Urbilaterian

clade|label1=Urbilaterian? ↓|1= clade|label2=Urbilaterian? ↓|1=Acoelomorphs|2= clade|label1=Deuterostomes|1=clade|1=Lophotrochozoa|2=Ecdysozoa|2=Protostomes The urbilaterian is the hypothetical last common ancestor of the bilaterian clade, which is generally considered to consist of all animals with bilateral symmetry.Verify source|date=September 2008

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 it would display largely depends on whether the acoelomorpha are included within the bilaterian clade. 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 Ma|570), and the first fully accepted fossil of a bilaterian "organism" is "Kimberella", dating to Ma|555. [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".] Interestingly, Fossil embryos are known from around the time of "Vernanimalcula" (Ma|580), 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.cite journal
author = Gostling, Neil J.
coauthors = Thomas, Ceri-Wyn; Greenwood, Jenny M.; Dong, Xiping; Bengtson, Stefan; Raff, Elizabeth C.; Raff, Rudolf A.; Degnan, Bernard M.; Stampanoni, Marco; Donoghue, Philip C. J.
year = 2008
title = Deciphering the fossil record of early bilaterian embryonic development in light of experimental taphonomy
journal = Evolution & Development
volume = 3
issue = 11
pages = 339–349
doi = 10.1111/j.1525-142X.2008.00242.x
month = June]

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.citation
author = Budd, G.E.; Jensen, S.
year = 2000
title = A critical reappraisal of the fossil record of the bilaterian phyla
journal = Biological Reviews
volume = 75
issue = 02
pages = 253–295
doi = 10.1017/S000632310000548X
url = http://journals.cambridge.org/production/action/cjoGetFulltext?fulltextid=624]

Characteristics of the urbilaterian

The urbilaterian must have possessed all those traits common to living bilateria that are thought only to have evolved 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.citation
author = Hejnol, A.; Martindale, M.Q.
year = 2008
title = Acoel development supports a simple planula-like urbilaterian
journal = Philosophical Transactions of the Royal Society B: Biological Sciences
volume = 363
issue = 1496
pages = 1493–1501
doi = 10.1098/rstb.2007.2239
url = http://journals.royalsociety.org/index/Q26K76211324T244.pdf]

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.

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 can only have evolved 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.citation
author = Telford, M.J.; Budd, G.E.
year = 2003
title = The place of phylogeny and cladistics in Evo-Devo research
journal = International Journal of Developmental Biology
volume = 47
issue = 7/8
pages = 479–490
url = http://www.ijdb.ehu.es/web/contents.php?vol=47
publisher = Univeristy of the Basque Country Press; 1999]

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 only have had 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).

References