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“Settled Science” and the Cambrian Explosion — Geologists Weigh In

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Yesterday we considered the “settled science” explaining the Cambrian explosion in light of one of two new papers in the Geological Society of America Bulletin. In the second paper, also by geologists from Harvard (Smith et al., with Francis A. MacDonald participating in both papers), we travel to Mongolia.

Until the 1990s, large regions of outcrops that cross the Ediacaran-Cambrian boundary were inaccessible to Western scientists. The Harvard team performed an extensive mapping of the area, correlating the outcrops with each other, measuring carbon and oxygen ratios for thousands of samples, and integrating their findings with outcrops in other parts of the world. They begin by introducing the problem:

Since Charles Darwin’s observation of the apparently rapid appearance of fossils in what is now known as Cambrian strata (Darwin, 1859), the Precambrian-Cambrian boundary has been widely regarded as one of the evolutionary pivot points in the history of life. Despite the persisting interest on this topic, the causes, triggers, and tempo of change remain controversial. Following the extinction of Ediacaran biota and the calcifying metazoan Cloudina, the early Cambrian (or Terreneuvian Series) encompasses an evolutionary interval characterized by increases in diversity and disparity (Marshall, 2006) that coincide with multiple carbon isotope excursions with amplitudes of <8‰ (e.g., Zhu et al., 2006; Porter, 2007; Maloof et al., 2010a; Erwin et al., 2011) over a geologically brief interval in Earth history (Valentine, 2002). Most recently, Maloof et al. (2010a) suggested that the Cambrian fossil first appearances occurred in three discrete pulses associated with rapid reorganizations in the carbon cycle. To test this hypothesis and others about mechanistic links between environmental change and evolutionary milestones across the Ediacaran-Cambrian transition, it is necessary to integrate records around the globe. However, previous global syntheses (e.g., Maloof et al., 2010a) have been limited to a small number of localities with large uncertainties in both local and global stratigraphic correlations and the lack of absolute ages directly linked to biostratigraphic and chemostratigraphic data, particularly with respect to critical data from Asia. Because the global data set is biased by just a few localities, refining local correlations and grounding them in regional geology are necessary before an accurate global synthesis of fossil occurrence data can be constructed and interpreted. [Emphasis added.]

Similar problems were found in Mongolia as in the first paper: previous studies were flawed, and geochemical evidence did not correlate with fossil evidence. Once again, it’s the local conditions that matter; “we suggest that this pattern is controlled largely by regional sedimentation and taphonomy [fossilization processes] rather than the rate of taxonomic origination,” they say. (“Taxonomic origination” is a euphemism for “abrupt appearance of complex animals.”)

One of their findings puts the squeeze on the evolutionary biologists. Their recalibration of the sequence puts the first appearance of small shelly fossils “hundreds of meters higher in the stratigraphy” than previously recorded, because earlier studies mistook an outcrop of phosphatic shale at one location with another fossil-bearing layer due to incorrect mapping. This compresses the time available for their evolution.

Reading these papers in detail, one gets the clear impression that geology is as much art as science. You can’t just walk up to a wall of strata and read it like a book, much less use it like a Rosetta Stone to correlate with similar outcrops in other parts of the world. A great deal of interpretation is involved, even with empirical data like carbon and oxygen ratios. The authors use the word “interpret” frequently, even alleging that previous geologists misinterpreted things.

For example, one of the second paper’s charts shows the small shelly fossils appearing in three pulses whose dates vary between Mongolia, Siberia, and China. Is this real, or an artifact of preservation?

We suggest that the apparent pulses of fossil first appearances are the result of intervals of nondeposition in the sections included in this compilation and do not represent global evolutionary patterns; FADs will not be found during periods in which sediment is not deposited. Charles Darwin (1859) suggested that the apparently rapid appearance of fossils found in Cambrian strata was a product of the incompleteness in the stratigraphic record — at a smaller scale, this indeed may be the case.

If it “may” be the case on the smaller scale, it is clearly not the case on the large scale. Every Cambrian expert agrees that the fossil record is complete enough to consider the Cambrian explosion a major unsolved problem in biology.

Other complications appear in the paper:

  1. Some of the strata are interpreted to be autochthonous (in their original position), and others are interpreted to be allochthonous (transported into place). There are flooding surfaces, intrusions, thrusts, bypass channels, subduction zones and unconformities. Much of the region is in a large basin that was infilled by sediments.

  2. Some formations are “highly variable both in terms of thickness and lithology,” with facies changes occurring over very short distances.

  3. The authors infer periods of “depositional hiatus” in certain areas, one of them possibly up to 6-10 million years in length (this is to keep their correlations in sync).

  4. They cannot account for the large “excursions” of carbon-isotope ratios (positive and negative) at certain levels. After considering various explanations, they say, “None of the hypotheses described above provides direct explanations for a mechanistic link between eustatic sea-level change and the isotopic variations.” The measurements cannot, therefore, serve as unambiguous proxies for changes occurring in the global carbon cycle at different times. (This undercuts Maloof’s 2010 hypothesis about three pulses of evolution tied to the carbon cycle and, instead, attributes the pulses to accidents of deposition.)

  5. The dates of the carbon-isotope ratio excursions do not always match between different parts of the world, even though they are assumed to represent correlation “tie points.” Mongolia has extra excursions, for instance, that do not appear in China or Siberia. “suggesting that the carbon cycle was oscillating even more rapidly than previously thought during the earliest Cambrian.”

  6. The first appearance of a trace fossil named Treptichnus pedum is considered diagnostic of the Ediacaran-Cambrian boundary around the world, but it appears at different dates in different locations. Because its appearance is strongly “lithofacies controlled” (dependent on the type of rock in the outcrop), “using T. pedum as a global chronostratigraphic marker has been problematic on most Cambrian paleocontinents, notably in Siberia, China, Mongolia, and Kazakhstan.”

  7. Some of the upper strata contain ultramafic minerals, representative of very high temperature volcanics that are atypical of the cooler mafic lavas observed today.

  8. Some of the strata are stratified; others are not. Some contain conglomerates are even large boulders, representative of transport. Some of the conglomerates contain pebbles that are rounded and well sorted; others are angular and unsorted.

  9. The strata contain chert, limestone, sandstone, siltstone, dolomite, ooids, and other minerals and structures that call for interpretation.

  10. The Harvard team ties their carbon-isotope ratios to absolute ages from Morocco (a third of the way around the globe), but Morocco lacks the earliest Cambrian fossils. “Because there is no one section globally in which it is possible to integrate the ichnofossil record, body fossil record, carbon isotope chemostratigraphy, and absolute ages, the calibration of this evolutionarily important transition remains piecemeal, resulting in much uncertainty in determining rates of origination and geochemical change.”

These complications make it likely that some future geologist will find flaws in this paper. If the science were settled, the Harvard team would not end with a call for “testable hypotheses that can be used to better constrain the relationships between biological and environmental change during this major transition in life.” In other words, we geologists just see “first appearances” of complex creatures in a confusing bundle of rocks. Ask the biologists where they came from.

Image credit: Golden spike indicating the base of the Ediacaran period, by Peter Neaum [CC BY-SA 3.0 or GFDL], via Wikimedia Commons.

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