Ichthyostegalia is an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

of extinct

amphibians Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...

, representing the earliest landliving vertebrates. The group is thus an evolutionary grade rather than a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

. While the group are recognized as having feet rather than fins, most, if not all, had internal

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

s in adulthood and lived primarily as shallow water fish and spent minimal time on land. The group evolved from elpistostegalian fish in the late

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

, or possibly in the middle Devonian. They continued to thrive as denizens of swampland and tidal channels throughout the period. They gave rise to the

Temnospondyli Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...

and then disappeared during the transition to the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

Classification

*Ichthyostegalia ** Acanthostegidae ***''

Acanthostega ''Acanthostega'' (meaning "spiny roof") is an extinct genus of stem-tetrapod, among the first vertebrate animals to have recognizable limbs. It appeared in the late Devonian period (Famennian age) about 365 million years ago, and was anatomic ...

'' ** Crassigyrinidae ***''

Crassigyrinus ''Crassigyrinus'' (from la, crassus, 'thick' and el, γυρίνος el, gyrínos, 'tadpole') is an extinct genus of carnivorous stem tetrapod from the Early Carboniferous Limestone Coal Group of Scotland and possibly Greer, West Virginia. Th ...

'' ** Densignathidae ***''

Densignathus ''Densignathus'' is an extinct genus of early stem-tetrapod from the Late Devonian of Pennsylvania in the United States. A lower jaw has been found from the Red Hill fossil site, which is known for a diversity of lobe-finned fishes and other earl ...

'' ** Elginerpetontidae ***'' Elginerpeton'' ***'' Obruchevichthys'' ** Ichthyostegidae ***'' Hynerpeton'' ***''

Ichthyostega ''Ichthyostega'' (from el, ἰχθῦς , 'fish' and el, στέγη , 'roof') is an extinct genus of limbed tetrapodomorphs from the Late Devonian of Greenland. It was among the earliest four-limbed vertebrates in the fossil record, and was o ...

'' ** Jakubsonidae ***'' Jakubsonia'' ** Metaxygnathidae ***''

Metaxygnathus ''Metaxygnathus'' is an extinct genus of ichthyostegalian found in Late Devonian deposits of New South Wales, Australia . It is known only from a lower jawbone. Previously thought to be a lobe-finned fish, it has now been reassigned to the ...

'' ** Sinostegidae ***'' Sinostega'' ** Tulerpetontidae ***''

Tulerpeton ''Tulerpeton'' is an extinct genus of Devonian four-limbed vertebrate, known from a fossil that was found in the Tula Region of Russia at a site named Andreyevka. This genus and the closely related ''Acanthostega'' and ''Ichthyostega'' represen ...

'' ** Ventastegidae ***'' Ventastega'' ** Ymeridae ***'' Ymeria''

Description

As first described, the order's sole member was ''

'', from which the group takes its name. ''Ichthyostega'' was seen as transitional between

fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

and the early

Stegocephalia Stegocephali (often spelled Stegocephalia) is a group containing all four-limbed vertebrates. It is equivalent to a broad definition of Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first ve ...

ns, in that it combines a flat, heavily armoured stegocephalian

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

with a

like tail bearing fin rays. Later work on ''Ichthyostega'' and other Devonian Labyrinthodonts shows that they also had more than 5 digits to each foot, in fact the whole foot being fin-like.

, later found in the same locations, appears to have had a soft operculum and both it and ''Ichthyostega'' possessed functional internal

s as adults. The feet are only known from ''Ichthyostega'',

, and

, but appear to be polydactyl in all forms with more than the usual five digits for tetrapods and were paddle-like. The tail bore true fin rays like those found in fish. The Ichthyostegalians were large to medium-sized, with an adult size form most genera on the order of a meter or more. Their heads were flat and massive, with a host of labyrinthodont teeth. They were

carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other ...

and probably mainly ate fish, but may also have fed on washed-up carcasses of fish and other marine life, and hunted unwary

arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, ...

and other invertebrate life along the tidal channels of the coal swamps. The

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

were complex and rather weak. At the close of the Devonian, forms with progressively stronger legs and vertebrae evolved, and the later groups lacked functional gills as adults. As adults, the animals would have been heavy and clumsy on land, and would probably appear more as fish that occasionally went ashore rather than proper land animals. All were however predominately aquatic and some spent all or nearly all their lives in water.

Genera

The order Ichthyostegalia was erected for ''Ichthyostega'', and contained until the 1980s only three genera (''Ichthyostega'', ''

'' and ''

''). While Ichthyostegalia in principle contain the most basal of animals with toes rather than fins, Clack and Ahlberg uses it for all finds more advanced than '' Tiktaalik'' (the closest relative of tetrapods known to have retained paired fins rather than feet). Under this use, the number of known Devonian tetrapods have increased dramatically, so that the group now contain 12

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

: Most of the newer finds are redescriptions of very fragmentary finds, usually just the lower jaw. These were thought to have been from fish when found, but

cladistic Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...

analyses indicate they are more advanced than ''Tiktaalik'', though whether they actually had feet rather than fins is unknown. In order of discovery: * ''

'' * ''

'' * '' Elginerpeton'' * '' Obruchevichthys'' * '' Ventastega'' * '' Hynerpeton'' * ''

'' * '' Sinostega'' * '' Jakubsonia'' * '' Ymeria''

References

{{Taxonbar, from=Q3364269 Devonian amphibians Paleozoic amphibians Paraphyletic groups