Crossopterygii is an infraclass (subclass) of Sarcopterygii fishes. An ancient group of bony fishes, up to 180 cm long. The axial skeleton is represented by chordae, vertebral bodies are absent. Paired fins in the fork of the blades are fleshy, covered with scales. Pectoral fins biserial type, i.e. with a central skeletal axis. The scales are cosmoidal. In the heart there is an arterial cone. In the intestine there is a spiral valve. There is a cloaca. Includes 1 order - Coelacanthiformes, 1 family - Latimeriidae, 1 genus - Latimeria with 1 species - L. chalumnae. Inhabits the Indian Ocean, in the Comoros Islands. Leads a benthic, semi-gluococeanic way of life. Egg-living fish. Predator.

     A peculiarity is the fins, at the base of which is a muscle blade. The only modern genus is Latimeria, in which two species are known. Appeared no later than 400 million years ago. Until the XX century it was believed that Crossopterygii became extinct 66 million years ago. The first specimen of Latimeria was caught in 1938 in the Indian Ocean off the southern coast of Africa from a depth of 70 meters and described by ichthyologist James Smith. A second specimen was caught on a rod from a depth of 15 m in the same area. By 1980, more than 70 latimeria had been caught. In 1998, a second species of the genus was discovered off the coast of Sulawesi. Originally, the killifish lived in shallow freshwater bodies of water, probably lacking oxygen. As a result, this line of fish developed musculature at the base of the fins (to allow movement with support on the substrate) and double respiration (including pulmonary respiration). Later, some representatives returned to the sea and the freshwater ones became extinct.

     A group of bony fishes. The paired fins serve for support on the bottom and are muscular blades with a skeletal axis consisting of several cyst-like branching segments, similar to the limb bones of quadrupedal vertebrates. Dorsal fins 2, they are short; many Crossopterygii have a third small caudal fin blade into which the body axis continues. The inner skull is divided into 2 parts - anterior and posterior, more or less mobile relative to each other. The brain occupies only a small part of the cranial cavity (in latimeria - 1/100 of the volume). Vertebral bodies are more often in the form of rings or semi-rings, but the chorda is preserved and reaches the posterior end of the anterior part of the skull. The body length of fossil Crossopterygii is up to 5 m. All Crossopterygii are predators. 3 orders. The first - Osteolepidiformes (Rhipidistia) - includes predominantly freshwater fishes known from the Early Devonian and extinct in the early Permian. They had internal nostrils - choanae, which, along with the device of paired fins, enabled some of them to go on land and become ancestors of four-legged vertebrates. Representatives of the order Coelacanthiformes (Actinistia) are known from the middle Devonian; during the Paleozoic lived in fresh waters, from the beginning of the Mesozoic moved to the sea. Now represented by one genus Latimeria, living in the Indian Ocean off the coast of the Comoros Islands. Do not have choanae. The recently discovered order Struniiformes is known only from marine sediments of the Devonian and along with features of Crossopterygii has features of similarity with Actinopterygii, which include Polyptera, previously classified as Crossopterygii.

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