nature.net
March 2006
First Animals
By Robert Anderson
The geologic timescale was one of the great achievements of nineteenth-century science. Yet the oldest named division of geologic time was the Cambrian, beginning 540 million years ago. All time before the Cambrian was a great unknown, simply designated Precambrian, even though life on Earth began more than 3.5 billion years ago. The earliest multicelled animals, for instance—enigmatic, mostly soft-bodied creatures that lived in the latest phase of the Precambrian—only rarely left fossils.
But recently the International Commission on Stratigraphy (ICS) came up with an official name for the 80 or so million years immediately preceding the Cambrian: the Ediacaran period. Interest in Ediacaran times has grown, because the fossils show that the period marks an unprecedented flowering in the diversity of life-forms following the end of a global freeze-over.
At the ICS Web site, click on one of the international charts to get the latest official update. Nail icons on the multicolor timechart indicate a specific layer of rock somewhere in the world that serves as the base of its period. The baseline for the Ediacaran, for instance, is a postglacial carbonate layer in the Enorama Creek section of the Flinders Ranges, some 250 miles due north of Adelaide, Australia. The name for the new period comes from the Ediacara Hills, a 570-million-year-old site in the Flinders Ranges where some of the first animals big enough to be seen with the unaided eye are preserved in good detail. The Palaeos site has more information on those animals, along with a list of links.
In “The Measure of Deep Time,” at the Web site of NASA’s Astrobiology Institute, writer David Morrison gives a summary of the new slice of time, with an illustration of the key events that define it. At The Age of the Earth on Talk.Origins Archive’s Web site you’ll find several pages related to the construction of the geologic timescale.
Go to The Neoproterozoic at Paleos.com to get a quick rundown on what it must have been like to live on our planet during the Ediacaran. At the Miller Museum of Geology in Kingston, Ontario, an online exhibit of “The Dawn of Animal Life” has many images of the latest Precambrian fossil discoveries. On display in the physical museum are the oldest animal fossils in the world, spongelike rings dating back 600 million years. At the Smithsonian’s early life exhibit you’ll find a wonderful diorama that brings the Ediacaran back to life.
The University of California Museum of Paleontology in Berkeley has another online exhibit of the earliest animals, along with a map (click on “Localities”) of where some of the fossils were discovered. To confuse matters, this site, like many others, has not yet adopted the name Ediacaran. Instead the sites use the alternate, Russian name—Vendian—a name derived from Siberian strata of the same age.
The most remarkable Ediacaran-age discovery comes from the Doushantuo Formation in southern China. Here, ancient tissue was replaced by calcium phosphate, preserving the cellular structure of small sponges and jellyfish in exquisite detail. Even microscopic eggs and embryos are visible. But most exciting of all is the 2004 discovery of the oldest known “bilateran,” an animal with bilateral symmetry, whose descendants include everything from worms to us. Investigators have so far identified ten bilaterans—each about the size of the period at the end of this sentence. Go to Pharyngula’s Pre-Cambrian coelomate page to read more about those remarkable fossils.
Robert Anderson is a freelance science writer living in Los Angeles.
Copyright © Natural History Magazine, Inc., 2006