benthic (paleo)ecology, ichnology, paleobiology
One of the major revolutions in the history of life on Earth took place some time around 540 million years ago and affected benthic ecosystems. This date marks the beginning not only of the Cambrian Period but also of the Paleozoic Era and the Phanerozoic Eon. Rocks below the Cambrian were considered by early Earth scientists as ‘azoic’, i.e. devoid of any evidence of life in contrast with the fossiliferous beds above. Modern paleontology demonstrated that the Precambrian was not ‘azoic’ but rather that fossils were present and included common stromatolites and microfossils, and, in its younger part, relatively frequent occurrences of macrofossils of the enigmatic Ediacaran faunas. Nevertheless, the base of the Cambrian was confirmed as a moment of major change in the Earth’s biosphere when most major groups of invertebrates evolved to transform the planet in something more resemblant to what it is today. That is commonly known as the ‘Cambrian Explosion‘. The Canadian trio ‘Brighter Lights, Thicker Glasses‘ offers an entertaining musicalized explanation of it in the following video:
The Cambrian Explosion represented a tremendous reorganization of marine benthic ecosystems (which were essentially everything alive on Earth by then) with the debut on stage of numerous new players and a generalized change in the fluxes of energy and matter as a result of the occurrence of new strategies for resource exploitation. A variety of causes have been proposed to try to explain this ‘sudden’ burst of life forms. Reviewing those would be too long for this post. What seems to be well proved is that burrowers played a very important role in the whole story. Latermost Precambrian (Ediacaran) ecosystem dynamycs were greatly controlled by the extensive presence of microbial mats on the sea floor, i.e. communities of microorganisms able to glue together the shallowest mm of sediment. Although Ediacaran trace fossils are known, assemblages are generally low diverse and include only traces of epifaunal or very shallow infaunal organisms probably mining on the microbial mats. In contrast, bioturbation in Cambrian marine sedimentary rocks is more intense and diverse and records deeper infaunal activity than anything seen before. The proliferation of bilateralians capable of extensive burrowing was one of the factors that put an end to the Precambrian ‘stromatolitic’ world (or maybe, and probably at the same time, one of the consequences of the ecological change). German paleontologists Adolf Seilacher and Friedrich Pfluger coined the term ‘agronomic revolution’ to name this phenomenon.
This long introduction is needed to understand the significance of the new paper published in Geology by a group of Russian scientists from the Institute of Petroleum Geology and Geophysics at Novosibirsk. They describe what they claim to be the oldest evidence of extensive bioturbation by infaunal organisms in carbonate rocks of the Late Ediacarian (ca. 555 Ma b.p.) of the Olenek Uplift in Siberia. This bioturbation resulted in a trace fossil named Nenoxites, which consists of sinuous tunnels bearing a meniscate backfill produced by excavation in the anterior part and repositioning of sediment at the posterior end. The authors interpret this trace fossil as the result of the food-seeking activity of a bilateralian. Identification of Ediacarian trace fossils may be tricky and some alleged bioturbation structures have been lately reinterpreted as body fossils of algae or protists or even inorganic structures (see Jensen, Droser and Gehling who reviewed in 2006 the Ediacaran ichnological record). The authors considered and ruled out the possibility that the structures were body fossils. Nenoxites bioturbation modified in many of the studied layers above 50% of the sedimentary fabric reaching in some cases 100%. The trace fossils are horizontal, inclined an subvertical what points to true infaunality. Authors estimate bioturbation depth in at least 5 cm.
Thus, Nenoxites ichnofabrics as described and interpreted in this recently-released paper constitute the oldest record of extensive infaunal bioturbation by an unknown bilaterian and predate the outburst of burrowers and the expansion of mixgrounds that took place 540 milion years ago in the Ediacarian-Cambrian transition. No doubt more findings will bring new light to better understand this phenomenon now described in the carbonate succesions of Siberia in the context of the evolution of benthic ecospace utilization in the times before the establishment of the Metazoan benthos in the Cambrian.
Ediacarian sedimentary rocks in the Fish River Canyon of southern Namibia.
Rogov V., Marusin V., Bykova N., Goy Y., Nagovitsin K., Kochnev B., Karlova G., and Grazhdankin, D. 2012. The oldest evidence of bioturbation on Earth. Geology 40 (5), 395–398, doi:10.1130/G32807.1.
Jensen S. Droser M.L., and Gehling J.G. 2006. A critical look at the Ediacaran fossil record. Neoproterozoic Geobiology and Paleobiology, Topics in Geobiology 27, 115-157, doi: 10.1007/1-4020-5202-2_5. Link to paper in Google books (partial).