Although it is common to hear references to the "Cambrian explosion", no-one who uses that expression thinks of it as an instant in time when the fuse was lit and - ZAP! - the phyla were born. It has always been recognised that some phyla appear stratigraphically later than others. The problem for Darwin was that the abrupt and early appearance of phyla in the fossil record did not fit his branching pattern of gradual evolution: his model extrapolates from diversification at the species level to produce the larger taxonomic categories. The different phyla should appear after, not before, extensive speciation. A detailed review paper has recently been published which has much useful information about the data relating to the Cambrian record of animals, but which unfortunately mixes this up with highly contentious interpretation. The authors introduce the issues in this way:
"These observations (of the great radiation of animal life during the Early Cambrian) led scientists to focus in particular on two puzzling aspects of the Cambrian radiation, both encompassed by the term "Cambrian explosion". The first is the dramatic increase in disparity (morphological distinctness) as represented by the supposed appearance of nearly all major animal body plans (equivalent to the animal phyla) within a geologically brief interval of time near the beginning of the Cambrian. This problem was compounded by an apparent lack of evidence for "intermediate" taxa - taxa that lie close to the last common ancestor of different phyla in the metazoan tree. The second difficulty is the high rate of diversification (increase in number of species) in the Early Cambrian, particularly the apparent spike in diversification during the Tommotian and Atdabanian ages, spanning an interval that seemed short relative to subsequent radiations."
"The big question that the Cambrian Explosion poses is where does all that new information come from?" says Dr. Stephen Meyer, a featured expert in the documentary. (source here)
Focussing on the analysis provided of the fossil record, the authors select sites that provide opportunities to do detailed stratigraphical work: in Morocco, Siberia, Mongolia and China. Much of the paper is devoted to fossil appearances and chemical isotope analyses drawn from these localities. They give particular attention to the small shelly fauna that characterises the Early Cambrian (the lowest two stages are the Nemakit-Daldynian and the Tommotian). A strong link is found between fossil appearances and seawater chemistry. This is their summary:
"The time line of small shelly fossil first appearances indicates the following.
(1) All aragonitic taxa appeared in the Nemakit-Daldynian, before the first appearances of calcitic taxa, confirming earlier studies and suggesting that the Mg/Ca ratio of seawater determines skeletal mineralogy at the time that carbonate skeletons first evolve in a clade. [. . .]
(2) The major groups of small shelly fossils appear early; five appear by 540-538 Ma, and all but one appear by 534-532 Ma.
(3) By the middle of the Nemakit-Daldynian (534-532 Ma), nearly half of the total number of small shelly fossil genera recorded in our data set had appeared, and by the end of the Nemakit-Daldynian, nearly three-quarters had appeared, suggesting that diversification of these animals occurred throughout the Nemakit-Daldynian, rather than being concentrated at the end of that time. [. . .]
(4) Three pulses in fossil first appearances, the smallest in the early Nemakit-Daldynian , ca. 540-538 Ma, the largest in the middle Nemakit-Daldynian, ca. 534-530 Ma, and the third in the Tommotian, ca. 524-522 Ma, may reflect peaks in small shelly fossil diversification, but could also reflect the influence of local or global preservational biases."
The pattern reported for the small shelly fossils is mirrored in the other animals studied. The above description is generic: there are three pulses of appearance of skeletal animals: a small one at the base of the Cambrian, the largest in the middle of the Nemakit-Daldynian and an intermediate pulse in the Tommotian. Prior to the Cambrian, the seawater is aragonitic; during the Nemakit-Daldynian it is described as aragonite-calcite transition; and in the Tommotian the seawater is calcitic. There is thus an ecological story to accompany the fossil appearance story: the big issue is whether the environmental change drives evolution or whether it constrains evolution or whether it limits the ecological options for animals to feed and breed. The authors recognise that their paper provides a foundation for such discussion to take place:
"An explanation for the processes responsible for the radiation of animals, and of whether the radiation was a consequence or a cause of associated geochemical changes, requires a thorough understanding of the pattern of that radiation, to which this paper contributes."
However, the authors go much further than this in their conclusions. They consider that Darwin's appeal to the imperfection to the fossil record has "turned out to be closer to the truth". In their judgment, the big puzzles are resolved:
"The problem of missing fossil ancestors was solved by the discovery of the Precambrian fossil record, the problem that nearly all the animal phyla appear in the Lower Cambrian with no evidence of intermediate taxa was solved by the recognition that most Lower Cambrian fossils represent stem-groups of living phyla, and the problem of the explosive diversification of animals at the start of the Tommotian was solved by improved correlation and radiometric dating of Lower Cambrian sequences - to which we contribute here - showing that this diversification was drawn out over more than 20 m.y."
It should be obvious that the problem of the early origin of the phyla is not solved by saying that the earliest Cambrian fossils are stem-group rather than crown-group fossils. The challenge to Darwinism posed by the abrupt origins of body plans is undiminished by this fresh analysis. Furthermore, saying that the diversification of animals was drawn out over 20 Ma may reduce the tension for some lineages, but there are still plenty of others where the diversification is inconsistent with Darwinian gradualism (as recently discussed for the echinoderms).
The authors appear to be too eager to sweep away the "Cambrian Explosion" challenge to Darwinism. They might be advised to refer to Meyer, et al. (2006): The Cambrian Explosion: Biology's Big Bang. They may wish also to refer to the work of Thomas Kuhn, who showed how easy it is for scientists to get in a rut and never subject their own presuppositions to critical scrutiny. This has been a real snare for Darwinists who have become experts at slotting every data element into their all-embracing theory. The remedy is to promote multiple working hypotheses. This allows one's own presuppositions to be challenged more easily - and this is healthy for science. The alternative hypothesis this blog has been exploring is that the fossil record is perfectly capable of an ecological perspective. It is there in the Cambrian Explosion data: as soon as environments were capable of being occupied by marine animals, they were colonised. The animals were not suited to aragonite seas, so they are absent from the Ediacaran. But as soon as calcitic seas became widespread, these animals were everywhere. For more on this, with further links, go here.
The earliest Cambrian record of animals and ocean geochemical change
Adam C. Maloof, Susannah M. Porter, John L. Moore, Frank O. Dudas, Samuel A. Bowring, John A. Higgins, David A. Fike, and Michael P. Eddy
Geological Society of America Bulletin, November 2010, v. 122, p. 1731-1774 | doi:10.1130/B30346.1
Abstract: The Cambrian diversification of animals was long thought to have begun with an explosive phase at the start of the Tommotian Age. Recent stratigraphic discoveries, however, suggest that many taxa appeared in the older Nemakit-Daldynian Age, and that the diversification was more gradual. [. . .] The time line suggests that the diversification of skeletal animals began early in the Nemakit-Daldynian, with much of the diversity appearing by the middle of the age. Fossil first appearances occurred in three pulses, with a small pulse in the earliest Nemakit-Daldynian (ca. 540-538 Ma), a larger pulse in the mid- to late Nemakit-Daldynian (ca. 534-530 Ma), and a moderate pulse in the Tommotian (ca. 524-522 Ma). These pulses are associated with rapid reorganizations of the carbon cycle, and are superimposed on long-term increases in sea level and the hydrothermal flux of Sr.
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