For the first time since 1891, the geologic timescale is getting a new period. In March the International Union of Geological Scientists added the Ediacaran Period to the Precambrian Eon, an enormous time span that covers 90 percent of geologic history.
The new period takes its name from Ediacaran fossils, remains of the oldest-known complex animal life, that were found in abundance in the Ediacaran Hills of South Australia. The period begins around 620 million years ago with the rapid end of
the global Marinoan glaciation—a great environmental calamity that entombed the planet in ice for several million years. It ends 543 million years ago with the onset of the Cambrian Period and the explosion of animal life.
As geologists readily admit, the table has its shortcomings. While the most recent 543 million years—the Phanerozoic Eon—are heavily subdivided into three eras and 11 periods, the remaining 4.1 billion years are lumped into a single eon, the Precambrian, and have few defining divisions. “This shows that we are now coming to understand at least the younger-most part of this hugely long interval of time in enough detail to treat it the way we treat younger rocks,” says Andrew Knoll, professor of biology at Harvard University. “It truly reflects our improving scientific understanding of the past.”
Although none of the other periods are named for fossils, they are key when it comes to geologic timing. Phanerozoic rocks are full of fossils; older rocks are not. A great achievement of 19th-century science, Knoll says, was learning to use fossils as distinctive time indicators. “That allowed this wonderful scale to come into being,” he says. The goal now, he adds, is to “carry this work back through the Precambrian, to try to bring eventually all of the Precambrian up to the same standard for a geologic timescale that we have for the Phanerozoic.” The next step for the Ediacaran Period is further subdivision. “People are constantly working to make a scale that is more reliable, more highly resolved,” Knoll says. “There’s a pretty good cottage industry in doing that.”