Oldest time division of the Paleozoic Era .
During the Cambrian, 543–490 million years ago, there were widespread seas and several scattered landmasses. The largest continent was Gondwana . The average climate was probably warmer than today, with less variation between regions. There were no land plants or animals, but there were marine organisms with either shells or skeletons. Because the dominant animals were trilobite s, the Cambrian is sometimes referred to as the Age of Trilobites.