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Taphonomy Study Shortens Fuse for the Cambrian Explosion

Photo credit: p.Gordon, CC BY 2.0 , via Wikimedia Commons.

The Cambrian Explosion problem to Darwinian evolution is well known to our readers, having been explicated by Stephen Meyer in his NY Times bestseller, Darwin’s Doubt. Objections to the case for intelligent design of the Cambrian phyla were answered in Debating Darwin’s Doubt in 2015, and we regularly post updates about the Cambrian Explosion. Since Darwin himself, evolutionists have wrestled with the question: how could 16 or more complex animal body plans arise in the geological blink of an eye? If Darwin’s theory were true, where is the evidence for ancestors in the Precambrian fossil record?

Faith in (Missing) Fossils

Evolutionary paleontologists have been trusting that the missing Cambrian ancestors did indeed exist, because genetic estimates put their origins hundreds of millions of years before the explosion. They admit fossils are lacking, but the molecular clock seemed to provide evidence for a long fuse leading up to the Cambrian radiation. Perhaps fossils of the ancestors would turn up some day to validate the molecular clock. The ancestral forms might have been too small to show up, or the material they were buried in was not suitable for preservation.

To investigate that last possibility, researchers at Oxford University led by Dr. Ross P. Anderson examined the taphonomic potential of Neoproterozoic (Precambrian) sediments from around the world. Their work is published (open access) in Trends in Ecology & EvolutionNews from Oxford calls it “the most thorough assessment to date of the preservation conditions that would be expected to capture the earliest animal fossils.”

The ‘molecular clock’ method, for instance, suggests that animals first evolved 800 million years ago, during the early part of the Neoproterozoic era (1,000 million years ago to 539 million years ago). This approach uses the rates at which genes accumulate mutations to determine the point in time when two or more living species last shared a common ancestor. But although rocks from the early Neoproterozoic contain fossil microorganisms, such as bacteria and protists, no animal fossils have been found.

This posed a dilemma for palaeontologists: does the molecular clock method overestimate the point at which animals first evolved? Or were animals present during the early Neoproterozoic, but too soft and fragile to be preserved? (Emphasis added.)

Anderson’s team first examined the mineralogy of the twenty best Cambrian fossil sites, such as the Burgess Shale. Using three analytical techniques, they determined that Burgess-Shale-Type (BST) rocks, notably Cambrian mudstones, are enriched in certain clays that appear responsible for the exceptional preservation. Then they asked if any Neoproterozoic rocks have similar BST mineralogy. Most do not, they concluded. But three of them do: one in Nunavut (Canada), one in Siberia, and one in Norway. These sites are assigned dates of 800 to 789 mya in the Tonian period.

Given that BST conditions preserve small, soft, and fragile animals in the Cambrian, a lack of widely accepted animal fossils in Neoproterozoic successions, even if BST preservation occurred, would suggest a real absence of animals at that time.

Guess the Result

No Cambrian animal ancestors were found in the three sites:

Microanalytical study of direct clay-microfossil associations from three of the most biodiverse Neoproterozoic mudstones, the ∼1000-million-year-old Lakhanda Group (Siberia, Russia), and the ∼800-million-year-old Svanbergfjellet (Svalbard, Norway) and Wynniatt (Nunavut, Canada) formations, suggests that the role of BST preservation promoted by clays was as important in some Neoproterozoic as in Cambrian settingsThese three deposits preserve multicellular and filamentous microorganisms, as well as forms with complex spines/processes that appear to be more fragile than typical spheroidal organic-walled forms common in Neoproterozoic assemblages. Elemental (EDS) and mineralogical mapping (synchrotron-based infrared microspectroscopy) revealed enrichments of kaolinite immediately adjacent to cell walls and forming protective haloes around the fossils.

Similarities in the distribution of clays in fossils from these three Neoproterozoic deposits and those from the Burgess Shale suggest that, in both cases, clays attached to or precipitated on decaying tissues, and that conditions conducive to BST preservation were available in both time periods. The diversity of fossil organisms and biopolymers preserved in this way shows no phylogenetic bias. Burgess Shale fossils representing stem taxa from a variety of groups (Canadia – annelid, Marrella and Opabinia – euarthropods, Ottoia – priapulid, Pikaia – chordate) are associated with kaolinite. Tonian microfossils associated with kaolinite include a chlorophyte, other undetermined eukaryotes, and probable cyanobacteria, organisms composed of a variety of biopolymers. However, no metazoan fossils have been reported from these Neoproterozoic deposits.

Their conclusion: animal ancestors “had not evolved by this time.”

Animal Affinities

Another constraint can be set at the Ediacaran period (600 to 574 mya). Most Ediacaran sites are of sandstone but show good taphonomic potential, as exemplified by detailed fossils of Dickinsonia, Kimberella and frondose organisms. The animal affinities of these are doubted, but the fossils prove that the mineralogy could have preserved Cambrian ancestors, had they existed. 

Comparing the role of clays in the preservation of Cambrian and Neoproterozoic soft-bodied fossil assemblages highlights the value of taphonomic data in substantiating the absence of animals. We have presented a new maximum constraint on animals of ∼789 Ma (Tonian), while unambiguous fossils from the Ediacara Biota place a minimum constraint at ∼574 Ma (Ediacaran).

This establishes two anchor points on the evolutionary timeline where mineralogy and paleontology converge on the conclusion, “No animals here, Mate.” Anderson’s team wants to continue investigating strata between those periods to see if the trend of “evidence of absence” continues. If so, it would constrain the “soft maximum ages on animals” further. In their concluding remarks, they say:

The antiquity of animals remains one of the most fundamental yet elusive questions in biology. Although the fossil record and molecular clocks often yield conflicting estimates for the origin of animals, a clearer understanding of fossilisation conditions (see Outstanding questions), particularly of BST preservation, may hold the key to reconciling these disparate data. Integration of soft maximum bounds constrained by taphonomic data into molecular clocks offers the prospect of a more robust chronology for early animal evolution.

Based on Assumptions

Delicately stated, but here’s the rub: to reconcile the conflict, the molecular clock will have to give. Fossils can be held in the hand and photographed. The molecular clock is based on assumptions of mutation rates. Fossils should calibrate the molecular clock, not the other way around.

This provides the first “evidence for absence” and supports the view that animals had not evolved by the early Neoproterozoic era, contrary to some molecular clock estimates.’

If the animal ancestors were not there 800 mya, and still not there 574 mya in the best possible taphonomic conditions, what are the chances they will be found in between? Slim to none is a common-sense guess. Otherwise, evolutionists are left with ghost stories: the animals appeared but left no trace. A similar argument can be made about the time between the Ediacaran and the Cambrian, since fossil animal ancestors are missing in that range, too. Science is supposed to be about what can be observed, not what is necessary to keep a popular theory from being falsified.

A reasonable conclusion from this paper is that the molecular clock is wrong, and there were no animal ancestors in Precambrian strata. This removes the “long fuse” argument and puts more bang in the Cambrian Explosion.