#actinistia

(Sources - 1, 2)

Order: Coelacanthiformes

Common Name: “coelacanths”

Families: 1 - Latimeriidae (only two living species are known today, both from the genus Latimeria)

Anatomy: large, plump body; many lobed fins; tapetum lucidum in eyes; two nostrils and four other external openings

Diet: cuttlefish, squid, snipe eels, small sharks, and other fish

Habitat: reef and volcanic slope habitats between 90 to 200 m (300 to 660 ft) deep, in the western Indian Ocean around the Comoro Islands, Madagascar, and Mozambique (L. chalumnae) and in the waters of north Sulawesi, Papua, Southwest Papua, and North Maluku (L. menadoensis)

Evolved in: Middle Triassic

Propaganda under the cut:

“ The fish had hollowed out its fins and would soon wish to survive. ”

– Jean Louis Rodolphe Agassiz

Coelacanthus granulatus is the extinct genus and species of coelacanths known from the late Permian period.

References

Allenypterus montanus was an unusual early coelacanth that lived during the late Carboniferous, around 324 million years ago, in a tropical bay covering what is now central Montana, USA.

Up to about 15cm long (~6"), its tapering tadpole-like body plan somewhat resembled that of modern knifefishes and featherbacks, with the top part of its tail fin highly elongated into a ribbon-like shape and the rest of its tail fins being vestigial. The distinctive humped shape of its back was also much more pronounced in larger, more mature individuals.

It was probably a fairly slow swimmer, and preserved gut contents suggest it mainly ate small soft-bodied prey.

Its closest known relative seems to have been the eel-like Holopterygius – but since around 60 million years and different continents separated them both, this suggests the existence of a whole ghost lineage of other tapering coelacanths yet to be discovered.

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Coelacanths are often thought of as a species untouched by time, due to their ancient evolutionary lineage and long lifespans. These fish evolve slow, move slowly, grow slowly, and even give birth slowly; pregnancies can last anywhere from three to five years, after which she gives birth to up to 25 live pups.

(Image: A pair of West Indian Ocean coelacanth (Latimeria chalumnae) (left) by Laurent Ballesta)

West Indian Ocean coelacanth (Latimeria chalumnae)

(Sources - 1, 2)

The Sarcopterygians (“Lobe-finned Fishes”), are the last of the three groups of “fish”, and are so named for the prominent muscular limb buds (lobes) within their fins. Of the Sarcopterygians living today, they are represented by the coelacanths, lungfish, and tetrapods (including humans), who all diverged in the Silurian. These next fish are closer related to us than they are to Actinopterygiians.

The class Actinistia, the “Coelacanths”, are an ancient group of fish that have been around since the Devonian but today are only represented by two remaining species: The West Indian Ocean Coelacanth (Latimeria chalumnae) and the Indonesian Coelacanth (Latimeria menadoensis).

Coelacanths can live as deep as 700 m (2,300 ft) below the sea, but are more commonly found at depths of 90 to 200 m (300 to 660 ft). They have sensitive eyes which include a tapetum lucidum and many rods which help them see better in dark water, as they are most active at night. They are opportunistic hunters, feeding on cuttlefish, squid, snipe eels, small sharks, and other fish found around their deep reef and volcanic slope habitats. Their abundance of fins allow for high maneuverability, and coelacanths can orient their body in almost any direction in the water. They have been seen doing headstands as well as swimming belly up. They are able to slow their metabolisms at will, sinking into less-inhabited depths and going into a hibernation mode to conserve energy.

Coelacanths are ovoviviparous, with the female retaining the fertilized eggs within her body while the embryos develop over a gestation period of five years. The female will give live birth to around 5-26 young. Young coelacanths resemble the adult, but carry an external yolk sac below their pelvic fins, and have larger eyes relative to body size. Individual coelacanths may live as long as 80 to 100 years.

Coelacanths get their name from Coelacanthus, a genus of Permian coelacanths and the first coelacanths to be described. Over 100 fossil species are known, and all of them were believed to have gone extinct in the Cretaceous. On December 23, 1938, the first Latimeria specimen was discovered among the catch of a South African fisherman, making coelacanths a “lazarus taxon.” While previously considered a “living fossil”, coelacanth body shapes were much more diverse in the Early Triassic, and Latimeria is not known from fossils, showing that it had to have gone through some changes to adapt to the modern day.

Propaganda under the cut:

Coelacanths are represented today by just two surviving species, one in East Africa and one in Indonesia, both very similar in appearance and ecology to each other.

For a long time their lineage was thought to be all "living fossils", retaining the same basic body plan for the last 400 million years – but more recent discoveries have revealed that these fish were actually much more diverse over the course of their evolutionary history.

Holopterygius nudus was a fairly early member of the group, living during the mid-Devonian about 385 million years ago. The only known fossil specimen was discovered in Germany in the 1970s, but it was originally thought to be a different type of fish entirely and wasn't identified as being a coelacanth until over 30 years later.

And compared to its living relatives it was tiny, just 7cm long (2.75"), with a distinctive tapering eel-like tail. Its convergent close resemblance to modern cusk-eels suggest it may have occupied a similar ecological niche, living near the sea floor and hiding in tight spaces like crevices and burrows.

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Coelacanths are famous for being “living fossils”, completely disappearing from the fossil record at the end of the Cretaceous but then being rediscovered alive just 80 years ago. But although they’re often thought to have physically changed very little over the last 300 million years or so, more recent discoveries are starting to show that coelacanth body forms and lifestyles were actually more varied in the distant past.

Meet the wonderfully-named Rebellatrix divaricerca, from the Early Triassic of British Columbia, Canada (~251-247 mya). Measuring around 1.3m long (4′3″), its body shape and large symmetrical forked tail suggest it was adapted for fast swimming. Unlike its slow-moving deep-water modern relatives this coelacanth was a speedy oceanic active predator, convergently similar to tuna or some sharks.