Cambrian

From Palaeos

two Cambrian lobopods: To the left is the lobopod Luolishania longicruris, from Chengjiang, and the right is the lobopod Aysheaia pendunculata, from the Burgess Shale. © Stanton Fink

The first eon of Earth’s history, from the first coalesence of the planet, about 4500 Mya, to about 542 Mya, is referred to as the Precambrian. From this hint, one might well suppose that the Cambrian comes next -- which it does, in a way. Actually, this is the biggest break point in all of geology. It marks the beginning of the Phanerozoic Eon, the Paleozoic Era, the Cambrian Period, the Early Cambrian Epoch, and what is sometimes called the Manykaian Age or Nemakit-Daldynian Age (currently the first age of the Cambrian has no official name). The Cambrian Period was named in 1835 by the geologist Adam Sedgwick, after the region of Cambria in North Wales, where rocks of this age were first found. The name "Cambria" is a version of Cumbria, a latinisation the Welsh Cymry (= countryman, compatriot against the invading Anglo-Saxons).

Long before it was a formal stratigraphic unit, the Cambrian was a concept about Earth history. It was understood to be the earliest period in which one could find the fossils of multi-celled animals (Metazoa). Since then, metazoans and their fossilized traces have been found well before 550 Mya. In particular, Ediacarans, a group of very strange and poorly understood creatures -- but obviously metazoans -- have been found in many parts of the world with ages pushing the 600 Mya mark.

Consequently, paleontologists now view the Cambrian as the period when the Bilateria first appeared and, almost at the same time, the first metazoans with shells. The Bilateria include all living metazoans except jellyfish, corals, and sponges. Bilaterians have a head at one end of an elongate body which is bilaterally symmetrical (hence the name). Their embryos all develop a separate layer of embryonic cells, called mesoderm, between the gut or coelom and the outer wall of the animal. If this whole description suggests a worm, you’ve got the right idea. A flatworm is the most basal living form of bilaterian.

Unlike many other, somewhat arbitrary, geological markers, the base of the Cambrian Period is defined with reference to the underlying paleontological concept. Small worms rarely leave body fossils, but their burrows are frequently preserved. The burrows of bilaterian worms are fairly distinctive. Trace fossils are often given names as if they were organisms, and the earliest well-known bilaterian trace fossil is a type of fossilized burrow referred to as Treptichnus pedum. The base of the Cambrian is currently defined as the first occurrence of T. pedum at Fortune Head, near the town of Fortune, on the north coast of western Newfoundland, Canada.

The three periods, followed by the time they began are:

Late Cambrian 501 Mya

Middle Cambrian 513 Mya

Early Cambrian 542 Mya

Palaeos com - Cambrian