The Ediacaran Period (635-542 million years ago) saw the first known occurrence of multicellular organisms, including the Ediacaran fauna and other early examples of animals. Over 100 genera of Ediacaran organisms have been described, with some possibly being the origin of phyla such as molluscs and annelids. Iconic Ediacaran organisms include Dickinsonia, Yorgia, and Charnia. Dickinsonia exhibited unlimited isometric growth, while Yorgia was mobile and Charnia was the most common Ediacaran fossil.
Ediacaran organisms lived during the Ediacaran Period, a geological period that spanned from about 635 to 542 million years ago. The Ediacaran includes the first known occurrence of multicellular organisms, starting to appear in the fossil record 600 million years ago with small cnidarians (relatives of jellyfish and corals), porifera (sponges), and early bilateral animals such as Vernanimalcula.
Fossilized Ediacaran organisms fall into two categories: the so-called Ediacaran fauna, a range of quilted, sack-like shapes sometimes called life’s “failed experiment,” which became extinct before the end of the period, and other fauna that represented early examples of animals that continued into the Cambrian (just after the Ediacaran period) and beyond, which includes cnidarians, poriferans, and early bilaterians such as segmented worms. Confusingly, sometimes the word “Ediacaran fauna” refers to all living organisms during the Ediacaran period, while other times it refers only to the initial offshoot of multicellular organisms with a distinct tufted appearance. Some cladists even classify them in their own realm within the Eukaryota domain.
More than 100 genera of Ediacaran organisms have been described, including Pteridinium, Ediacaria, Marywadea, Charnia, Dickinsonia, Arkarua, Onega, and Yorgia. The relationship of Ediacaran organisms to contemporary forms is much debated. While no one has yet argued that chordates (animals with a backbone or notocord) can be found in the Ediacaran fossil record, it is argued that phyla such as molluscs, annelids, flatworms, nematodes, and others may have originated during this period.
One of the iconic organisms of Ediacara is Dickinsonia, a bilaterally symmetrical ribbed oval of unknown affinity. Dickinsonia ranged in size from 4 to 1400 millimeters, a very large degree of variation. It is thought to have exhibited unlimited isometric growth, i.e. it continued to grow continuously until it died. The internal parts of Dickinsonia consist of 0.5-1 mm diameter spindle-shaped fibers of unknown function.
Another iconic Ediacaran organism, Yorgia, looked like a cross between Dickinsonia and a segmented worm. Very long tracks have been found from Yorgia, ranging up to 43 m (141 ft), strongly suggesting that the organism was mobile.
Another of the famous Ediacaran organisms was Charnia, a frond-like animal whose discovery in 1957 caused the scientific community to take the idea of pre-Cambrian multicellular life seriously for the first time. Charnia, some specimens of which exceed 2 m (6.5 ft) in length, is both temporally and geographically the most common Ediacaran fossil. Although initially classified as a relative of sea pens, this interpretation has since been discredited. Very little is known about the ecology or life cycle of Charnia.
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