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.

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Crossopterygii 

a group of bony fishes.

The Crossopterygii typically have paired fins, which serve for support against the floor of the body of water and consist of muscular paddles with a skeletal axis of several penicillate segments resembling the bones of the extremities of quadrupeds. There are two short dorsal fins. Many Crossopterygii have a small third lobe of caudal fin, into which the body axis continues. The inner cranium is subdivided into two parts, anterior and posterior, which are more or less movable in relation to one another. The brain occupies only a small part of the cranial cavity (in Latimeria, 1/100 of the volume). The vertebrae are usually in the shape of a ring or half-ring, but the notochord is preserved and reaches the posterior end of the anterior part of the cranium. The body of extinct forms was as long as 5 m. All members of the group are predators.

There are three orders of Crossopterygii. The first group— Osteolepidiformes, or Rhipidistia—comprises predominantly freshwater fishes that appeared during the early Devonian and became extinct at the beginning of the Permian. The Osteolepidiformes had internal nares, or choanae, which, along with the paired fins, made it possible for some of the group to emerge onto dry land (becoming the forerunners of the quadrupeds). Representatives of the order Coelacanthiformes, or Actinistia, appeared during the middle Devonian; they lived in fresh waters during the Paleozoic but moved to the sea at the beginning of the Mesozoic. They are now represented by a single genus (Latimeria), members of which live off the shores of the Comoro Islands in the Indian Ocean. The Coelacanthiformes have no choanae. The order Struniiformes was discovered only recently from marine deposits of the Devonian. Along with characteristics of the Crossopterygii they have features resembling the Actinopterygii (a subclass that includes superorder Cladistia, once considered to be part of Crossopterygii).

REFERENCES

Osnovy paleontologii: Bescheliustnye, ryby. Moscow, 1964.
Nikol’skii, G. V. Chastnaia ikhtiologiia, 3rd ed. Moscow, 1971.

D. V. OBRUCHEV