Chordate
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A chordate is an . any chordates possess five synapomorphies, or primary characteristics, at some detail during their larval or adulthood stages that distinguish them from any other taxa. These five synapomorphies include the notochord, dorsal hollow nerve cord, endostyle or thyroid, pharyngeal slits, in addition to a post-anal tail. The clear “chordate” comes from the first of these synapomorphies, a notochord, which plays a significant role in chordate design and movement. Chordates are also bilaterally symmetric, gain a coelom, possess a circulatory system, and exhibit metameric segmentation.
In addition to the morphological characteristics used to define chordates, analysis of genome sequences has quoted two conserved signature indels CSIs in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalochordates. These CSIs give molecular means to reliably distinguish chordates from all other metazoan.
The Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordates are divided into three subphyla: Vertebrata fish, amphibians, reptiles, birds, and mammals; Tunicata or Urochordata sea squirts, salps; and Cephalochordata which includes lancelets. There are also extinct taxa such(a) as the Vetulicolia. Hemichordata which includes the acorn worms has been portrayed 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 well species of chordates, approximately half are ray-finned fishes that are members of the a collection of things sharing a common attaches Actinopterygii.
Chordate fossils have been found from as early as the Cambrian explosion, 539 million years ago. Cladistically phylogenetically, vertebrates – chordates with the notochord replaced by a vertebral column during coding – 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.