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The Heterostraci, or heterostracans, is a large clade of the Pteraspidomorphi, i.e. a group of fossil, armored jawless vertebrates, which lived from the Early Silurian to the Late Devonian (about 430 to 370 million years ago). They generally possess a fusiform head armor and a fan-shaped tail. Some forms, however, can have a rather depressed armor, with broadly expanded branchial plates. Some text-books still include the Astraspida, Eriptychiida and Arandaspida in the Heterostraci, but heterostracans clearly differ from these groups in having a single, common branchial opening on each side.
Heterostracans are represented by nearly 300 species. They were marine but lived in sandy lagoons or deltas. Some species, however, are regarded as fresh water. They are known exclusively from North America, Europe and Siberia. They probably fed by scraping the bottom with their fan-shaped oral plates that armed their lower lip. They were poor swimmers and probably bottom-dewellers. It has been suggested that their posteriorly placed common gill opening could serve as a jet propulsion device and that made them moved like Cousteau's diving saucer.
Although most heterostracans are relatively small (5 to 30 cm in total length), some of them, the Psammosteidae, could grow to a very large size (up to 1.5 m in length) and developed steer-like branchial plates.