The Holly Jolly of December. Precambrian Era: Ediacaran fossils

Metazoans developed rapidly from the beginning of the Cambrian, when organisms acquired the ability to produce the protein collagen and, thus, skeletons and shells. However, more-primitive metazoans without skeletons—the Ediacara fauna—appeared earlier (more than 600 million years ago), after the end of the Varanger-Marinoan ice age at 580 million years ago and before the onset of the Cambrian Period at 541 million years ago. They are found as impressions of soft-bodied, multicellular animals in the rocks and have the form of tiny blobs, circular discs, or plantlike fronds ranging from less than 1 cm (less than 0.4 inch) to more than 1 metre (about 3 feet) long. The type locality is the Ediacara Hills in South Australia, where over 1,500 well-preserved specimens have been collected, resulting in the naming of more than 60 species and 30 genera. They occur in a quartzite that is stratigraphically situated some 500 metres (1,600 feet) below the base of the Cambrian System. These organisms resemble modern jellyfish, worms, sponges, seaweed, sea anemones, and sea pens. Comparable impressions in the youngest Precambrian sediments have been found in over 30 localities from every continent except Antarctica. Ediacaran fossils have been discovered in areas such as Charnwood in central England, Ukraine, Iran, the Ural Mountains and the White Sea coast in Russia, Namibia, Newfoundland, the Mackenzie and Wernecke mountains in northwestern Canada, the Yangtze valley in China, and North Carolina in the southeastern United States. Ediacaran fossils have been deposited in environments ranging from tidal marine habitats to the deep seafloor. Some forms show evidence of sophisticated adaptations, such as the use of multiple modes of reproduction. The Ediacaran organisms were probably the ancestors of shelled organisms that mark the beginning of the Phanerozoic.