Does the "Great Unconformity" Explain the Missing Cambrian Ancestors? - Evolution News & Views

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Does the "Great Unconformity" Explain the Missing Cambrian Ancestors?

Ediacaran

During the Q&A after Stephen Meyer's lecture at his recent debut event for Darwin's Doubt, a questioner asked whether the "Great Unconformity" explains why the fossil record does not bear fossils that are ancestors to the Cambrian animals.

DebatingDD.jpegAn unconformity is an erosional surface representing a gap in the geological record, where time is missing. We can recognize an unconformity when a geological layer sits directly on top of much older strata, with a time-gap in between. The "Great Unconformity" is probably the most famous such gap in the geological record, and is found in some (though certainly not all) locations around the world. The exact timespan it represents is hard to define precisely (and probably varies significantly from location to location), though scientists generally suggest that it extends from sometime in the Cambrian (perhaps as late as the middle Cambrian) back hundreds of millions of years into the Precambrian world (perhaps as far back as 1.7 billion years ago). Thus, in some places the "Great Unconformity" might represent over a billion years of missing time.

Stephen Meyer pointed out in response to the questioner that the "Great Unconformity" may be "worldwide" in the sense that it's found in many parts of the world. But that doesn't mean it's found everywhere. As Meyer explained, the Great Unconformity cannot be universal, otherwise we wouldn't have strata from the Ediacaran period, and we wouldn't know about Ediacaran-age fossils, such as the Precambrian sponge embryos Meyer talks about in Darwin's Doubt.

In the same way, the "Great Unconformity" in the Grand Canyon is thought to have erased significant parts of the Cambrian period. If this were the case everywhere, we would have no knowledge of the Cambrian explosion itself. This shows that while the "Great Unconformity" is a significant geological feature, that hardly means it's ubiquitous.

In response to the questioner, Meyer also pointed out that this is an unorthodox objection to the Cambrian explosion. I agree. Though I've read numerous articles struggling to explain away the Cambrian explosion, I'd never heard of anyone arguing that the alleged Precambrian ancestors to the Cambrian animals were missing because the strata containing their fossils had been completely eroded from the face of the earth in such a "Great Unconformity." But after hearing this objection, I did some research. I discovered a single place where this argument has been made: It was over 100 years ago by Charles Doolittle Walcott, the famous geologist who discovered the Burgess Shale. Apparently the argument was never adopted by subsequent geologists, for good reasons.

In a 2006 paper in Earth Science History, Walcott biographer and former National Museum of Natural History paleobiologist, Ellis L. Yochelson (now deceased), noted that Walcott gave up on finding biological explanations for the absence of Precambrian animal ancestors, and thus turned to geological ones. Yochelson quotes Walcott as follows:

I have for the past eighteen years watched for geological and paleontological evidence that might aid in solving the problem of pre-Cambrian life. The great series ... have been studied and searched for evidences of life until the conclusion has gradually been forced upon me that on the North American Continent we have no known pre-Cambrian marine deposits containing traces of organic remains, and that the abrupt appearance of the Cambrian fauna results from geologic and not biotic consequences.
Yochelson explains that Walcott dubbed this unconformity the "Lipalian interval," and used it as a geological explanation for the lack of Precambrian animal fossils:
Whether Walcott was right or wrong in his interpretation, the Lipalian was a grand synthesis based on years of virtually fruitless searching for fossils in pre-Cambrian sedimentary deposits. There is an old saying that the absence of evidence is not evidence of absence. Somehow, that applies to the concept of the Lipalian and, equally, to the speculation as to what may have triggered that concept.

(Ellis L. Yochelson, "The Lipalian Interval: A Forgotten, Novel Concept in the Geologic Column," Earth Sciences History, Vol. 25: 251-269 (2006).)

This is just about the only source anywhere I can find where someone made this argument. Yochelson himself notes that Walcott's very notion of the "Lipalian interval" was never adopted or promoted by subsequent geologists. Much less have any recent geologists attempted to use it to explain away the lack of Precambrian fossils. Yochelson continues:
The endeavor [Walcott's proposal of the Lipalian Interval] was for naught! So far as one can tell, the net result of Walcott's notion was nothing whatsoever. Perhaps this constitutes an example of "history repeats itself." Walcott (1893) produced a significant paper on geologic time that was widely distributed (Yochelson 1989) yet that had elucidated no discussion. The most optimistic interpretation is that the geologic community accepted the interpretation as so satisfactory that no comment was needed.

The first major bibliography of North American geology indexes Walcott's 1910 "Abrupt Appearance" paper under "Paleontology Cambrian," probably as a consequence of the title. The Lipalian is not separately indexed nor is there any sublisting that would lead to it. In the following bibliography, covering 1919-1928, Lipalian is not indexed. The Lipalian is not listed in the compilation of Wilmarth (1925). It was only just mentioned in a few textbooks and then, except for rare appearances in abstracts, the term has vanished.

In one sense this disappearance is understandable, for though the approach of naming items before there is physical evidence of them has worked well in theoretical physics, probably the idea was never embraced by the geologic community because absence of data did not appeal to geologists. Theoretical concepts for aspects of geology that no longer existed, such as Pangea or Gondwana, were in the literature. They, at least, had some evidence to support their establishment and were grounded in past geography, even if there was no mechanism at the time to explain the observations that supported these notions. Time is ephemeral and basing a concept on lack of data within a continuous sequence is difficult to explain and more difficult to accept.

So we see that this objection to Meyer's arguments is not endorsed by geologists. Again, there are good reasons for that.

To reiterate, yes, there is evidence of the "Great Unconformity" in some parts of the world. However in other parts there are plenty of relevant Precambrian strata, from the period just before the Cambrian. Today that is called the Ediacaran period, and the fossils known from it are not thought to represent ancestors to the animals that appear in the Cambrian. In Darwin's Doubt, Meyer cites multiple authorities who adopt this view. He also quotes Douglas Erwin and James Valentine who make a crucial point showing why this objection fails:

In their 2013 book, The Cambrian Explosion, paleontologists James Valentine and Douglas Erwin go further. They note that many late Precambrian depositional environments actually provide more favorable settings for the preservation of fossils than those from the Cambrian period. As they write, "a revolutionary change in the sedimentary environment -- from microbially stabilized sediments during the Ediacaran [late Precambrian] to biologically churned sediments as larger, more active animals appeared -- occurred during the early Cambrian. Thus, the quality of fossil preservation in some settings may have actually declined from the Ediacaran to the Cambrian, the opposite of what has sometimes been claimed, yet we find a rich and widespread explosion of [Cambrian] fauna." (p. 69)
The Ediacaran strata, just below the Cambrian, do not yield ancestors to the Cambrian fauna. So the mystery remains.

There is one recent paper, however, that tries to explain the Cambrian explosion using the Great Unconformity. But it doesn't cite the gap in the fossil record as supposed evidence that Precambrian animal fossils were destroyed. Instead, it makes an even weirder argument -- that the weathering of rock, evidence of which we see in the Great Unconformity, dumped a bunch of sediment into the oceans, and that sediment triggered the Cambrian explosion. Last year, I discussed the paper:
Citing increased chemical weathering around the time of the Cambrian explosion doesn't explain the abrupt appearance of new genes and other genetic information needed to generate new body plans. If they expect us to believe that sedimentation rates explain the sudden origin of new body plans, then it would seem that the Cambrian explosion is still a "mystery."
Compared to this wacky argument, I'd almost prefer the fanciful (and conveniently unfalsifiable) idea that all the Precambrian ancestors disappeared because all their strata weathered away worldwide. That, despite having been discarded by scientists, at least has a veneer of remote plausibility.