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The urbilaterian (from German ur- 'original') is the hypothetical last common ancestor of the clade, i.e., all having a bilateral symmetry.
Molecular clock 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 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.
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 Cladistics which is being discussed; consequently the urbilaterian in this context is farther out the evolutionary tree and is more Synapomorphy 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 functions. For instance, the gene Pax6 has a function 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 do not have. This means that even if a gene can be identified as present in the urbilaterian, we cannot necessary tell what the gene's function was. Before this was realised, genetic reconstructions implied an implausibly complex urbilaterian.
The evolutionary developmental biologist Lewis Held notes that both and use the oscillating mechanism based on the Notch signaling pathway to produce segments from the growing tip at the rear of the embryo. Further, both groups make use of "the obtuse process of 'resegmentation', whereby the phase of their metameres shifts by half a unit of wavelength, i.e. somites splitting to make vertebrae or parasegments splitting to form segments." Held comments that all this makes it difficult to imagine that their urbilaterian common ancestor was not segmented.
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.
This proposal has little or no support in the existing data, and has been commonly used as a justification against the sedentary/semi-sedentary models of urbilaterians as a whole.
This proposes that the urbilaterian is an organism whose adult life is sessile-sedentary with a juvenile or free and pelagic larval phase. This hypothesis is a derivative of Claus Nielsen's larval hypothesis, but now also considering the homology of the adult forms of Choanozoa (except Ctenophora). It also considers various phylogenetic, paleontological and molecular data, relates the adult and ancestral form of anthozoans (from which Medusozoa, Placozoa, Nephrozoa, and perhaps Proarticulata are derived), in turn derived from an ancestral organization shared between Choanoflagellate, Sponge and ParaHoxozoa.
This argues that theories involving a mobile urbilaterian cause problems with palaeontological and morphological data in relation to groups within and outside Bilateria.
So members of Proarticulata are an evolutionary dead end rather than the ancestors of nephrozoans. It is possible that the Cloudinidae ( Cloudina, Conotubus and Multiconotubus) are basal (and therefore bilateral) nephrozoans, because they have considerable similarity with the Tubarium of sedentary Pterobranchia, as well as with the Exoskeleton of semi-mobile Hyolitha and mobile Mollusca, this taking into account the ontogeny of the cloudinids.
This implies that the Cloudinomorpha is not a polyphyletic group as would have been proposed but rather is a paraphyletic grade from which several taxa derive that may or may not conserve the ancestral clonality of basal metazoans, but instead of cloudinids having an annelid-type gut, it would instead be a U-shaped digestive tube, in fact the relationship between Cloudina and Annelid is denied.
The hypothesis of annelid-like ancestor is rejected, due to the independent evolution of segmentation and complete metamerism of several groups of bilaterians (Annelid, Panarthropoda, Chordate and Proarticulata); On the other hand, the urbilaterian would be an animal with a U-shaped gut, with Deuterostome that hemichordates and lophophorates among other groups conserve, a stolon that holds the organism inside a tube secreted from the embryonic form as a Graptolite or protoconch, a semi-metamerism derived from the formation of mesoderm from the gastrovascular cavity of an anthozoan-like animal.
The common ancestor of modern bilaterals would then be more similar to modern pterobranchs, although they would not be completely identical to them.
The location of Ctenophora (Myriazoa) should not change the hypothesis since it has been left aside taking only into account the molecular and morphological development of Choanoflagellatea, Porifera and Cnidaria.
Complexity
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.
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, one can reasonably assume that the urbilaterian was too.
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 a 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).
Possible models of the Urbilaterian
It is possible that the common ancestor of all bilaterals looked similar to:
Colonial-Pennatulacean hypothesis: (Colonialy fusion of cnidarian-like)
The proposal that bilaterals arose from the fusion between pennatulacean-like cnidarian zooids was granted by Dewel, implies that the body plans of bilaterals originated from a colonial ancestor.
Larval Hypothesis (Pelagic larvae and adult ancestor)
Panarticulata hypothesis: (Segmentated annelid-like ancestor)
Cloudinomorpha hypothesis: (Biphasic Sedentary sessile adult and Pelagic larvae)
The recent model by Alexander V. Martynov and Tatiana A. Korshunova revives the idea of a sessile sedentary biphasic ancestor.
See also
External links
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