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Crossopterygii (class Osteichthyes) Subclass of bony fish comprising both fossil and living lobe-finned or tassel-finned fish, including the Coelacanthiformes and Rhipidistia. The former are well known from Palaeozoic and Mesozoic rocks, but were thought to have become extinct by the end of the Cretaceous until living specimens were netted this century in the Indian Ocean. The Rhipidistia did become extinct, although not before they gave rise, in the Devonian, to the amphibians. The Crossopterygii are characterized by the fact that all fins (except the tail-fin) are based on movable stalks or lobes. The tail fin is either heterocercal or diphycercal.
Sarcopterygii Subclass of fleshy-finned fish comprising two main groups (super-orders): the coelacanths (Crossopterygii), and the lungfish (Dipnoi). The Crossopterygii appeared in the Devonian, were widely distributed in the Mesozoic, and are now represented by a single genus, Latimeria. One order of the Crossopterygii, the Rhipidistia, is credited with being the root stock for more advanced vertebrates. The Dipnoi also appeared in the Devonian and are now represented by three species. They inhabit fresh water and have developed organs by which they can breathe atmospheric air.