Actinistia is a subclass of mostly fossil lobe-finned fishes. This subclass contains the coelacanths (Order Coelacanthiformes), including the two living species of coelacanths, both of the genus Latimeria: the West Indian Ocean coelacanth and the Indonesian coelacanth.
Temporal range: Devonian–Recent
|West Indian Ocean coelacanth|
The coelacanths ( (listen) SEE-lə-kanth) constitute a now-rare order of fish that includes two extant species in the genus Latimeria: the West Indian Ocean coelacanth (Latimeria chalumnae) primarily found near the Comoro Islands off the east coast of Africa and the Indonesian coelacanth (Latimeria menadoensis). They follow the oldest-known living lineage of Sarcopterygii (lobe-finned fish and tetrapods), which means they are more closely related to lungfish and tetrapods than to ray-finned fishes. They are found along the coastlines of the Indian Ocean and Indonesia. The West Indian Ocean coelacanth is a critically endangered species.
Coelacanths belong to the subclass Actinistia, a group of lobed-finned fish related to lungfish and certain extinct Devonian fish such as osteolepiforms, porolepiforms, rhizodonts, and Panderichthys. Coelacanths were thought to have become extinct in the Late Cretaceous, around 66 million years ago, but were rediscovered in 1938 off the coast of South Africa.The coelacanth was long considered a "living fossil" because scientists thought it was the sole remaining member of a taxon otherwise known only from fossils, with no close relations alive, and that it evolved into roughly its current form approximately 400 million years ago. However, several recent studies have shown that coelacanth body shapes are much more diverse than previously thought.Coelacanthopsis
Coelacanthopsis is an extinct genus of lobe-finned fish which lived during the Carboniferous period.
The Coelacanth is the only living example of the fossil Coelacanth fishes Actinistia. They are also the closest link between fish and the first amphibian creatures which made the transition from sea to land in the Devonian period (408-362 Million Years Ago). That such a creature could have existed for so long is nearly incredible, but some say that the cold depths of the West Indian Ocean at which the Coelacanth thrives, and the small number of predators it has, may have helped the species survive eons of change.
The Coelacanth was first discovered in 1938 by Marjorie Courtenay Latimer, the curator of a small museum in the port town of East London, as she was visiting a fisherman who would let her search through his boat's catch for interesting specimens. Ironically, Marjorie was only visiting the sea captain to wish him a happy Christmas when she first spotted the Coelacanth's oddly shaped, blue-gray fin protruding from beneath a mountain of fish. Marjorie brought back the specimen to the museum where she compared it against images of known species, and ultimately realized what she had was no ordinary fish.
After sending a rough drawing of the fish to Professor J.L.B. Smith, at Rhodes University, Grahamstown, who in turn confirmed that the creature she had discovered on the boat's deck was indeed a prehistoric fish, a Coelacanth to be exact.
Since then, Coelacanth populations have been found near Indonesia, South Africa, and other unexpected places. While there have been enough sightings of the creature to indicate that there is more than one area where the species exists, it remains a highly protected and mysterious animal, a living fossil which may, or may not be the only creature from our past which has survived millions of years of evolution.
Some place it in the family Rhabdodermatidae.Diplocercides
Diplocercides is a genus of prehistoric lobe-finned fish belonging to the coelacanth group (Actinistia or Coelacanthimorpha) which lived during the Late Devonian period (between 370 and 397 million years BCE). Fossils of Diplocercides have been found in Germany, Iran, Ireland, Australia and Poland (and possibly in South Africa in 1937).Recently three-dimensional fossils of Diplocercides were uncovered from the Gogo Formation of Western AustraliaDumfregia
Dumfregia is a genus of prehistoric lobe-finned fish which lived during the Carboniferous period.Euteleostomi
Euteleostomi is a successful clade that includes more than 90% of the living species of vertebrates. Euteleostomes are also known as "bony vertebrates". Both its major subgroups are successful today: Actinopterygii includes the majority of extant fish species, and Sarcopterygii includes the tetrapods.
"Osteichthyes" in the paleontological sense (i.e., "bony vertebrates"), is synonymous with Euteleostomi. However, in ichthyology and Linnaean taxonomy Osteichthyes, literally "bony fish," refers to the paraphyletic group that differs by excluding tetrapods. The name Euteleostomi, coined as a monophyletic alternative that unambiguously includes the living tetrapods, is more widely used in bioinformatics and related fields. The term Euteleostomi comes from Eu-teleostomi, where "Eu-" comes from the Greek εὖ meaning well or good, so the clade can be defined as the living teleostomes.
Euteleostomes originally all had endochondral bone, fins with lepidotrichs (fin rays), jaws lined by maxillary, premaxillary, and dentary bones composed of dermal bone, and lungs. Many of these characters have since been lost by descendant groups, however, such as lepidotrichs lost in tetrapods, and bone lost among the chondrostean fishes. Lungs have been retained in dipnoi (lungfish), and many tetrapods (birds, mammals, reptiles, and some amphibians). In many ray-finned fishes lungs have evolved into swim bladders for regulating buoyancy, while in others they continue to be used as respiratory gas bladders.Hadronectoridae
Hadronectoridae is an extinct family of prehistoric coelacanth fishes which lived during the Carboniferous period. However, according to Actinistia, it could include Laugiidae, Rhabdodermatidae and not extinct Coelacanthiformes.Holostei
Holostei are bony fish. There are eight species divided among two orders, the Amiiformes represented by a single living species, the Bowfin (Amia calva), and the Lepisosteiformes, represented by seven living species in two genera, the gars. Further species are to be found in the fossil record and the group is often regarded as paraphyletic. Holosteians are closer to teleosts than are the chondrosteans, the other group intermediate between teleosts and cartilaginous fish. The spiracles are reduced to vestigial remnants and the bones are lightly ossified. The thick ganoid scales of the gars are more primitive than those of the bowfin.Megalocoelacanthus
Megalocoelacanthus dobiei is an extinct species of giant latimeriid coelacanth lobe-finned fish which lived during the Lower Campanian epoch until possibly the early Maastrichtian in the Late Cretaceous period in the Western Interior Seaway and Mississippi Embayment. Its disarticulated remains have been recovered from the Eutaw Formation, Mooreville Chalk Formation, and Blufftown Formation of Alabama, Mississippi, and Georgia, and also from the Niobrara Formation of Kansas. Although no complete skeleton is known, careful examination of skeletal elements demonstrate it is closely related to the Jurassic-aged coelacanthid Libys. The species is named for herpetologist James L. Dobie.Onychodontida
Onychodontida (= Onychodontiformes or Struniiformes) is an order of prehistoric sarcopterygian fish that lived between Late Silurian to Late Devonian period. The onychodontiformes are probably a paraphyletic group of basal sarcopterygians.Paraconodontida
Paraconodontida is an extinct order of conodonts. It contains two superfamilies, Amphigeisinoidea and Furnishinoidea.Parnaibaia
Parnaibaia is a genus of coelacanth fish which lived during the Late Jurassic period. Fossils of Parnaibaia have been found in the Pastos Bons Formation in Maranhão, Brazil. Parnaibaia was described for the first time by palaeontologist Yoshitaka Yabumoto in 2008.Prioniodontida
Prioniodontida, also known as the "complex conodonts", is a large clade of conodonts that includes two major evolutionary grades; the Prioniodinina and the Ozarkodinina. It includes many of the more famous conodonts, such as the giant ordovician Promissum (Prioniodinina) from the Soom Shale and the Carboniferous specimens from the Granton Shrimp bed (Ozarkodinina). They are euconodonts, in that their elements are composed of two layers; the crown and the basal body, and are assumed to be a clade.Pteraspidomorphi
Pteraspidomorphi is an extinct class of early jawless fish. They have long been regarded as closely related or even ancestral to jawed vertebrates, but the few characteristics they share with the latter are now considered as primitive for all vertebrates.Rhabdodermatidae
Rhabdodermatidae is a family of prehistoric, coelacanthimorph, lobe-finned fishes which lived during the Carboniferous period (about 359 - 299 million years ago).Rhipidistia
The Rhipidistia, also known as dipnotetrapodomorphs (formally Dipnotetrapodomorpha) are a clade of lobe-finned fishes which include the tetrapods and lungfishes. Rhipidistia formerly referred to a subgroup of Sarcopterygii consisting of the Porolepiformes and Osteolepiformes, a definition that is now obsolete. However as cladistic understanding of the vertebrates has improved over the last few decades a monophyletic Rhipidistia is now understood to include the whole of Tetrapoda and the lungfishes.
Rhipidistia includes porolepiformes and dipnoi. Extensive fossilization of lungfishes has contributed to many evolutionary studies of this group. Evolution of autostylic jaw suspension, in which the palatoquadrate bone fuses to the cranium, is unique to this group.
The precise time at which the choana evolved is debated, with some considering early rhipidistians as the first choanates.Sarcopterygii
The Sarcopterygii () or lobe-finned fish (from Greek σάρξ sarx, flesh, and πτέρυξ pteryx, fin)—sometimes considered synonymous with Crossopterygii ("fringe-finned fish", from Greek κροσσός krossos, fringe)—constitute a clade (traditionally a class or subclass) of the bony fish, though a strict cladistic view includes the terrestrial vertebrates.
The living sarcopterygians include two species of coelacanths and six species of lungfish.Spermatodus
Spermatodus is an extinct genus of prehistoric coelacanth, or lobe-finned fish.Synaptolus
Synaptolus is a genus of prehistoric lobe-finned fish which lived during the Carboniferous period.Whiteiidae
Whiteiidae is an extinct family of prehistoric coelacanth fishes which lived during the Triassic period.