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Coelacanthiformes

Article by: Schultze, Hans-Peter Museum of Natural History, University of Kansas, Lawrence, Kansas. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097-8542.757501 (http://dx.doi.org/10.1036/1097-8542.757501)

Content

•Anatomy • Reproduction • Fossil record • Relationship • Bibliography • Additional Readings

An order of lobefin fishes placed in the subclass Coelacanthimorpha and well known as fossils. Coelacanthiformes and the fossil suborders Rhizodontimorphi and Osteolepimorpha (rhipidistians) were once grouped as the subclass Crossopterygii. However, since it is now believed this assemblage is artificial, the coelacanth fraction was elevated to the subclass Coelacanthimorpha, with one order (Coelacanthiformes), four fossil families, and one extant family (Coelacanthidae).

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 (Fig. 1). 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.

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Fig. 1 Overall shape and internal structures of Latimeria chalumnae.

Anatomy

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).

On the snout are the three paired openings of the rostral organ in addition to the two paired openings of the nasal sacs (Fig. 2). The rostral organ has a central chamber from which three tubes diverge to both sides. It is an electroreceptive organ, unique in its shape and concentration of electroreceptors among fishes.

Fig. 2 Snout of Latimeria.

The lateral side of the lower jaw is formed by angular and splenial bones; a short toothed dentary lies above the splenial bone. The coronoid bones on the dorsal margin of the lower jaw and the prearticular on the inner side of the jaw carry small pointed teeth. The deep postcoronoid reaches high above the dorsal margin of the lower jaw and does not carry teeth. The posterior end of the lower jaw forms a double groove for the articulation (joint) of the double condyle (the rounded process at the end of a bone) of the quadrate and another groove for the symplectic bone (Fig. 3).

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Fig. 3 Lower jaw of Latimeria with external bones.

The posterior moiety (otico-occipital region) of the endocranium carries the brain with labyrinth (the hollow in the temporal bone); a long extension of the pituitary gland (pituitary foot) surpasses the articulation between posterior and anterior moieties and reaches to the base of the anterior. Below the brain cavity the notochord extends in a tube to the posterior end of the anterior moiety. A paired basicranial muscle connects ventrolaterally the anterior with the posterior moiety. The basicranial muscle together with jaw and gill arch muscles acts in the movement of the anterior moiety against the posterior (lifting the snout) and in the opening of the mouth (Fig. 4).

Fig. 4 Endocranium of Latimeria.

The shoulder girdle is composed of anocleithrum, cleithrum, clavicle, and extracleithrum (Fig. 5). An unpaired interclavicle unknown in Latimeria is known from at least two fossil forms. The endoskeletal scapulocoracoid forms a knob on which the first mesomer of the axial endoskeletal elements of the pectoral fin articulates. Superficial muscles reach from the scapulocoracoid to the distal end of the axial skeleton and the bases of the fin rays; muscles connect internally the axial elements. This fleshy lobe is covered by small scales. The pelvic, second dorsal, and anal fins show the same structure. The first dorsal fin lacks axial elements and the fleshy lobe. The fin rays are articulated but not branched except for one Devonian genus. The exposed surface of the round scales is covered by ridges or tubercles converging toward the median posterior tip of the scales. The vertebral column is formed by an unrestricted notochord with neural arches and spines dorsally and with hemal arches and spines ventrally in the caudal region; ribs occur in some fossil forms. The notochordal fluid is considered by some Far Eastern people to be a life-prolonging elixir. The lung of Latimeria is an elongated oil organ well adapted to a deep-water existence. In contrast, fossil coelacanths were shallow-water dwellers with a much larger ossified lung. The intestine is formed as a scroll or longitudinal valve. The rectal gland is a salt-excreting organ which developed in parallel to that of cartilaginous fishes and serves for osmoregulation with high retention of urea. Coelacanths are carnivorous; fish are known to be stomach content in the extant Latimeria and in fossil forms.

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Fig. 5 Shoulder girdle of Latimeria.

Reproduction

Latimeria shows sexual dimorphism; the female is longer (measuring up to 1.8 m or 6 ft) than the male (up to 1.4 m or 5 ft). Reproduction is by internal fertilization even though the male has no intermittent organ. Latimeria has the largest eggs (9 cm or 3.6 in. in diameter) of any bony fish. It is ovoviviparous; up to 30 developing juveniles have been found in one female. Ovovivipary occurs also in Mesozoic coelacanths. Yolk-sac-carrying juveniles of comparatively small Paleozoic coelacanths have been found fossilized; these finds, in addition to the occurrence of unhatched specimens, indicate egg-laying early coelacanths.

Fossil record

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. The last known fossil coelacanths from the Upper Cretaceous of North America are also the largest known (estimated length of 3.5 m or 11 ft for Megalocoelacanthus). Fossil coelacanths are distributed worldwide; Paleozoic forms are known exclusively from the Northern Hemisphere, whereas Mesozoic forms have been found also in southern continents. Most fossil records refer to marine forms; nevertheless, some coelacanths were able to enter freshwater. See also: Fossil (/content/fossil/270100)

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Relationship

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. Even the possibility that both coelacanthiforms and lungfish are more closely related to each other than each is to tetrapods is postulated. In any case, both are not close relatives of tetrapods; the latter are related to rhipidistian fishes, especially Elpistostegalia. The living coelacanth is not, as often written, a survivor of human ancestry. See also: Osteichthyes (/content/osteichthyes/478500); Sarcopterygii (/content/sarcopterygii/601800)

Hans-Peter Schultze

Bibliography

P. L. Forey, History of the Coelacanth Fishes, Chapman & Hall, London, 1998

J. A. Musick, M. N. Bruton, and E. K. Balon (eds.), The Biology of Latimeria chalumnae and Evolution of Coelacanths, vol. 32 of Environmental Biology of Fishes, Kluwer Academic, Dordrecht, the Netherlands, 1991

J. S. Nelson, Fishes of the World, 4th ed., Wiley, New York, 2006

Additional Readings

C. R. A. Candeiro et al., Continental fossil vertebrates from the mid-Cretaceous (Albian–Cenomanian) Alcântara Formation, Brazil, and their relationship with contemporaneous faunas from North Africa, J. Afr. Earth Sci., 60(3):79–92, 2011 DOI: 10.1016/j.jafrearsci.2011.02.004 (http://dx.doi.org/10.1016/j.jafrearsci.2011.02.004)

G. Helfman et al. (eds.), The Diversity of Fishes: Biology, Evolution, and Ecology, 2d ed., John Wiley & Sons, Hoboken, NJ, 2009

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