635-542Ma

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Ediacaran Period

The Ediacaran Period (from the Ediacara Hills of South Australia) is the last geological period of the Neoproterozoic Era, just before the Cambrian. It ranges from approximately 635 to 542 million years before the present. Historically this name has been variously used by researchers, but its status as an official geological period was ratified in March 2004 by the International Union of Geological Sciences (IUGS) and announced on May 13, 2004, the first new such period declared in 120 years. It was also called the Vendian period; this name arose in Russia.

The Ediacaran period marks the end of the last ice age in a 250 million year-long series of glaciations that covered most of the planet and froze the oceans from pole to pole - a time commonly known as Snowball Earth.

Paleogeography

Nearly 600 million years ago, an asteroid hit what is now South Australia. Over time, the crater has been eroded but Lake Acraman, a dry lake, marks its location.

The Beaverhead crater spans central Idaho and western Montana. It is estimated to be 600 million years old. Although the crater has become weathered, there are geological features such as shatter cones and shocked rocks.

Life

A fossil of a 600 million-year-old multicellular organism displays evidence of complexity. This is the earliest record currently available of multicellular organisms.

Shelled mollusks show up in the fossil record 545 million years ago. The origins and earliest evolution of this diverse group, which includes clams, snails, squid, and octopuses, remain unclear. Among the first fossil specimens were the monoplacophorans—the ancestors of many shelled mollusks alive today. Thought to be descended from annelids, monoplacophorans lacked eyes and moved on a rounded foot under a simple, limpet-like shell. In Cambrian times, they inhabited warm, shallow seas. Monoplacophorans were believed long extinct until a living species was discovered deep in the Pacific Ocean in the 1950s.

The microscopic fossils examined were recovered from rocks collected in southern China. The finely detailed X-ray images exposed features pattern that led researchers to conclude the organisms were, “the reproductive spore bodies of single-celled ancestors of animals.” Study co-author Phil Donoghue of Britain’s University of Bristol said the results mean much of what has been written about the fossils for the last 10 years is “flat wrong.” Scientists say the micro-organisms lived during the Ediacaran geologic period between 600 million and 543 million years ago when multi-celled life was just starting to evolve. One theory proposed that climate shocks during the planet's Snowball Earth phase initiated the evolution of complex, multicellular life that emerged in the Ediacaran period.

Ediacaran Dating

The animal fossil record from this era is sparse, possibly because animals had yet to evolve hard shells, which make for easier fossilization. The period is unusual because its beginning is not defined by a change in the fossil record. Unusual soft bodied fossils do occur in the Ediacaran Period, but these are limited to the latter parts of the Period, after about 580 million years ago. Rather, the beginning is defined by the appearance of a new texturally and chemically distinctive carbonate layer that indicates a climatic change (the end of a global ice age). There is an unusual depletion of 13C that marks the end of the global ice ages of the preceding Cryogenian period. The date of the boundary is fairly well constrained at 635 million years ago based on U-Pb (uranium-lead) dates from Namibia and China.

Sources

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