What is "urbilaterian"

n. (context biology English) The hypothetical last common ancestor to all bilaterians.

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 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.