The Ediacaran Era May Be a Flawed Concept

Posted on March 26, 2013 in Darwin and Evolution, Dating Methods, Fossils, Geology, Marine Biology, Philosophy of Science

Before humans arrived, no strata came with “Ediacaran” stamped on them.  Does this human-invented name have any real meaning?  Does it tell time?

The so-called Ediacaran era came just before the Cambrian, when all the animal phyla exploded onto the scene.  Actually, that sentence requires believing they were in a time sequence.  To the observer’s eye, Cambrian strata have certain kinds of fossils in them interpreted to represent an era when the first complex animal body plans emerged.  In certain places on earth below strata designated Cambrian, there are odd frond-like organisms that have been named the Ediacaran fauna, so named after a location in Australia where they were first identified.  Below Cambrian implies before Cambrian, simply enough.  But two papers in Geology raised new questions about these beliefs.

Buatois and a team harking from Canada and South Africa, writing in Geology, consider the Ediacaran-Cambrian interface “arguably the most important in the stratigraphic column,” noted that the ichnofossil Treptichnus pedum is commonly used to indicate the interface.   They went hunting for this fossil in South Africa and found it in a wide variety of environments:

Our study in Fortunian units in the Vanrhynsdorp Group of South Africa shows a broad environmental tolerance for the T. pedum producer in shallow-marine clastic settings. This ichnotaxon is not only present in low-energy offshore wave-dominated marine settings, but it also occurs at considerably shallower water in intertidal and shallow-subtidal zones of tide-dominated systems. T. pedum seems to have high values of peak abundance in the upper offshore and lower intertidal sand flats. In many sections, the Ediacaran-Cambrian transition occurs in successions containing a sequence boundary due to incision of valleys that were filled with coarse-grained deposits of fluvial or estuarine origin, both representing facies that are unsuitable for T. pedum. The range offset of this ichnotaxon is typically greater above sequence boundaries and within transgressive systems tracts, providing some constrains [sic] on its use.

Despite these concerns, they believe that “the broad environmental tolerance of T. pedum in shallow-marine clastic settings supports evolutionary innovations rather than facies controls as the main mechanism underlying the observed vertical pattern of distribution of this ichnospecies” in the study area.  In their mind, it “supports” one interpretation over another, despite the constraints.

A second paper in Geology asks a more fundamental question: “How well do fossil assemblages of the Ediacara Biota tell time?”  Notice the frequent use of the word interpret in their opening sentences:

Patterns of origination, evolution, and extinction of early animal life on this planet are largely interpreted from the fossils of the Precambrian soft-bodied Ediacara Biota, spanning nearly 40 m.y. of the terminal Ediacaran period. Localities containing these fossils are loosely considered as part of either the Avalon, White Sea, or Nama Associations. These associations have been interpreted to have temporal, paleobiogeographic, preservational, and/or paleoenvironmental significance. Surprisingly, elements of all three associations occur within the Ediacara Member of the Rawnsley Quartzite of South Australia.

What does it mean to have three associations in one location?  It means serious questions are in order about the interpretation of observed strata as markers of time and evolution:

An analysis of over 5000 specimens demonstrates that fossil distribution is strongly controlled by facies and taphonomy rather than time or biogeography and that individual taxa vary considerably in their environmental tolerance and taphonomic integrity. The recognition that these taxa represent organisms living in various distinct environments, both juxtaposed and shared, holds strong implications for our interpretation of the record of early animal life on this planet and questions the biostratigraphic utility of the three associations. Furthermore, although in situ soft-bodied preservation provides a unique perspective on composition of benthic fossil assemblages, the record should not be interpreted as a simple “snapshot”. Fossil beds represent a range of preservational modifications varying from current winnowed census samples of benthic communities at different depths and ecological maturity, to entirely transported assemblages. Unless the appropriate environments and taphonomic conditions are present for certain taxa, the absence of a particular taxon may or may not indicate its extinction in space or time.

There’s a lot in that paragraph for doubt.  Geologists cannot simply count fossils.  Unseen geological processes can scramble the mix, winnowing the census, transporting whole assemblages elsewhere.  They ask their fellow geologists to identify “appropriate” environments and taphonomic (fossil-creating) conditions, but they don’t identify who decides what is appropriate.  Eyebrows should rise at their statement that the considerable variation in the environment and “taphonomic integrity” at Ediacaran sites has not just implications, but “strong implications for interpretation of the record of early animal life on this planet”.  That warning could just as well be sounded for the remainder of the fossil record.

Notice how they ended by saying that “the absence of a particular taxon may or may not indicate its extinction in space or time.”  The absence of evidence for the notorious Precambrian Rabbit is not evidence of absence.  Who knows without an eyewitness?  Precambrian rabbits may have been hopping in another country at the same time Ediacarans were fossilizing under the sea.  Remember?  You need appropriate taphonomic conditions.  You can’t look at a stratum, they said, as a simple “snapshot” and decide there were no rabbits in this era.  Unseen processes may have been at work winnowing the census or transporting entire assemblages.

The key point is that strata designations and their interpretations are highly theory-laden.  It doesn’t matter that most geologists accept the current theory.  The consensus has been wrong many times before.  When you look at evidence, learn to scrape off the labels humans attach to them, and especially the canned interpretations.  Science progresses by perceptive observers who see the anomalies those within the paradigm have been trained to ignore.

 

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