Ediacaran organisms lived during the Ediacaran period, a geologic period extending from approximately 635 to 542 million years ago. The Ediacaran includes the earliest known occurrence of multicellular organisms, which begin to appear in the fossil record 600 million years ago with small cnidarians (relatives of jellyfish and coral), poriferans (sponges), and early bilateral animals like Vernanimalcula.
Fossilized ediacaran organisms fall into two categories: the so-called Ediacaran fauna, a range of quilt and bag-like forms sometimes called "life's failed experiment," which went extinct before the end of the period, and other fauna that represented early examples of animals that continued into the Cambrian (immediately 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 organisms living during the Ediacaran period, while other times it refers just to the initial offshoot of multicellular organisms with a distinct quilted appearance. Some cladists even classify these in their own kingdom within the domain Eukaryota.
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 highly debated. Although 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 mollusks, annelids, flatworms, nematodes, and others may have originated during this time.
One of the iconic Ediacaran organisms is Dickinsonia, a bilaterally symmetrical ribbed oval of unknown affinity. Dickinsonia ranged in size from 4 to 1400 millimetres, a very large degree of variation. It is thought to have displayed unbounded isometric growth, that is, it kept growing continuously until it died. The internals of Dickinsonia consist of spindle-like fibers 0.5-1 mm in diameter of unknown function.
Another iconic Ediacaran organism, Yorgia, looked like a cross between Dickinsonia and a segmented worm. Very long trails from Yorgia have been found, ranging up to 43 m (141 ft) have been found, suggesting strongly that the organism was mobile.
Another one 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 samples of which exceed 2 m (6.5 ft) in length, is both temporally and geographically the most widespread Ediacaran fossil. Although initially classified as a relative of the sea pens, this interpretation has since been discredited. Very little is known about the ecology or life cycle of Charnia.