|Phylogenetic position of the urbilaterian|
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.
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 . 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 ( ), 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.
Characteristics of the urbilaterian
The urbilaterian must have possessed all those traits common to living bilateria that are thought to have evolved only once. The defining features are a three-layered (triploblastic) embryo and two main axes of symmetry — a top-to-bottom axis and a front-to-back axis.
A traditional approach to reconstructing the urbilaterian considers it as a "roundish flatworm" consisting of as few features as possible. This concept of a very simple animal was abandoned when it was realised that the vast array of traits common to bilaterians would produce quite a complex animal, but has had a resurgence with the inclusion of a phylum of flatworms within the bilateria.
The urbilaterian is often considered to have possessed a gut and internal organs, a segmented body and a centralised nervous system, as well as a biphasic life cycle (i.e. consisting of larvae and adults) and some features of embryonic development. However, this need not necessarily be the case. If these features were of key importance to the evolution of large size, it is possible that bilaterian lineages each stumbled upon them independently, and perhaps even co-opted the same underlying genetic machinery from a different role.
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.
Complex or simple?
Proponents of a complex urbilaterian point to the shared features and genetic machinery common to all bilateria. They argue that (1) since these are similar in so many respects, they could have evolved only once; and (2) since they are common to all bilateria, they must have been present in the ancestral bilaterian animal.
However, as biologists' understanding of the major bilaterian lineages increases, it is beginning to appear that some of these features may have evolved independently in each lineage. Further, the bilaterian clade has recently been expanded to include the acoelomorphs — a group of relatively simple flatworms. This lineage lacks key bilaterian features, and if it truly does reside within the bilaterian "family", many of the features listed above are no longer common to all bilateria. Instead, some features — such as segmentation and possession of a heart — are restricted to a sub-set of the bilateria, the deuterostomes and protostomes. Their last common ancestor would still have to be large and complex, but the bilaterian ancestor could be much simpler. However, some scientists stop short of including the acoelomorph clade in the bilateria. This shifts the position of the cladistic node which is being discussed; consequently the urbilaterian in this context is farther out the evolutionary tree and is more derived than the common ancestor of deuterostomes, protostomes and acoelomorphs.
Genetic reconstructions are unfortunately not much help. They work by considering the genes common to all bilateria, but problems arise because very similar genes can be co-opted for different roles. For instance, the gene Pax6 has a key role in eye development, but is absent in some animals with eyes; some cnidaria have genes which in bilateria control the development of a layer of cells that the cnidaria don't even possess. This means that even if a gene can be identified as present in the urbilaterian, we can't tell what the gene was coding for. Before this was realised, genetic reconstructions implied a surprisingly complex urbilateria.
Reconstructing the urbilaterian
The absence of a fossil record gives a starting point for the reconstruction — the urbilaterian must have been small enough not to leave any traces as it moved over or lived in the sediment surface. This means it must have been well below a centimetre in length. As all Cambrian animals are marine, it is reasonable to assume that the urbilaterian was too.
Furthermore, a reconstruction of the urbilateria must rest on identifying morphological similarities between all bilateria. While some bilateria live attached to a substrate, this appears to be a secondary adaptation, and the urbilaterian was probably mobile. Its nervous system was probably dispersed, but with a small central "brain". Since acoelomorphs lack a heart, coelom or organs, the urbilaterian probably did too — it would presumably have been small enough for diffusion to do the job of transporting compounds through the body. A small, narrow gut was probably present, which would have had only one opening — a combined mouth and anus. Functional considerations suggest that the surface of the bilaterian was probably covered with cilia, which it could have used for locomotion or feeding.
There is still no consensus on whether the characteristics of the deuterostomes and protostomes evolved once or many times. Features such as a heart and blood circulation system may therefore not have been present even in the deuterostome-protostome ancestor, which would mean that this too could have been small (hence explaining the lack of fossil record).
- Further details are available at Ediacara biota
- Further details are available at Vernanimalcula.
- Further details are available at Fossil embryos.
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