Don't miss listening to Prof. Simon Conway Morris on the Burgess Shale fauna on Paleocast hosted by Dave Marshall. The Burgess Shale is an important Middle Cambrian deposit in the British Columbia Rocky Mountains. It is a Lagerstatte, i.e. it contains exceptionally well preserved fossils and therefore gives us rich details about the animal life and biodiversity of the early Paleozoic oceans and some insights into the geologically rapid diversification of early metazoans. (the Cambrian "explosion").
Why is preservation so exquisite in the Burgess Shale? Reconstructions of the sedimentary basin indicate that the mud that became the Burgess Shale was deposited at the base of a high relief limestone reef which essentially formed a sort of an underwater sea cliff. Periodic turbidity currents swept in fauna living in shallower areas and buried them rapidly in the deeper water at the base of the cliff. These currents form deposits a few cm thick, encasing animal remains a few mm in dimensions. The waters were oxygen starved, thus there was less aerobic bacterial degradation of soft tissue. Add to that were some peculiar geochemical conditions of the Cambrian ocean. One was a paucity of sulphate which retarded degradation by sulphate reducing bacteria. The other, as some recent work by Robert Gaines and colleagues suggest, was the high calcium carbonate saturation levels of the ocean, which lead to rapid cementation of the sea floor in between episodes of turbidity flows.
The image on the left shows CaCO3 cement rich layers in the Burgess shale (source: Gaines et al 2012). These cemented crusts on the sea floor formed an impermeable barrier and reduced the influx of sulphate and oxygen bearing sea water in to the sediment, further slowing down microbial activity. How do we know there was less activity of sulphate reducing bacteria? The researchers analyzed the patterns of sulphur isotopes in the fossil rich turbidity layers and the background sediment. Sulphate reducing bacteria preferentially take up the lighter isotope of sulphur from sea water. Thus, background deposits with normal or enhanced microbial activity have a lighter isotope signature relative to the Cambrian sea water standard. On the other hand, less microbial activity means less fractionation of the lighter isotope into bacteria and ultimately into the sediment matrix. In Burgess Shale type deposits the fossil rich turbidity layers capped by CaCO3 cements show an enriched or heavier sulphur isotope signal indicating less microbial activity. Finally, since the bottom waters were anoxic, there was little benthic fauna living there. This meant that the cement crusts were not disturbed and broken by bioturbation and remained effective seals throughout the crucial first few weeks of burial when degradation is at its peak. Soft tissue does break down due to slowed microbial activity and fermentation and methanogenesis. The three dimensional carcass collapses into a nearly two dimensional carbon rich film. The final result is that recalcitrant extracellular organic material like cuticles, chaetae, and jaws are preserved as compressed thin carbonaceous films often just a few microns thick, the soft fine grained mud encasing the carcass helping preserve fine morphological details. This preservation style also meant that some animals lacking recalcitrant tissues like flatworms, mesozoans, nemerteans and unshelled molluscs are less well represented in the Burgess Shale style deposits ( Butterfield 2003). Peculiar preservational styles by their very exceptional and localized nature impose a bias on the fossil record that palaeontologists must recognize to understand true evolutionary patterns.
The examples on the left shows Burgess Shale style preservation of Arthropod (B), Polychaetae worm (C) and Arthropod (D) [source: Gaines 2014]. This style of preservation actually appears first in the early Neo-Proterozoic and then disappears for about 150 million years until the earliest Cambrian. It again declines by late Cambrian with the earliest Ordovician being the last recorded example of this taphonomic style. A unique combination of geological conditions and early diagenesis of sediment prevailing in the latest Proteozoic and earliest Cambrian resulted in these fossil deposits. This time period also has other forms of detailed preservation of soft tissues, the two most important being the Edicaran style preservation wherein the remains of macroscopic plants and animals deposited in sandy and silty sediment were draped by microbial mats and compressed to form impressions (death masks) on the sediment surface. The other important style is the Doushantuo style preservation (named after the Doushantuo fossil beds of late NeoProterozoic age, China, containing preserved algae and putative embryos and larval stages of early animals ) where phosphate minerals are attracted to and precipitate around organic tissue preserving delicate cell outlines and internal organs. Very occasionally, the same fossil will show two different preservational styles, for example, the extracellular tissue preserved in the Burgess Shale style while internal organs preserved in the Doushantuo style. These taphonomic "windows", as they are referred to, appear and disappear through the Neo-Proterozoic to Cambrian period. For example, the Edicaran style preservation first appears in the late NeoProterozic around 580 million years ago or so. Considering that microbial mats which play an important role in this style of preservation are pervasive through the late Archaean and the Proterozoic, the first appearance of the Edicaran remains is then likely an evolutionary signal of the first appearance of macroscopic multicellular eucaryotes on earth. The disappearance of Edicaran style by the earliest Cambrian also suggests a biological feedback. The evolution of macroscopic benthic animals burrowing and grazing on bacterial mats may have destroyed the cover protecting the faunal remains. Preservational styles are controlled not just by geological conditions but due to contemporaneous evolutionary innovations too.
Coming back to the talk! Prof Simon Conway Morris describes the history of research on the Burgess Shale, how he got into researching it, details of some of the animals found in it including the famous Pikaia. This has been interpreted as an early representative of the chordates from which the vertebrates evolved. Overall, Conway Morris gives a masterly authoritative talk.
I would have loved to hear him talk a little more about the broader questions that arise from this deposit. Are the origins of the Burgess animals to be found in the earlier Edicaran fauna? Does the Cambrian have greater morphological disparity than later periods in earth history? Has life followed a contingent unique pathway or does examples of convergence tell us something deeper about the general principles of evolution? ..or an intelligence which frames the ultimate laws and guides evolutionary processes. Simon Conway Morris has indicated elsewhere his thoughts that the Universe is the product of a rational mind and that evolution is but a search engine and I wish Dave Marshall had pressed him on his theist beliefs. But I guess the topic was the fauna of the Burgess Shale and in particular that iconic quarry in British Colombia.
He did mention in passing something which I think is an important aspect of this story. Just as he had finished his Master's degree from Bristol University in 1972, a project headed by Prof. Harry Whittington on the Burgess Shale was starting at Cambridge. Simon Conway Morris saw this as a good opportunity. At around the same time, the Chicago school of palaeontologists lead by David Raup (who died last week), Jack Sepkoski and Tom Schopf had started a program to broaden the scope of palaeontology to include rigorous quantitative methods on large sample sets to understand biodiversity and patterns of evolution, bringing the field of palaeontology, as John Maynard Smith famously said, to the "high table of evolutionary theory". These events underscore the important point that for all your brilliance in something, circumstances and timing matter. Simon Conway Morris was present at the right time at the right place. And he did the Burgess Shale fauna justice.
Why is preservation so exquisite in the Burgess Shale? Reconstructions of the sedimentary basin indicate that the mud that became the Burgess Shale was deposited at the base of a high relief limestone reef which essentially formed a sort of an underwater sea cliff. Periodic turbidity currents swept in fauna living in shallower areas and buried them rapidly in the deeper water at the base of the cliff. These currents form deposits a few cm thick, encasing animal remains a few mm in dimensions. The waters were oxygen starved, thus there was less aerobic bacterial degradation of soft tissue. Add to that were some peculiar geochemical conditions of the Cambrian ocean. One was a paucity of sulphate which retarded degradation by sulphate reducing bacteria. The other, as some recent work by Robert Gaines and colleagues suggest, was the high calcium carbonate saturation levels of the ocean, which lead to rapid cementation of the sea floor in between episodes of turbidity flows.
Coming back to the talk! Prof Simon Conway Morris describes the history of research on the Burgess Shale, how he got into researching it, details of some of the animals found in it including the famous Pikaia. This has been interpreted as an early representative of the chordates from which the vertebrates evolved. Overall, Conway Morris gives a masterly authoritative talk.
I would have loved to hear him talk a little more about the broader questions that arise from this deposit. Are the origins of the Burgess animals to be found in the earlier Edicaran fauna? Does the Cambrian have greater morphological disparity than later periods in earth history? Has life followed a contingent unique pathway or does examples of convergence tell us something deeper about the general principles of evolution? ..or an intelligence which frames the ultimate laws and guides evolutionary processes. Simon Conway Morris has indicated elsewhere his thoughts that the Universe is the product of a rational mind and that evolution is but a search engine and I wish Dave Marshall had pressed him on his theist beliefs. But I guess the topic was the fauna of the Burgess Shale and in particular that iconic quarry in British Colombia.
He did mention in passing something which I think is an important aspect of this story. Just as he had finished his Master's degree from Bristol University in 1972, a project headed by Prof. Harry Whittington on the Burgess Shale was starting at Cambridge. Simon Conway Morris saw this as a good opportunity. At around the same time, the Chicago school of palaeontologists lead by David Raup (who died last week), Jack Sepkoski and Tom Schopf had started a program to broaden the scope of palaeontology to include rigorous quantitative methods on large sample sets to understand biodiversity and patterns of evolution, bringing the field of palaeontology, as John Maynard Smith famously said, to the "high table of evolutionary theory". These events underscore the important point that for all your brilliance in something, circumstances and timing matter. Simon Conway Morris was present at the right time at the right place. And he did the Burgess Shale fauna justice.
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