Urbilaterian

AcoelomorphsLophotrochozoaEcdysozoaDeuterostomesThe urbilaterian (from German ur- 'original') is the hypothetical last common ancestor of the bilaterian clade, i.e., all animals having a bilateral symmetry. The urbilaterian (from 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. 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. 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. The urbilaterian (from 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. Light detection (photosensitivity) is present in organisms as simple as seaweeds; the definition of a true eye varies, but in general eyes must have directional sensitivity, and thus have screening pigments so only light from the target direction is detected. Thus defined, they need not consist of more than one photoreceptor cell. The presence of genetic machinery (the Pax6 and Six genes) common to eye formation in all bilaterians suggests that this machinery - and hence eyes - was present in the urbilaterian. The most likely candidate eye type is the simple pigment-cup eye, which is the most widespread among the bilateria. Since two types of opsin, the c-type and r-type, are found in all bilaterians, the urbilaterian must have possessed both types - although they may not have been found in a centralised eye, but used to synchronise the body clock to daily or lunar variations in lighting.