Coelacanthiformes

(vertebrate zoology) An order of lobefin fishes in the subclass Crossopterygii which were common fresh-water animals of the Carboniferous and Permian; one genus, Latimeria, exists today.

An order of lobefin fishes placed in the subclass Crossopterygii and well known as fossils. Members are easily recognized by the two dorsal fins, by the paired pectoral and pelvic fins, and the anal fin, and by their symmetrical caudal fin with small central prolongation (see illustration). The only living fish with such features is Latimeria chalumnae; it is also the only extant fish with an intracranial joint, which otherwise occurs only in fossil rhipidistians. In 1952 its habitat was discovered to be the deep waters around the Comoro Islands. A new population of Latimeria was discovered in 1998 north of Sulawesi in the Indonesian archipelago.

Overall shape and internal structures of Latimeria chalumnae.

The Coelacanthiformes (or Actinistia) are the only fishes with a special rostral organ, a deep postcoronoid in the lower jaw, a tandem double articulation of the lower jaw, a postspiracular bone, and an additional bone, the extracleithrum, in the shoulder girdle. Like lungfish, coelacanths lack the marginal upper jaw bone, the maxilla, and possess a short dentary. Except for two Devonian (Miguashaia and Gavinia with heterocercal tail) and a Carboniferous (Allenypterus with diphycercal tail) genus, coelacanths have a caudal fin with equal-sized upper and lower lobe of unbranched fin rays separated by an axial notochordal lobe (triphycercal tail).

Latimeria shows sexual dimorphism; the female is longer than the male. Reproduction is by internal fertilization even though the male has no intermittent organ. Latimeria has the largest eggs of any bony fish. It is ovoviviparous; up to 30 developing juveniles have been found in one female.

Coelacanths are known since the Middle Devonian. They reach high diversity in the Early Triassic and Late Jurassic. Coelacanths acquired their common structure in the Carboniferous. The number of morphological changes is minor thereafter. There is no fossil record for the last 80 million years. Most fossil records refer to marine forms; nevertheless, some coelacanths were able to enter fresh water. See also Fossil.

The phylogenetic position of the coelacanthiforms within sarcopterygian fishes is debated. Within extant fishes, they may be closer to tetrapods than to the lungfish, or vice versa. The living coelacanth is not, as often written, a survivor of human ancestry. See also Crossopterygii; Osteichthyes; Sarcopterygii.