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Coelacanth ( , adaptation of Modern Latin Cœlacanthus: cœl-us + acanth-us from Greek κοῖλ-ος [hollow] + ἄκανθ-α [spine]), which is a reference to the hollow spines of the fins, 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. They were considered the "missing link" between the fish and the tetrapods until the first Latimeria specimen was found off the east coast of South Africa, off the Chalumna Rivermarker in 1938. They are, therefore, a Lazarus taxon. Since 1938, Latimeria chalumnae have been found in the Comorosmarker, Kenyamarker, Tanzania, Mozambiquemarker, Madagascarmarker, and in iSimangaliso Wetland Parkmarker, Kwazulu-Natal in South Africa. The second extant species, L. menadoensis, was described from Sulawesimarker, Indonesiamarker in 1999 by Pouyaud et al. based on a specimen discovered by Erdmann in 1998 and deposited in Indonesian Institute of Sciences (LIPI). The first specimen of this species was only photographed at a local market by Arnaz and Mark Erdmann before being bought by a shopper. The coelacanth has no real commercial value, apart from being coveted by museums and private collectors. As a food fish the coelacanth is almost worthless as its tissues exude oils even when dead, imparting the flesh with a foul flavor.

Natural history

Fossil of Axelrodichthys araripensis, an extinct coelacanthiform
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.

Description

This fish has some unique traits among vertebrates:

  • The kidneys are fused into a single organ, which is located on the floor of the abdomen instead of just under the backbone.
  • A trilobate (three-lobed) caudal fin, with upper and lower halves separated by a small secondary tail.
  • A tiny heart that is more-or-less a straight tube
  • A brain that occupies only 1.5% of the braincase, the rest of the cavity is filled with fat.
  • The pectoral, pelvic and anal fins set on muscular lobes.


Fossil record

Undina penicillata
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. Before the living specimens were discovered, it was believed by evolutionists that the coelacanth was a "missing link" between the fish and the tetrapods. 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.

Taxonomy

Latimeria menadoensis, Tokyo Sea Life Park (Kasai Rinkai Suizokuen), Japan
[[Image:Fishapods.jpg|thumb|right|350px|In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish – like Eusthenopteron – exhibited a sequence of adaptations:
 
  • Acanthostega which had feet with eight digits,
  • 
     
    
  • Ichthyostega with limbs. Descendants also included pelagic lobe-finned fish such as coelacanth species.]]


    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



    See also



    References

    1. Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
    2. A fossil coelacanth jaw found in a stratum datable 410 mya that was collected near Buchan in Victoria, Australia's East Gippsland, currently holds the record for oldest coelacanth; it was given the name Eoactinistia foreyi when it was published in September 2006. [1]
    3. The Coelacanth - a Morphological Mixed Bag
    4. Nelson, Joseph S. (2006). Fishes of the World. John Wiley & Sons, Inc. ISBN 0471250317


    External links




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