reported are not typical Ediacara morphologies.”

  • “A few Ediacaran fossils have been interpreted as stem group metazoans, but the Cambrian period marks the unequivocal appearance of most major phyla.”

  • “The lack of forms with clear morphologic ties to even Cambrian fauna further complicates this issue.”

  • “However, although fossils of the Ediacara Biota are not easily classified with modern taxa, they nonetheless provide the record of early animals. One of the primary issues is that they are soft bodied and preserved in a manner that is, in many cases, unique to the Ediacaran.”

  • “The apparent lack of taxonomic continuity between the Precambrian and Cambrian fossil records has led to controversial and conflicting interpretations about the Ediacara biota and their place in the evolution of metazoan life on this planet. This has been further complicated by the absence of similar modes of construction between these faunas and the rarity of Precambrian skeletonized fossils.” [Then they discuss Coronacollina, mentioned above — hardly a clear case of a skeleton.]

  • “Taken as a whole, the Ediacara Biota represents an enigmatic assemblage of fossils that are not easily related to modern taxa.”

In short, despite confident statements in the abstract and conclusion, the evidence they provide for motility, sex, skeletons, and other “modern” traits in Ediacaran creatures is not clear at all. They seem to be grasping at straws to fill in the gap before the Cambrian explosion. But those quotes above show that they recognize the stark difference between Ediacaran and Cambrian life.


Image: Dickinsonia costata, by Verisimilus at English Wikipedia [GFDL, CC-BY-SA-3.0 or CC BY 2.5], via Wikimedia Commons.