The Specials of June. The Paleozoic Era(The Cambrian Period) evolution of life

The Cambrian truly is an astonishing period in evolution of life on earth. Most major groups of animals first appear in the fossil record, an event popularly and scientifically called the "Cambrian Explosion". The name largely derives from the hypothesized explosion of diversity of life that occurred very rapidly, but that this actually occurred is not a consensus among scientists. Darwin and others once believed that the Cambrian rocks contained the first and oldest fossil animals. We now know that these occur in the earlier Vendian strata.

Many marine metazoans having mineralized exoskeletons flourish in the Cambrian, including sponges, corals, molluscs, echinoderms, bryozoans, brachiopods and arthropods. It is commonly believed that there were no organisms at the very base of Cambrian that had hard parts, either as an external skeleton or simply spicules. This is, however, remains in dispute. The first shelled metazoans that are characteristic of the Cambrian occur well after the earliest complex trace fossils. This suggests that hard parts evolved later. Hence, trilobites, archaeocyaths, and small shelly animals did not evolve before the middle part of the Early Cambrian. The evolution of shelled metazoans is reflected by the appearence of successively more advanced shelly fossils.