Ediacaran

The Ediacaran is a geological period of the Neoproterozoic Era that lasted 96 million years, from 635 million years ago to 538.8 million years ago. It was the last period of the Proterozoic Eon and the Precambrian supereon, ending just before the start of the Cambrian Period, which marks the beginning of the Phanerozoic Eon. During this time, recognizable fossil evidence of life becomes more common.

The Ediacaran Period is named after the Ediacara Hills in South Australia, where trace fossils of many unusual lifeforms, called the Ediacaran biota, were first found by geologist Reg Sprigg in 1946. It became an official geological period in 2004, the first new one declared in 120 years. Although it takes its name from the Ediacara Hills, the main reference point is actually located in the bed of Enorama Creek within the Ikara-Flinders Ranges National Park.

The Ediacaran marks the first widespread appearance of complex multicellular fauna after the end of a global glaciation known as Snowball Earth. This period saw an evolutionary radiation known as the Avalon Explosion, with many simple soft-bodied animal groups appearing, such as Proarticulata, Petalonamae, Aspidella, and Trilobozoa. Most of these organisms died out by the end of the Ediacaran, but some modern animal groups, like cnidarians and early bilaterians, also appeared. Hard-bodied organisms with mineralized shells also began to appear near the end of this period.

During the Ediacaran, the supercontinent Pannotia formed and later broke apart. There were also several glaciation events, such as the Gaskiers and Baykonurian glaciations. Another notable event was the Shuram excursion, though it likely did not have a glacial origin.

Ediacaran vs. Vendian

See also: Riphean age and Jotnian

The Ediacaran Period overlaps with the Vendian Period but is shorter. The Vendian Period, which lasted from 650 to 543 million years ago, was first suggested by a Russian geologist in 1952. The two periods share some of the same time but have different ways of marking their start and end.

Scientists use different clues from rocks and fossils to decide where these periods begin and end. Some of these clues come from special kinds of tiny organisms and larger fossils found in certain areas. These clues help us understand how and when these ancient times began.

Upper and lower boundaries

The Ediacaran Period marks the time from the end of a big cold period called the Marinoan glaciation until the first appearance of more complex trace fossils called Treptichnus pedum.

The start of the Ediacaran is not marked by new fossils, but by a special kind of rock layer called a "cap carbonate." This layer shows signs of sudden climate change after the ice age. Scientists have chosen a specific spot in the Ikara–Flinders Ranges National Park in South Australia to mark the beginning of this period.

The end of the Ediacaran is marked by the beginning of the Cambrian Period, which scientists decided based on these trace fossils found on the southeast coast of Newfoundland. However, there is some debate about these methods for defining the borders of the Ediacaran.

Subdivisions

The Ediacaran Period has not been officially split into smaller parts yet. However, scientists have suggested some ways to divide it. One idea is to have an Upper Ediacaran starting with the Gaskiers glaciation. There is also a idea for a Terminal Ediacaran Stage that began about 550 million years ago, and an earlier stage starting around 575 million years ago when the first widespread Ediacaran biota fossils appeared. Scientists are still discussing whether to split the earlier parts into an Early and Middle Ediacaran or not, because they are not sure if the Shuram excursion is a separate event from the Gaskiers, or if the two events are related.

Absolute dating

The exact age of the Ediacaran Period in South Australia is hard to determine because there isn’t enough special rock to study. Scientists figure the Ediacaran lasted from about 635 to 538.8 million years ago by comparing it to rocks in other places like Africa, China, North America, and Tasmania. They use methods such as U–Pb (uranium–lead) and Re–Os (rhenium–osmium) dating to find these ages.

Biota

The Ediacaran Period had few fossils, because animals with hard shells had not yet appeared. The Ediacaran biota includes the oldest known multicellular organisms, which had special tissues. These organisms often looked like worms, leaves, disks, or bags. Some of these were early forms of cnidarians like Auroralumina and Haootia, and others were early bilaterians like Yilingia. Sponges also existed by the end of this period, such as Helicolocellus.

Many Ediacaran organisms look very different from modern life. It is hard to see how they are related to the life that appeared later in the Cambrian explosion. Over 100 groups of these organisms have been named, including well-known ones like Arkarua, Charnia, Dickinsonia, Ediacaria, Marywadea, Cephalonega, Pteridinium, and Yorgia. Some fossils from this time are also foraminifera, which are not animals.

The Ediacaran had four main periods of life. The first, from 635 to 575 million years ago, had tiny organisms called large ornamented Ediacaran microfossils. The second, from 575 to 560 million years ago, featured the Avalon biota. The third, from 560 to 550 million years ago, is called the White Sea biota, named after fossils found near the White Sea. The last period, from 550 to 539 million years ago, is known for the Nama biotic assemblage.

There was a big change in life during this time, possibly because early animals changed the environment, or maybe because of an anoxic event.

Astronomical factors

Because the Moon was closer to Earth during this time, the tides were stronger and changed faster than they do today. A day back then lasted about 21.9 hours, and there were around 13.1 months and 400 days in a year.

Documentaries

Several documentaries have explored the Ediacaran Period and its unique lifeforms.

One documentary is called Australia: The Time Traveller's Guide from 2012, part of the ABC Science series. Another is The Geological History of Canada, shown on The Nature of Things in 2011.

The famous narrator David Attenborough has also presented two documentaries about early life. The first episode of his series Life on Earth looks at the very beginning of life on our planet. In another program called First Life, he shows animations of fascinating creatures like Charnia, Dickinsonia, Spriggina, Funisia, and Kimberella, created using special CGI technology.