A feud between two Victorian geologists took a third period to settle
Adam Sedgwick named the Cambrian after the Latin form of Cymru, the Welsh word for Wales, where he studied its rocks in 1831. His friend Roderick Murchison named the Silurian just above it. Then each man began claiming the other's layers, and their quarrel outlived them both before anyone found a compromise.
The Cambrian is the first period of the Paleozoic Era, running for almost 52 million years, from 538.8 to 486.85 million years ago. Before it, most living things were small, single-celled and rarely preserved. During it came the Cambrian explosion, which produced early members of most modern animal groups, and an unusually large number of sites where soft body parts fossilised alongside shells.
Sedgwick and Murchison defined the boundary between their two systems together in 1836, but the peace did not hold. Fossils were scarce in Sedgwick's rocks, so he relied on rock types and published slowly. Murchison, with abundant Silurian fossils, correlated strata across Europe and Russia and pushed the base of his system downward to swallow Sedgwick's upper Cambrian. In 1852 Sedgwick hit back after fieldwork found a break inside the Silurian. Resolution came only in 1879, when Charles Lapworth assigned the contested layers to a new period he called the Ordovician.
Pinning down the start proved equally slow. It was once equated with the first trilobites, but small shelly fossils and the older Ediacaran organisms blurred that line. Only in 1994 was the base formally ratified, at Fortune Head in Newfoundland, marked by the burrow traces of a creature called Treptichnus pedum. An early estimate of 570 million years gave way to 538.8, taken from volcanic ash in Oman, though some argue that horizon reflects a change in sediment rather than the true boundary. The period ends at Green Point in western Newfoundland, where a tiny tooth-like fossil called a conodont first appears.
The world looked very different. Most continents sat in the southern hemisphere, surrounded by the huge Panthalassa Ocean, with Gondwana stretching from the south polar region past the equator and Laurentia straddling it. Asteroids left marks too, including Estonia's Neugrund crater, about 20 kilometres across. Between two of its epochs came the oldest known mass extinction of trilobites.
Source: Cambrian
