Vertebrates are members of the Phylum Chordata in the animal kingdom. At some point in their life cycle, all chordates possess a notochord, a hollow neural tube dorsal to the notochord, and lateral muscle blocks. They are said to be the most highly evolved group of animals, at the same time the most complex and complicated.
But what separate the vertebrates from other chordates are the innovations including having neural crest cells and their derivative, elaboration of placode-derived structures, an elaborate segmented brain, cartilage, the axial and head skeleton, and an increase in the number of genes in the genome (Shimeld and Holland, 2000). Shimeld and Holland identified innovations present in the vertebrate characteristics not found in other chordates, as well as in other Phyla in their article “Vertebrate Innovations.
” They explained that the neural crest cells are a key feature to the vertebrate character which contributes to the formation of structures that are vertebrate novelties. These novelties, only found in the vertebrata subphylum, include the cranium, branchial skeleton, and sensory ganglia. The placodes, on the other hand, contribute to the formation of specialized structures of the vertebrate head including the eye, ear, lateral line and olfactory organs (sensory placodes), and the sensory neurons to the cranial ganglia (neurogenic placodes).
Shimeld and Holland (2000) explained that sensory placodes have homologues in basal chordates but it is “apparent they were elaborated considerably during early vertebrate evolution” and that neurogenic placodes are entirely novel in vertebrates. The vertebrate brain differs from any others in that it consists of specialized fore-, mid-, and hindbrain regions.
There are also no similar tissues in other chordates that are comparable to vertebrate cartilage and endoskeletal support tissues. Shimeld and Holland has identified that development in vertebrate structures not found in other chordates are attributed to changes in the regulation of specific genes such as Hox, Krox, FGF-8, Pax-1/9.
Shimeld, S. M. and Holland, P. W. H. (2000). Vertebrate innovations. Proceedings of the National Assembly of Sciences vol. 97, no. 9, 4449-4452.