Dawn of Animal Life
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|Multi-celled Animal Fossils|
The Metazoa multi-celled animal life.
The term "multi-celled" when applied to a plant or animal means that the organism is made up of several different cell types. The first multi-celled animals (metazoa) evolved over 600 million years ago.
Primitive metazoa can be grouped in three basic categories: sponge-like animals, cnidarians, and worms. The sponges, and cnidarians (corals and sea anemones), are the most primitive with about 11 specialized cell types. Worms and higher metazoa have approximately 55 specialized cells.
The Oldest Known Animal Fossils
Single-celled prokaryotes and eukaryotes in the oceans were eventually joined by metazoa, the first multi-celled animal life. The first metazoa were entirely soft-bodied (without shells or hard parts of any kind) and they evolved during a time called the Precambrian (older than 545 million years). It was originally thought that these early animals would not leave a fossil record at all because soft parts of animals are only very rarely fossilized.
Since 1946, however, paleontologists have been finding fossils in rocks of Precambrian age. Collectively they are known as the Ediacarian Fauna, and it is accepted by many paleontologists that these are fossils of the oldest known multi-celled animals on Earth
Up until the discovery of the fossils pictured below, all Ediacarian fossils had been found in rocks that were younger than a major world-wide glaciation that occurred roughly 590 million years ago. The glaciers deposited very distinctive beds of tillite (a rock formed from the mud/gravel/boulders left behind as a glacier melts). It was theorized that multi-celled life did not evolve until after the world's climate moderated, and the glaciers receded.
In 1990, paleontologists from Queen's University and the University of Montreal teamed up to seach for metazoan fossils in rocks older than the ancient glaciation. Through careful geological detective work, they identified an area with the highest probability of having such fossils. Their target was a rock unit known as the Twitya formation in the remote Mackenzie Mountains, Northwest Territories, Canada.
Near the end of the field season, they were finally rewarded as they discovered several slabs of rock containing tiny disk and ring-like impressions. These fossils are now accepted as the oldest known multi-celled animal fossils in the world. As can be seen in the images below, the rocks clearly show impressions from what are interpreted as soft, cup-shaped animals that lived on a muddy sea floor around 600 million years ago.
The Miller Museum at Queen's University is the only place in the world where these samples are on public display.
How were they preserved?