|
The Ediacaran (; formerly Vendian) biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms that 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.〔Simple multicellular organisms such as red algae evolved at least .〕 The Ediacaran biota radiated in an event called the Avalon explosion, ,〔(Two Explosive Evolutionary Events Shaped Early History Of Multicellular Life )〕 after the Earth had thawed from the Cryogenian period's extensive glaciation. The biota largely disappeared with the rapid increase in biodiversity known as the Cambrian explosion. Most of the currently existing body plans of animals first appeared in the fossil record of the Cambrian rather than the Ediacaran. For macroorganisms, the Cambrian biota appears to have completely replaced the organisms that populated the Ediacaran fossil record, although relationships are still a matter of debate. The organisms of the Ediacaran Period first appeared around and flourished until the cusp of the Cambrian , 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 of 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, or 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 palaeontologists have suggested that these represent completely extinct lineages that do not resemble any living organism. One palaeontologist 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 evolving independently from unrelated single-celled organisms. Breandán MacGabhann argues that the concept of "Ediacara Biota" is artificial and arbitrary as it can not be defined geographically, stratigraphically, taphonomically nor biologically. He points out that 8 particular fossils or groups of fossils considered "Ediacaran" have 5 taphonomic modes (preservation styles), occur in 3 geological periods, and have no phylogenetic meaning as a whole. ==History== The first Ediacaran fossils discovered were the disc-shaped ''Aspidella terranovica'' in 1868. Their discoverer, Scottish geologist Alexander Murray, found them useful aids for correlating the age of rocks around Newfoundland.〔 However, since they lay below the "Primordial Strata" of the Cambrian that was then thought to contain the very first signs of animal life, a proposal four years after their discovery by Elkanah Billings that these simple forms represented fauna was dismissed by his peers. Instead, they were interpreted as gas escape structures or inorganic concretions.〔 No similar structures elsewhere in the world were then known and the one-sided debate soon fell into obscurity. In 1933, Georg Gürich discovered specimens in Namibia but the firm belief that life originated in the Cambrian led to them being assigned to the Cambrian Period and no link to ''Aspidella'' was made. In 1946, Reg Sprigg noticed "jellyfishes" in the Ediacara Hills of Australia's Flinders Ranges but these rocks were believed to be Early Cambrian so, while the discovery sparked some interest, little serious attention was garnered. It was not until the British discovery of the iconic ''Charnia'' in 1957 that the pre-Cambrian was seriously considered as containing life. This frond-shaped fossil was found in England's Charnwood Forest,〔(【引用サイトリンク】title=Leicester’s fossil celebrity: Charnia and the evolution of early life )〕 and due to the detailed geological mapping of the British Geological Survey there was no doubt these fossils sat in Precambrian rocks. Palaeontologist Martin Glaessner finally, in 1959, made the connection between this and the earlier finds and with a combination of improved dating of existing specimens and an injection of vigour into the search many more instances were recognised. All specimens discovered until 1967 were in coarse-grained sandstone that prevented preservation of fine details, making interpretation difficult. S.B. Misra's discovery of fossiliferous ash-beds at the Mistaken Point assemblage in Newfoundland changed all this as the delicate detail preserved by the fine ash allowed the description of features that were previously undiscernible. Poor communication, combined with the difficulty in correlating globally distinct formations, led to a plethora of different names for the biota. In 1960 the French name "Ediacarien" – after the Ediacaran Hills in South Australia, which take their name from aborigine ''Idiyakra'', "water is present" – was added to the competing terms "Sinian" and "Vendian" for terminal-Precambrian rocks, and these names were also applied to the life-forms. "Ediacaran" and "Ediacarian" were subsequently applied to the epoch or period of geological time and its corresponding rocks. In March 2004, the International Union of Geological Sciences ended the inconsistency by formally naming the terminal period of the Neoproterozoic after the Australian locality.〔Reprint, 2004 original available here () (PDF).〕 The term "Ediacaran biota" and similar ("Ediacara"/"Ediacaran"/"Ediacarian"/"Vendian", "fauna"/"biota") has, at various times, been used in a geographic, stratigraphic, taphonomic, or biological sense, with the latter the most common in huge modern literature.〔MacGhabhann, 2014, Geosciences Frontiers, 5: 53–62〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ediacaran biota」の詳細全文を読む スポンサード リンク
|