Coelacanth

Etymology

The word Coelacanth is an adaptation of the Modern Latin Cœlacanthus ('hollow spine'), from the Ancient Greek κοῖλ-ος (grc, 'hollow') and ἄκανθ-α (grc, 'spine'), referring to the hollow caudal fin rays of the first fossil specimen described and named by Louis Agassiz in 1839, belonging to the genus Coelacanthus.

Discovery

The earliest fossils of coelacanths were discovered in the 19th century. Coelacanths were believed to have become extinct at the end of the Cretaceous period. More closely related to tetrapods than to the ray-finned fish, coelacanths were considered a transitional form between fish and tetrapods.

On 22 December 1938, the first Latimeria specimen was found off the east coast of South Africa, off the Chalumna River (now Tyolomnqa). Museum curator Marjorie Courtenay-Latimer discovered the fish among the catch of a local fisherman.

The Comoro Islands specimen was discovered in December 1952. Between 1938 and 1975, 84 specimens were caught and recorded.

The second extant species, the Indonesian coelacanth, was first recognized in Manado, North Sulawesi, Indonesia, by Mark V. Erdmann and his wife Arnaz Mehta at a local fish market in September 1997, but were only able to take a few photographs of the first specimen of this species before it was sold. After confirming that it was a unique discovery, Erdmann returned to Sulawesi in November 1997 to interview fishermen and look for further examples. A second specimen was caught by a fisherman in July 1998 and was then handed to Erdmann. The species was described in 1999 by Pouyaud et al. based on Erdmann's 1998 specimen and deposited at a facility of the Indonesian Institute of Sciences (LIPI).

Distribution

Prehistorically, Actinistians ranged throughout the world, being found in geological formations of Europe, the Americas, Australia, and Greenland.

Some species of Actinistians, especially the Mawsoniids, were found in deposits corresponding to brackish and even freshwater environments, suggesting an anadromous ability.

The two extant Latimeria species, the West Indian Ocean coelacanth and the Indonesian coelacanth, are restricted to a few locales within the Indo-Pacific and are named base on their range.

Description

Coelacanths are a part of Sarcopterygii or the lobe-finned fishes, the same clade as the lungfish and tetrapods, and they all possess lobed fins as opposed to rayed fins. Externally, several characteristics distinguish coelacanths from other lobe-finned fish: coelacanths have eight fins – two dorsal fins, two pectoral fins, two pelvic fins, one anal fin and one caudal fin. The tail is very nearly equally proportioned and is split by a terminal tuft of fin rays that make up its caudal lobe; this is alternatively termed a trilobate fin (three-lobed) or a diphycercal tail. A secondary tail extending past the primary tail separates the upper and lower halves of the coelacanth. Ctenoid elasmoid scales act as thick armor to protect the coelacanth's exterior. Several internal traits also aid in differentiating coelacanths from other lobe-finned fish. At the back of the skull, the coelacanth possesses a hinge, the intracranial joint, which allows it to open its mouth extremely wide. Coelacanths also retain an oil-filled notochord, a hollow, pressurized tube which is replaced by a vertebral column early in embryonic development in most other vertebrates. The body is covered in ctenoid elasmoid scales that act as armor.

The soft tissue of coelacanths is mostly known from Latimeria, the relictual extant genus.

Evolution and taxonomy

Coelacanths are members of the class Actinistia, with many researchers considering the term "coelacanth" to cover all members of Actinistia. The order Coelacanthiformes has been used for a subgroup of actinistians, containing the modern coelacanths, as well as other extinct closely related actinistians spanning from the Permian onwards. Based on the fossil record, the divergence of coelacanths, lungfish, and tetrapods is thought to have occurred during the Silurian. Over 100 fossil species of coelacanth have been described. The oldest identified coelacanth fossils are around 420–410 million years old, dating to the Pragian stage of the early Devonian. These include Eoactinistia from Australia, known only from a fragmentary jaw, as well as Euporosteus yunnanensis from China, known from a partial skull that indicates it to be the earliest anatomically modern coelacanth. Some authors have also suggested that the slightly older onychodont Styloichthys may also be an early coelacanth.

Coelacanths were never a diverse group in comparison to other groups of fish, and reached a peak diversity during the Early Triassic (252–247 million years ago), coinciding with a burst of diversification between the Late Permian and Middle Triassic. Most Mesozoic coelacanths belong to the suborder Latimerioidei, which contains two major subdivisions, the marine Latimeriidae, which contains modern coelacanths, as well as the extinct Mawsoniidae, which were native to brackish, freshwater as well as marine environments.

Paleozoic coelacanths are generally small (~30–40 cm in length), while Mesozoic forms were larger. Several specimens belonging to the Jurassic and Cretaceous mawsoniid coelacanth genera Trachymetopon and Mawsonia likely reached or exceeded 5 m in length, making them amongst the largest known fishes of the Mesozoic, and amongst the largest bony fishes of all time.

The most recent fossil latimeriid is Megalocoelacanthus dobiei, whose disarticulated remains are found in late Santonian to middle Campanian, and possibly earliest Maastrichtian-aged marine strata of the Eastern and Central United States, the most recent mawsoniids are Axelrodichthys megadromos from early Campanian to early Maastrichtian freshwater continental deposits of France, as well as an indeterminate marine mawsoniid from Morocco, dating to the late Maastrichtian A small bone fragment from the European Paleocene has been considered the only plausible post-Cretaceous record, but this identification is based on comparative bone histology methods of doubtful reliability.

Living coelacanths have been considered "living fossils" based on their supposedly conservative morphology relative to fossil species; however, recent studies have expressed the view that coelacanth morphologic conservatism is a belief not based on data. Fossils suggest that coelacanths were most morphologically diverse during the Devonian and Carboniferous, while Mesozoic species are generally morphologically similar to each other.

Cladogram showing the relationships of coelacanth genera after Torino, Soto and Perea, 2021.

Timeline of genera

After Ferrante and Cavin (2025):[[File:Coelacanth phylogeny.png|center|frameless|750x750px]]

References

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