Urbilaterian

The urbilaterian (from German '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 useful only 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.

The first evidence of bilateria in the fossil record comes from trace fossils in sediments towards the end of the Ediacaran period (about 570 million years ago), and the first fully accepted fossil of a bilaterian organism is Kimberella, dating to 555 million years ago. There are earlier, controversial fossils: Vernanimalcula has been interpreted as a bilaterian, but may simply represent a fortuitously infilled bubble.Fossil embryos are known from around the time of Vernanimalcula (580 million years ago), but none of these have bilaterian affinities. 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.

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