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A chordate (/ ˈ k ɔːr d eɪ t / KOR-dayt) is a deuterostomal bilaterian animal belonging to the phylum Chordata (/ k ɔːr ˈ d eɪ t ə / kor-DAY-tə).All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa.
This article contains a list of all of the classes and orders that are located in the phylum Chordata. The subphyla Tunicata and Vertebrata are in the unranked Olfactores clade, while the subphylum Cephalochordata is not. Animals in Olfactores are characterized as having a more advanced olfactory system than animals not in it.
Another feature present in both the Hemichordata and Chordata is pharyngotremy — the presence of spiracles or gill slits into the pharynx, which is also found in some primitive fossil echinoderms . [17] [18] A hollow nerve cord is found in all chordates, including tunicates (in the larval stage). Some hemichordates also have a tubular nerve cord.
Olfactores represent the overwhelming majority of the phylum Chordata, as the Cephalochordata are the only chordates not included in the clade. This clade is defined by a more advanced olfactory system which, in the immediate vertebrate generation, gave rise to nostrils .
Chordate genomics is the study of the evolution of the chordate clade based on a comparison of the genomes of several species within the clade. The field depends on whole genome data (the entire DNA sequence) of organisms.
The idea of a tree of life arose from ancient notions of a ladder-like progression from lower into higher forms of life (such as in the Great Chain of Being).Early representations of "branching" phylogenetic trees include a "paleontological chart" showing the geological relationships among plants and animals in the book Elementary Geology, by Edward Hitchcock (first edition: 1840).
Below is a phylogenetic tree of the phylum Chordata. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some groups in this tree (lancelets and tunicates) are invertebrates. The positions (relationships) of the lancelet, tunicate, and craniate clades are as reported.
The calcichordate family tree in its original incarnation, with all chordates originating from a soft bodied ancestor. Later versions of the theory had each chordate group having a mitrate ancestor each, requiring three episodes of loss of stereom. Mitrates are assumed to have evolved from the cornutes. [6]