End of an Era
Prologue
We follow up on our last article in the Forgotten Origins series with a look at the late Ediacaran mass extinction, an event with relevance to today.
Mass Extinctions
The popular position on mass extinctions holds that we are currently in the midst of an Anthropocene extinction event, Earth’s sixth mass extinction. Most literature places the first mass extinction in the Ordovician about 445 million years ago. But this accounting seems to fly in the face of paleontological evidence from the Late Ediacaran. Have we miscounted?
Mass extinctions are defined by the rapid disappearance of a majority of the biosphere’s species. These catastrophic die-offs are recorded in the fossil record, forming boundaries between distinct geological periods. One of the most definitive boundaries for early geologists was the transition from the Precambrian to the Cambrian, AKA the dividing line between the Proterozoic and the Paleozoic. As previously mentioned, in Darwin’s time, the fossil record appeared to tap out 541 million years ago at the beginning of the Cambrian. For all appearances, we leaped from four billion years of no life to an abundance of life in the blink of an eye.
Now we know that the beginning of the Cambrian followed the long history of animal’s living during the Ediacaran. Mass extinctions are dramatically negative events when viewed purely from the perspective of the lost species. But mass extinction events don’t purport to be total extinctions; life has always found its way to the other side. Throughout geological history, the period following a mass extinction corresponds to the rise of new species that fill the ecological void created by a massive die-off.
The Other Side
The Cambrian explosion corresponds to an explosion of new life after a massive die-off. Out with the old and in with the new. In this case, the old consisted of Ediacaran species thriving in the shallow oceans of our planet. During a brief period in the early Cambrian, almost all of today’s animal phyla came into existence. Simultaneously, the world of Ediacaran biota slipped into the void, never to be seen again.
The global distribution of fossilized Ediacaran animals, along with their diversity, speaks to a biosphere rich in soft-bodied life. Preservation of some Ediacaran fossil assemblages is excellent, but relating them to modern phyla remains difficult. Still, we know life was thriving in Earth’s shallow oceans for almost 40 million years before the Cambrian explosion.
Ediacaran evolution produced critical changes to animal life and introduced skills and characteristics like mobility, sexual reproduction, digestive systems, and more. The components of complex organisms developed in Ediacaran animals, preparing the biosphere for its jump into the Cambrian.
The Precambrian-Cambrian boundary marks the change from soft-bodied lifeforms to complex animals with hard body parts. This change was rapid, and for the most part, fossils of Ediacaran animals are not found in Cambrian strata. Ediacaran ecosystems fell extinct and were replaced by the hard-shelled predators of the Cambrian. It sounds like a mass extinction to me.
A common theory is the new Cambrian creatures were predators with mobility, protective shells, and voracious appetites. They moved quickly compared to their sluggish predecessors and were able to win the daily battle between who eats and who gets eaten. Creatures on the Ediacaran seafloors stood no chance and were devoured into extinction. It’s not an unreasonable theory, but perhaps more was going on.
The End of an Era
Discerning the cause, or various causes, of the end Ediacaran extinction is a puzzle that may never be completely solved, but not for lack of trying. Researchers in 2018 proposed the Ediacaran ended when Earth’s oceans went completely anoxic. They analyzed uranium isotope variations in limestone samples from three well-known Ediacaran locales in China. The samples covered the last 10 million years of the Ediacaran Period, and once the isotope data was integrated with the fossil data, an episode of extreme anoxia was identified. The anoxia coincided with the disappearance of the Ediacaran biota. But this data is for one locale and doesn’t necessarily represent worldwide conditions.
Another slightly more subtle take on the idea of being devoured into extinction is biotic replacement. The earliest of the hard-shelled Cambrian creatures were probably burrowers and grazers. Their activities would have severely disrupted the Ediacaran lifestyle, interfering with food chains and destroying the existing seabed ecosystems. The theory is based on observing progressively lower species diversity during the last several million years of the Ediacaran, using fossil assemblages from a locale in southern Namibia. This scenario points to a progressive extinction as opposed to a catastrophic one.
The weight of evidence points to an end Ediacaran mass extinction caused by species competition. It forms a partial analog for the ongoing Anthropocene extinction in this respect. A single species currently dominates Earth’s ecosystems, Homo sapiens. Supporting a world population of over eight billion humans stresses the environment as the demand for natural resources soars and hungry mouths need feeding. Mass extinctions don’t require catastrophic asteroid strikes or massive outpourings of magma from the mantle; they just need biotic replacement — by humans in the case of the Anthropocene.
(The Forgotten Origins series is also available on ArcheanWeb)
Sources:
What caused the mass extinction of Earth’s first animals? Unravelling mystery of the Ediacaran-Cambrian transition(Source Science Daily)
Earth’s First Mass Extinction Was Caused by The Emergence of Animals, Scientists Say (by Peter Dockrill; Science Alert)
Mass Extinctions: The Case of the Vanishing Ediacarans (by Andrew Alden; KQED)
Biotic replacement and mass extinction of the Ediacara biota (by Simon A. F. Darroch, Erik A. Sperling, Thomas H. Boag, Rachel A. Racicot, Sara J. Mason, Alex S. Morgan, Sarah Tweedt, Paul Myrow, David T. Johnston, Douglas H. Erwin and Marc Laflamme; The Royal Society)
