“MOVERS AND SHAKERS”
THE FIRST DEFINITE MULTICELLULAR ANIMALS
600 million years ago
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| Early multicellular animals. This is a reconstruction of the Ediacaran fauna from Australia, dated to approximately 600 million years ago. Credit: Illustration by John Sibbick, courtesy of John Sibbick Illustration. This illustration is used with permission. All rights reserved. |
For the first three billion years, the fossil record mainly documents stromatolites, rocks built up by the slow growth of microscopic single-celled bacterial colonies. Quite interesting, but very, very monotonous. With the end of the Second Snowball Earth about 600 million years ago, and the general warming of the earth’s climate that followed, things began to get really, really interesting…
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| Dickinsonia is an Ediacaran fossil, found in 545-600 million year old deposits. Credit: Photo by Smith609/ Wikipedia. |
Between about 600 million and 545 million years ago, at the end of the Precambrian, the first large advanced animals are known. They are called the from the Ediacaran fauna (named after the Ediacara Hills of southern Australia where they were first described in the 1940’s, also known as the Vendian). These fossils are primarily the impressions of soft-bodied animals lacking skeletons (before shells and bones had evolved), and are now known from Australia, Europe, Asia, Africa, and North America. Between fifty and seventy species have been found in deposits of this age. They range in size from about half an inch to over 3 feet long. Some of these forms apparently lived by anchoring themselves to the sea floor, while others may have crawled on the sea floor or floated or swam in the seas.
Their affinities with later forms are still controversial. Some scientists think that some of these strange forms have similarities with jellyfish or worms (usually with different patterns of symmetry), but other palaeontologists think that they are very different from any living forms today, and are perhaps not ancestral to later forms. In any case, some contemporary organisms during this time period were ancestral to the explosion of new animal forms of the Cambrian starting some 545 million years ago.
This time period also documents the earliest known fossil embryos or fertilized eggs of animals, especially in rocks from China. These remarkable embryos, about 1/20th of an inch in diameter, show various stages of cell division and would eventually develop into the adult animal forms that are represented in the fossil record of this time.
HOW DO WE KNOW?
Fossils from rocks dated to between 600 million and 545 million years ago have been found on almost all continents. They are by far the most advanced animal forms known to have existed at that time, known exclusively from remarkable high-resolution fossil impressions, as they were all soft-bodied animals without hard parts to fossilize.
WHY SHOULD I CARE?
Some of these early multicellular animals from this time period would ultimately give rise to all of the animal forms that would come later, including ourselves. From one of these strange ancient primitive body plans lived an animal that is the direct ancestor of all living humans, including you.
WEB RESOURCES
A video about carbon dating:
https://www.huffingtonpost.com/2012/05/22/carbon-dating-science-history-dinosaurs-human_n_1536274.html?ref=topbar
A pdf with images and descriptions of an “Ediacaran Garden”:
https://www.ccsf.edu/Departments/History_of_Time_and_Life/PDFs/EdiacaranGarden36x36.pdf
An article on the oldest fossils of animals found in south China:
https://www.livescience.com/animals/090727-first-life.html
Early animal fossils of sponges found in the Arabia peninsula:
https://www.wired.com/wiredscience/2009/02/earliestanimal/
University of California Museum of Paleontology web site on very simple animals, the placozoa:
https://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html
BOOK RESOURCES
Cockell, Charles, ed. An Introduction to the Earth-Life System. Cambridge: Cambridge University Press. 2007 (Chapter Five mainly)
Cowen, Richard. History of Life (5 ed.). Chicester, West Sussex: Wiley-Blackwell. 2013.
Dawkins, Richard. The Ancestor’s Tale: A Pilgrimage to the Dawn of Evolution. New York: Houghton Mifflin Company (Mariner Books). 2004. (Has elements that may be useful here as well as any Event dealing with evolution/life)
Fedonkin, Mikhail A, James G Gehling, Kathleen Grey, Guy M Narbonne, and Patricia Vickers-Rich. The Rise of Animals: Evolution and Diversification of the Kingdom Animalia. Baltimore: Johns Hopkins University Press. 2007.
McMenamin, Mark A. and Dianna L. Schulte McMenamin. The Emergence of Animals: The Cambrian Breakthrough. New York: Columbia University Press. 1990 (Chapter Two mainly)
Raff, Rudolf A. The Shape of Life: Genes, Development, and the Evolution of Animal Form. Chicago: University of Chicago Press. 1996 (especially Chapters 2 and 3 which are more accessible than later chapters)
Stinchcomb, Bruce L. World’s Oldest Fossils. Atglen, Pennsylvania: Schiffer Publishing, Limited. 2007.
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