|
}} }} }} The urbilaterian (< German ur- 'original') is the hypothetical last common ancestor of the bilaterian clade, i.e., all animals having a bilateral symmetry. Its appearance is a matter of debate, for no representative has been (or is ever likely to be) identified in the fossil record; the reconstructed morphology that it would display largely depends on whether the bilaterian clade is defined as including the acoelomorpha or not. Since all protostomes and deuterostomes share features, such as blood circulation systems and guts, that are only useful in relatively large (macroscopic) organisms, their common ancestor ought also to have been macroscopic. However, such large animals should have left traces in the sediment in which they moved, and evidence of such traces first appear relatively late in the fossil record — long after the urbilaterian would have lived. This leads to suggestions of a small urbilaterian, which is the supposed state of the ancestor of protostomes, deuterostomes and acoelomorphs. ==Dating the urbilaterian== The first evidence of bilateria in the fossil record comes from trace fossils in sediments towards the end of the Ediacaran period (about ), and the first fully accepted fossil of a bilaterian ''organism'' is ''Kimberella'', dating to .〔Further details are available at ''Ediacara biota''〕 There are earlier, controversial fossils: ''Vernanimalcula'' has been interpreted as a bilaterian, but may simply represent a fortuitously infilled bubble.〔Further details are available at ''Vernanimalcula''.〕 Fossil embryos are known from around the time of ''Vernanimalcula'' (), but none of these have bilaterian affinities.〔Further details are available at Fossil embryos.〕 This may reflect a genuine absence of bilateria, but caution is due — it could be that bilateria didn't lay eggs in sediment, where they would be likely to fossilise. Molecular techniques can generate expected dates of the divergence between the bilaterian clades, and thus an assessment of when the urbilaterian lived. These dates have huge margins of error, though they are becoming more accurate with time. More recent estimates are compatible with an Ediacaran bilaterian, although it is possible, especially if early bilaterians were small, that the bilateria had a long cryptic history before they left any evidence in the fossil record. 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「urbilaterian」の詳細全文を読む スポンサード リンク
|