Fossils from the Ediacaran period, before the Cambrian, have been found, with over 100 different types of organisms dating back 600 million years. Some of these ancient forms are related to modern animals, including representatives of modern phyla. Only three modern phyla are mostly accepted by scientists as having existed in the Ediacaran period, but there is substantial reason to believe that others were also represented.
For many decades, scientists have thought that multicellular life did not exist before the dawn of the Cambrian period 542 million years ago. Then, in 1967, a careful geological study of Pre-Cambrian rock beds unearthed frond-like organisms called Charnia. This was the first time definitive Pre-Cambrian metazoan fossils had been found, and it caused a sensation. Since then, over 18 fossil beds from the Ediacaran period (before the Cambrian) have been found, with over 100 different types of organisms dating back 600 million years. Scientists debate deeply whether these ancient forms are related to modern animals, and the consensus is that some of them are.
Although common knowledge states that modern phyla have their origin in the Cambrian explosion about 520 million years ago, recent discoveries have confirmed the representatives of modern phyla in the Ediacaran period. Some of these probably represent stem groups that have little or no relation to living forms. The phyla having representatives in the Ediacaran period are Porifera (sponges), Cnidaria (Inaria.), Ctenophora (comb jellies), probable Mollusca (Kimberella), probable Onychophora (Xenusion), probable Arthropoda (Parvancorina), probable Annelida (Cloudina) , probable Echinodermata (Arkarua), and members of a proposed extinct phylum of bilateral animals, Proarticulata (Dickinsonia).
Only three modern phyla (Porifera, Cnidaria and Ctenophora) are mostly accepted by scientists as having existed in the Ediacaran period, and there is substantial reason to believe that Mollusca, Onychophora, Arthropoda, Annelida and Echinodermata were also represented. Early molluscs are among the oldest of the small shell fauna marking the beginning of the Cambrian, and the Ediacara Kimberella fossil has mollusc-like characteristics, such as a univalve shell, and is found in combination with scrapings suggesting strongly a radula, the defining characteristic of molluscs. Recent images of Xenusion strongly suggest that it is an ancient onychophore (velvet worm), while Parvancorina clearly has a head and, despite the absence of fossil limbs, appears to be a stem-group arthropod. Annelida and Echinodermata are more uncertain, but the numerous tube-shaped skeletons found from the Ediacaran period suggest polychaete worms, and Arkarua’s fivefold symmetry makes it a probable echinoderm.
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