Coelacanth

Coelacanth is the common name for an order of fish that includes the oldest living lineage of gnathostomata known to date. The coelacanths, which are related to lungfishes and tetrapods, were believed to have been extinct since the end of the Cretaceous period, until the first Latimeria specimen was found off the east coast of South Africa, off the Chalumna River in 1938. They are, therefore, a Lazarus taxon.

Natural history
They first appeared in the fossil record in the Middle Devonian. Prehistoric species of coelacanth lived in many bodies of water in Late Paleozoic and Mesozoic times.

Coelacanths are lobe-finned fish with the pectoral and anal fins on fleshy stalks supported by bones, and the tail or caudal fin diphycercal (divided into three lobes), the middle one of which also includes a continuation of the notochord. Coelacanths have modified cosmoid scales, which are thinner than true cosmoid scales. Coelacanths also have a special electroreceptive device called a rostral organ in the front of the skull, which probably helps in prey detection. The small device also could help the balance of the fish, as echolocation could be a factor in the way this fish moves.

Fossil record
Although now represented by only two known living species, as a group the coelacanths were once very successful with many genera and species that left an abundant fossil record from the Devonian to the end of the Cretaceous period, at which point they apparently suffered a nearly complete extinction. It is often claimed that the coelacanth has remained unchanged for millions of years, but, in fact, the living species and even genus are unknown from the fossil record. However, some of the extinct species, particularly those of the last known fossil coelacanth, the Cretaceous genus Macropoma, closely resemble the living species. The most likely reason for the gap is the taxon having become extinct in shallow waters. Deep-water fossils are only rarely lifted to levels where paleontologists can recover them, making most deep-water taxa disappear from the fossil record. This situation is still under investigation by scientists.

Taxonomy


Subclass Coelacanthimorpha (Actinistia) are sometimes used to designate the group of Sarcopterygian fish that contains the Coelacanthiformes. The following is a classification of known coelacanth genera and families:

Class Sarcopterygii

Subclass Coelacanthimorpha


 * Order COELACANTHIFORMES
 * Family Coelacanthidae (extinct)
 * Axelia (extinct)
 * Coelacanthus (extinct)
 * Ticinepomis (extinct)
 * Wimania (extinct)
 * Family Diplocercidae (extinct)
 * Diplocercides (extinct)
 * Family Hadronectoridae (extinct)
 * Allenypterus (extinct)
 * Hadronector (extinct)
 * Polyosteorhynchus (extinct)
 * Family Mawsoniidae (extinct)
 * Alcoveria (extinct)
 * Axelrodichthys (extinct)
 * Chinlea (extinct)
 * Diplurus (extinct)
 * Mawsonia (extinct)
 * Family Miguashaiidae (extinct)
 * Miguashaia (extinct)
 * Family Latimeriidae
 * Holophagus (extinct)
 * Libys (extinct)
 * Macropoma (extinct)
 * Macropomoides (extinct)
 * Megacoelacanthus (extinct)
 * Latimeria (James Leonard Brierley Smith, 1939)
 * L. chalumnae (Comorese coelacanth) (James Leonard Brierley Smith, 1939)
 * L. menadoensis (Indonesian coelacanth) (Pouyaud, Wirjoatmodjo, Rachmatika, Tjakrawidjaja, et al., 1999)
 * Undina (extinct)
 * Family Laugiidae (extinct)
 * Coccoderma (extinct)
 * Laugia (extinct)
 * Family Rhabdodermatidae (extinct)
 * Caridosuctor (extinct)
 * Rhabdoderma (extinct)
 * Family Whiteiidae (extinct)
 * Whiteia (extinct)