Actinistians, also called coelacanths, are a group of sarcopterygians recognizable by their stocky form and three-lobed tail, with the exception of the very earliest representatives of the group.(40 kb)
They are also distinguished by the presence of a rostral gland in the snout, which allows them to detect tiny organisms in sediments, a second dorsal fin, and an anal fin with joints and a bone structure reminiscent of paired fins. The first dorsal fin is also placed farther forward than in other sarcopterygians.
Actinistians appeared toward the end of the Devonian. From an anatomical perspective, they seem to have been highly varied at the beginning of their evolutionary history. Living mainly in seas, they prospered for 215 million years until the end of the Cretaceous when they were they were not spared the effects of the mass extinction that hit the dinosaurs and numerous other groups. All actinistian fossils date from before the end of the Cretaceous.
In 1938, fishermen caught a strange 1.5-metre long fish at the mouth of the Chalumna River in South Africa. This stunning discovery brought the coelacanth out of the depths of time and into the spotlight of biology and paleontology. The shock was felt among specialists around the world; after all, they now had the incredible and completely unexpected chance to unravel many of the mysteries surrounding actinistians, which were known only as fossils up to that time.
The identification of the first coelacanth catch was made by South African ichthyologist James Leonard Brierley Smith. Smith had been alerted to its discovery by Marjorie Courtenay-Latimer, who oversaw the unloading of the creature at the dock in East London, South Africa.
Since that time, more than 200 specimens have been caught, mostly off-shore of the Comoros Islands. Baptized Latimeria chalumnae
in honour of the lady and the river, the species enjoys widespread recognition and has enlightened paleontologists on sarcopterygian anatomy. Fifty years after the capture of the first specimen, another species was discovered in Indonesia, 10,000 km from South Africa, and was named Latimeria menadoensis
. Both species are threatened by extinction and are protected.
The existence of these living fossils, despite their absence in the fossil records for the past 65 Ma, could be explained by the stable environment at the bottom of the ocean where they live. During the extinction event at the end of the Cretaceous, species living near the surface were wiped out, while others, evolving in the ocean depths, were less affected by the event.
Miguasha sediments have yielded specimens of the most primitive of all actinistians, the ancestor of the two living species and all those that preceded them.