Crossopterygii

Crossopterygii [krä‚säp·tə′rij·ē‚ī]

(paleontology)

A subclass of the class Osteichthyes comprising the extinct lobefins or choanate fishes and represented by one extant species; distinguished by two separate dorsal fins.

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Crossopterygii

An infraclass of the bony fishes (class Osteichthyes), also known as fringe-finned fishes, that forms one of the two major divisions of the lobe-finned fishes (Sarcopterygii). The group first appeared as fossils in the Early Devonian; in the Paleozoic they were mostly small to medium-sized carnivorous fish living in shallow tropical seas, estuaries, and fresh waters. There were two principal groups: a diverse set of fishes termed Rhipidista and the Coelacanthini. Their principal radiations were in the Devonian, and by the Mississippian they were in sharp decline. The Rhipidista were wholly extinct by the Middle Permian, but the coelacanths underwent a second, smaller, Mesozoic radiation and managed to survive to the present day as the most famous lobe-fin of all, the living species Latimeria chalumnae. Crossopterygii are characterized by a unique hinge in the skull that allowed the front portion to be raised and lowered during feeding and respiratory movements.

Members of the order Rhipidistia were principally fusiform, fast-swimming carnivores that flourished in the rivers and lakes of the Late Devonian. They could breathe air, and the use of lungs as well as gills gave them an advantage in warm, shallow-water environments where dissolved oxygen was often low. Members of the order Coelacanthini are characterized by a special trifid tail and scales ornamented with tubercles. They are not thought to be close to the ancestors of tetrapods.