Small Shelly Fossils, and the Length of the Cambrian Explosion - Evolution News & Views

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Small Shelly Fossils, and the Length of the Cambrian Explosion

ScienceCover-1.gifIn his review of Darwin's Doubt in the journal Science, UC Berkeley paleontologist Charles Marshall claims that Stephen Meyer "completely omits mention of the Early Cambrian small shelly fossils," which he claims causes Meyer "to exaggerate the apparent suddenness of the Cambrian explosion." Yet both of Marshall's claims are false. Meyer does not fail to mention the small shelly fossils and he does not exaggerate the brevity of the Cambrian explosion.

In the first place, Marshall has his facts wrong. Meyer does discuss the small shelly fossils on page 425 of Darwin's Doubt. Meyer writes as follows:

The Cambrian period 543 mya is marked by the appearance of small shelly fossils consisting of tubes, cones, and possibly spines and scales of larger animals. These fossils, together with trace fossils, gradually become more abundant and diverse as one moves upward in the earliest Cambrian strata (the Manykaian Stage, 543-530 mya).

Nevertheless, although Meyer discusses the small shelly fossils, he does not treat them as a solution to the problem of the explosion of morphological novelty that arises later in in the Cambrian period. The small shelly fossils appear in the fossil record at the base of the Cambrian period about 542-543 million years ago. The main pulse of morphological innovation that Cambrian paleontologists commonly refer to as the "Cambrian explosion" first begins about 530 million years ago and then lasts about 10 million years through the Tommotian and Atdabanian stages of the Cambrian period. During the first 5-6 million year stage (the Tommotian) of the explosion, between 14-16 novel phyla first appear in the fossil record.1 Without actually asserting that the small shelly fossils somehow explain the subsequent explosion of all these novel forms of animal life (or even that the small shelly fossils represent ancestors to all, or some, of these forms), Marshall faults Meyer for not treating them as part of the Cambrian explosion.

But does Meyer's decision not treat them as clear ancestors of the later forms mean that that he exaggerated the brevity of the explosion and, in so doing, overlooked a possible explanation for the missing ancestral fossils to the animals that arise in the crucial Tommotian and Atdabanian periods?

It doesn't -- as Marshall's own technical writing has made clear. For example, in a 2006 paper in Annual Reviews of Earth and Planetary Sciences, Marshall acknowledges that these fossils are of unclear evolutionary affinities and importance. He calls them "largely problematic fossils" that are "hard to diagnose, even at the phylum level."2 Figure 1 of his paper portrays them as apparently disconnected to the later radiation of Cambrian animals. This impression is reinforced in the text of his article where he notes that the small shelly fossils for the most part are "problematic" organisms of unknown classification:

While many represent individual animals, others represent individual components of the armor of much larger animals (Conway Morris & Peel 1995). Some of the described genera belong to known phyla such as Brachiopoda and Mollusca. However, many are problematic, including the cambroclaves, coeloscleritopherans, cribricyatheans, machaeridians, tommotiids, as well as a diverse array of incertae sedis [fossils of uncertain taxonomic placement].3

Other authorities agree that these small shelly fossils [SSFs] are of unclear evolutionary significance and affinity. In his book On the Origin of Phyla, James Valentine argues that the SSFs "are very difficult indeed to interpret."4 Valentine's 2013 book, The Cambrian Explosion, co-written with Douglas Erwin, notes that "many SSFs are still poorly understood."5 Simon Conway Morris found them so unimportant that he does not mention them in either of his authoritative books on the Cambrian explosion (Crucible of Creation or Life's Solution).

Nevertheless, Marshall faults Meyer for failing to mention the SSFs and claims his oversight resulted in his understating the length of the Cambrian explosion:

Meyer completely omits mention of the Early Cambrian small shelly fossils and misunderstands the nuances of molecular phylogenetics, both of which cause him to exaggerate the apparent suddenness of the Cambrian explosion.

Now Marshall never mentions any specific errors in Meyer's treatment of molecular phylogenetics so we must await his further critique on that subject. But what about the claim that Darwin's Doubt exaggerated the brevity of the Cambrian explosion? Should Meyer have included the appearance of the early Cambrian small shelly fossils as part of the explosion when he estimated the length of that event? Not according to a very recent paper by Marshall himself. In 2010, Marshall co-wrote with James Valentine in the journal Evolution (emphases added):

By the beginning of the Cambrian Period, near 543 million years ago, a few kinds of "small shelly" fossils are found, <2mm in largest dimension. The small shellys rose to a peak in abundance and diversity during the period from 530 to 520 million years ago, when representatives of living phyla are found among them. During that same period, a chiefly larger-bodied invertebrate fauna of up to a dozen phyla, and including many soft-bodied forms, is also first represented by fossils. This geologically abrupt appearance of fossils representing quite disparate bodyplans of many living metazoan phyla is termed the Cambrian explosion...6

Let's unpack the construction of this paragraph, in which Marshall explains the length of the Cambrian explosion in relation to the small shelly fossils. Starting at the end of the quote, Marshall and Valentine equate "the Cambrian explosion" with the "geologically abrupt appearance of fossils representing quite disparate body plans." They further identify this period with "that same period" wherein "a chiefly larger-bodied invertebrate fauna of up to a dozen phyla, and including many soft-bodied forms, is also first represented by fossils." Marshall and Valentine also equate that period of time with "the period from 530 to 520 million years ago" and distinguish it from the earlier time in which the first small shelly fossils arose. Thus, according to Marshall -- in a co-authored technical paper written in 2010 -- the Cambrian explosion does not begin with the first appearance of the small shelly fossils 543 million years ago, or during the earliest part of the Cambrian period. Rather, he and fellow paleontologist James Valentine affirm that the explosion begins about 530 million years ago and lasted to about 520 million years -- a date consistent with what Valentine has written elsewhere, including in his recent book with Erwin that Marshall cites approvingly in his review of Meyer.7

Thus, by Marshall's own admission, (a) the appearance of small shelly fossils around 543 million years ago does not mark the beginning of the Cambrian explosion, and (b) the Cambrian explosion should be dated to 530 to 520 million years when we see the "abrupt appearance" of many disparate body plans, long after the small shellies appear. This means that Marshall has acknowledged in print that the "Cambrian explosion" itself lasted only about 10 million years -- just as Meyer affirmed in Darwin's Doubt. Indeed, Marshall and Valentine affirm that SSFs appear long before the primary explosive radiation of Cambrian animals and they affirm a 10-million year duration for the Cambrian explosion. In response to Nick Matzke (see here), I documented many scientific papers written by other Cambrian experts that also assign an approximately 10 million year period for the main pulse of morphological innovation that paleontologists typically call the Cambrian explosion.8 So here again we see one of Meyer's critics criticizing Meyer for holding a position9 about a factual matter that leading Cambrian paleontologists also hold -- in this case, a position that Marshall himself has sometimes publicly affirmed.

It's revealing that Marshall doesn't actually claim that the small shelly fossils solve the problem of the explosion of morphological novelty that occurs later in the Cambrian period. Instead, he seems content to use the small shelly fossils as a rhetorical cudgel, knowing, I suspect, that these fossils do little if anything to diminish the real problem of morphological novelty that makes the subsequent stages of the Cambrian period so vexing from a Darwinian point of view.

References Cited:

(1) Erwin, et al., "The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals," Science, 334 (2011) 1091-1097. Samuel A. Bowring, John P. Grotzinger, Clark E. Isachsen, Andrew H. Knoll, Shane M. Pelechaty, Peter Kolosov, "Calibrating Rates of Early Cambrian Evolution," Science, 261 (September 3, 1993): 1293-1298.

(2) Charles R. Marshall, "Explaining the Cambrian 'Explosion' of Animals," Annual Reviews of Earth and Planetary Sciences, Vol. 34:355-384 (2006).

(3) Charles R. Marshall, "Explaining the Cambrian 'Explosion' of Animals," Annual Reviews of Earth and Planetary Sciences, Vol. 34:355-384 (2006).

(4) James W. Valentine, On the Origin of Phyla, p. 304 (University of Chicago, 2004).

(5) Douglas Erwin and James Valentine, The Cambrian Explosion: The Construction of Animal Biodiversity, p. 151 (Roberts and Co., 2013).

(6) Charles R. Marshall and James W. Valentine, "The Importance of Preadapted Genomes in the Origin of the Animal Bodyplans and the Cambrian Explosion," Evolution, Vol. 64-5: 1189-1201 (2010). (Emphasis added).

(7) "[A] great variety and abundance of animal fossils appear in deposits dating from a geologically brief interval between about 530 to 520 Ma, early in the Cambrian period. During this time, nearly all the major living animal groups (phyla) that have skeletons first appeared as fossils (at least one appeared earlier). Surprisingly, a number of those localities have yielded fossils that preserve details of complex organs at the tissue level, such as eyes, guts, and appendages. In addition, several groups that were entirely soft-bodied and thus could be preserved only under unusual circumstances also first appear in those faunas. Because many of those fossils represent complex groups such as vertebrates (the subgroup of the phylum Chordata to which humans belong) and arthropods, it seems likely that all or nearly all the major phylum-level groups of living animals, including many small soft-bodied groups that we do not actually find as fossils, had appeared by the end of the early Cambrian. This geologically abrupt and spectacular record of early animal life is called the Cambrian explosion." Douglas Erwin and James Valentine, The Cambrian Explosion: The Construction of Animal Biodiversity, p. 5 (Roberts and Co., 2013) (emphases added).

(8) See Casey Luskin, "How 'Sudden' Was the Cambrian Explosion? Nick Matzke Misreads Stephen Meyer and the Paleontological Literature; New Yorker Recycles Misrepresentation," Evolution News & Views, July 16, 2003 at http://www.evolutionnews.org/2013/07/how_sudden_was_074511.html.

(9) Meyer equates the Cambrian explosion with the most explosive period of the Cambrian radiation (as most Cambrian experts do) in which the vast majority of the higher taxa arose. As he argues, the re-dating of critical Cambrian strata in 1993 established that the strata documenting the first appearance of the majority of the Cambrian phyla and classes took place within a 10 million year period -- a period he calls "the explosion of novel Cambrian animal forms." (pp. 71-72) As Meyer writes: "these studies [i.e., radiometric analyses of zircon crystals in Siberian rocks] also suggested that the explosion of novel Cambrian animal forms" took about 10 million years. (p. 71)


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