Vertebrates

A **chordate** is an animal constituting the phylum **Chordata**. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail: these five anatomical features define this phylum. Chordates are also bilaterally symmetric; and have a coelom, metameric segmentation, and a circulatory system.

The Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata (fish, amphibians, reptiles, birds, and mammals); Tunicata (salps and sea squirts); and Cephalochordata (which includes lancelets). There are also extinct taxa such as the Vetulicolia. Hemichordata (which includes the acorn worms) has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Osteichthyes.

Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Cladistically (phylogenetically), vertebrates – chordates with the notochord replaced by a vertebral column during development – are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull. The Craniata and Tunicata compose the clade Olfactores. (See diagram under Phylogeny.)

**Vertebrates** comprise all species of animals within the subphylum **Vertebrata** (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described. Vertebrates include such groups as the following:

* jawless fishes
* jawed vertebrates, which include the cartilaginous fishes (sharks, rays, and ratfish)
* tetrapods, which include amphibians, reptiles, birds and mammals
* bony fishes

Extant vertebrates range in size from the frog species *Paedophryne amauensis*, at as little as 7.7 mm (0.30 in), to the blue whale, at up to 33 m (108 ft). Vertebrates make up less than five percent of all described animal species; the rest are invertebrates, which lack vertebral columns.

The vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution, though their closest living relatives, the lampreys, do. Hagfish do, however, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology.

Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys, and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of craniata.

## Urbilaterian

From Wikipedia: Urbilaterian

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. Two reconstructed urbilaterian morphologies can be considered, first the less complex ancestral form forming the common ancestor to Xenacoelomorpha and Nephrozoa, and the second, more complex (coelomate) urbilaterian ancestral to both Protostomes and Deuterostomes, sometimes referred to as the "urnephrozoan". 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.

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 an implausibly complex urbilaterian.