Ediacaran Biota

Ediacaran Biota

The Ediacara ( /ˌiːdiˈækərə/; formerly Vendian) biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms which lived during the Ediacaran Period (ca. 635–542 Ma). Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The Ediacara biota radiated in an event called the Avalon Explosion, 575 million years ago, after the Earth had thawed from the Cryogenian period's extensive glaciation. The biota largely disappeared contemporaneously with the rapid appearance of biodiversity known as the Cambrian explosion. Most of the currently existing body-plans of animals first appeared only in the fossil record of the Cambrian rather than the Ediacaran. For macroorganisms, the Cambrian biota completely replaced the organisms that populated the Ediacaran fossil record.

The organisms of the Ediacaran Period first appeared around 600 million years ago and flourished until the cusp of the Cambrian 542 million years ago when the characteristic communities of fossils vanished. The earliest reasonably diverse Ediacaran community was discovered in 1995 in Sonora, Mexico, and is approximately 600 million years in age, pre-dating the Gaskiers glaciation about 580 million years ago. While rare fossils that may represent survivors have been found as late as the Middle Cambrian (510 to 500 million years ago) the earlier fossil communities disappear from the record at the end of the Ediacaran leaving only curious fragments of once-thriving ecosystems. Multiple hypotheses exist to explain the disappearance of this biota, including preservation bias, a changing environment, the advent of predators and competition from other life-forms.

Determining where Ediacaran organisms fit in the tree of life has proven challenging; it is not even established that they were animals, with suggestions that they were lichens (fungus-alga symbionts), algae, protists known as foraminifera, fungi or microbial colonies, to hypothetical intermediates between plants and animals. The morphology and habit of some taxa (e.g. Funisia dorothea) suggest relationships to Porifera or Cnidaria. Kimberella may show a similarity to molluscs, and other organisms have been thought to possess bilateral symmetry, although this is controversial. Most macroscopic fossils are morphologically distinct from later life-forms: they resemble discs, tubes, mud-filled bags or quilted mattresses. Due to the difficulty of deducing evolutionary relationships among these organisms some paleontologists have suggested that these represent completely extinct lineages that do not resemble any living organism. One paleontologist proposed a separate kingdom level category Vendozoa (now renamed Vendobionta) in the Linnaean hierarchy for the Ediacaran biota. If these enigmatic organisms left no descendants their strange forms might be seen as a "failed experiment" in multicellular life with later multicellular life independently evolving from unrelated single-celled organisms.

The Ediacara biota in context -650 — – -640 — – -630 — – -620 — – -610 — – -600 — – -590 — – -580 — – -570 — – -560 — – -550 — – -540 — – -530 — – -520 — – -510 — – -500 — – -490 — Cryogenian Ediacaran Cambrian ← Last Ediacaran communities ← Last putative Ediacaran ← Embryos? ← Last extensive glaciation ← First Ediacaran megafossil ← Gaskiers
Glaciation Aspidella
discs Charnia  Neoproterozoic

(last era of the Precambrian)
 Palæozoic

(first era of the Phanerozoic)

Axis scale: millions of years ago.
References: Waggoner 1998, Hofmann 1990