Miller Hall Museum of Geology Queen's University Department of Geology W.G. Miller Miller Hall Museum of Geology
Multi-celled Animal Fossils PDF Print E-mail

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.

Sponge image courtesy of Smithsonian Institute

Sponges

Sponges are the simplest grade of multi-celled animals. In general, sponges have open-topped, sack-like bodies which are fixed to the sea floor. Water is pulled through the body, and food is filtered out.

Cnidarians

The cnidarians include corals, sea anemones, and jellyfish.Their basic body plan is also a sack-like form, but at one end there is a mouth which can be opened and closed, and tentacles which direct food to the mouth.

Worm image courtesy of Ambros Lab, Dartmouth College

Worms

Worms are the most advanced grade of simple metazoan, and it is this body plan from which all higher animals evolved. Worms have a fluid-filled cavity called a coelom inside the body and variations of this cavity can be seen in all higher animals. In many animals, it has become the sack which holds the internal organs.


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?

As the diagram to the left shows, these animals lived in relatively deep water and they were occasionally quickly buried by sand transported from shallower water. Eventually the loose sand and mud became alternating beds of sandstone and shale in what is called a turbidite sequence. Imprints of the cup-shaped animals are found on the bottoms of sandstone beds.

They don't look like animals. Why do scientists think they were?

Although faint, the circular impressions are very similar to more obvious animal fossils found in Proterozoic rocks that are slightly younger. This is how they were recognized in the first place. By studying these rocks, scientists can tell that they formed in fairly deep water. The water was probably deeper than the photic zone (the depths to which enough light penetrates that photosynthetic plants can survive). This, and the fact that there is no carbon preserved, implies that the impressions were not made by plants. The simplest metazoans that are living today are sea anemones and other simple cnidaria (corals, jellyfish). These are cup-shaped animals that would form fossils remarkably like the discs and rings that are preserved on these two samples. Although there are still many questions to be answered, most scientists are convinced that in this display you are seeing the oldest fossilized remains of multi-celled animals yet discovered.

These first multi-celled animals were followed by a much more diverse group of organisms known as the Ediacara Biota.

 

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