CROSSOPTERYGIAN

any member of the subclass Crossopterygii, a group of lobe-finned, bony fishes believed to have given rise to the amphibians and all other land vertebrates. The only living representative is the coelacanth (Latimeria chalumnae). The major characteristic of crossopterygians is that the skull is divided into anterior and posterior units. These two chambers are strongly joined at the sides. A notochord is present in varying degrees in the skull base and in the vertebral column, where ossification is not complete. The rostral organ, a specialized sensory organ, is located in the snout. The heart of the coelacanth is bilaterally symmetrical (the two sides are mirror images) and quite primitive; it is located within a pericardial cavity that is not separated from the peritoneal cavity. Blood exits the heart through a series of small valves and is pushed through the branchial arteries by contractile organs located along these vessels. Lying dorsal to the abdominal organs is a large fat deposit, probably the degenerative remnant of an ancestral lung. Large roughened scales cover the exterior of these fish. The tail is strong and divided into three lobes. In this group the paired fins (pectoral and pelvic) are connected to their respective girdles and are carried on fleshy stalks with bone or cartilage support structures. The posterior dorsal fin and the anal fin are structurally of this type also, but the anterior dorsal fin has bony rays and lacks the fleshy stalk. There are three orders of crossopterygians: Rhipidistia, Actinistia, and Struniiformes. The rhipidistians were ancestral to land vertebrates. In addition to a gill system, they had a lung system and a pair of nostrils that enabled them to breathe air; the paired fins were also capable of land locomotion. The Actinistia have shown almost no change through time; the abundant forms of the Devonian Period (408 million years ago) closely resemble the modern coelacanth. The Struniiformes are quite different structurally from the other crossopterygians but retain the division of the skull into two units. any member of the subclass Crossopterygii, a group of primitive, lobe-finned, bony fishes believed to have given rise to the amphibians and all other land vertebrates. They appeared at the beginning of the Devonian Period (395,000,000 years ago) but are now represented only by the coelacanth (Latimeria chalumnae). Additional reading There is virtually no popular literature dealing with this group of fishes. The following are technical in nature: E. Jarvik, On the Structure of the Snout of Crossopterygians and Lower Gnathostomes in General, Zool. Bidr. Upps., 21:235675 (1942); J.P. Lehman, Crossopterygii, in Trait de palontologie IV, 3:301412 (1966); J. Millot, and J. Anthony, Latimeria chalumnae, dernier des Crossoptrygiens, in Trait de zoologie, vol. 13, pp. 25532597 (1958); Anatomie de Latimeria chalumnae, 3 vol. in 4 (195878); J.L.B. Smith, A Living Coelacanthid Fish from South Africa, Trans. R. Soc. S. Afr., 28:1106 (1940); K.S. Thomson, The Comparative Anatomy of the Snout in Rhipidistian Fishes, Bull. Mus. Comp. Zool. Harv., 131:313357 (1964).