A tetrapod is any four-limbed vertebrate animal of the superclass Tetrapoda. Tetrapods include all extant and extinct amphibians and amniotes, with the latter in turn evolving into two major clades, the sauropsids and synapsids. Some tetrapods such as snakes, legless lizards, and caecilians had evolved to become limbless via mutations of the Hox gene, although some do still have a pair of vestigial spurs that are remnants of the hindlimbs.
Sarcopterygii — sometimes considered synonymous with Crossopterygii — is a clade of bony fish commonly referred to as lobe-finned fish. They are characterised by prominent muscular limb buds (lobes) within their fins, which are supported by articulated appendicular skeletons. This is in contrast to the other clade of bony fish, the Actinopterygii, which have only skin-covered bony spines supporting the fins.
"Labyrinthodontia" is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras. Traditionally considered a subclass of the class Amphibia, modern classification systems recognize that labyrinthodonts are not a formal natural group (clade) exclusive of other tetrapods. Instead, they consistute an evolutionary grade, ancestral to living tetrapods such as lissamphibians and amniotes. "Labyrinthodont"-grade vertebrates evolved from lobe-finned fishes in the Devonian, though a formal boundary between fish and amphibian is difficult to define at this point in time.
Eusthenopteron is a genus of prehistoric sarcopterygian fishes known from several species that lived during the Late Devonian period, about 385 million years ago. It has attained an iconic status from its close relationship to tetrapods. Early depictions of animals of this genus show them emerging onto land, but paleontologists now widely agree that eusthenopteron species were strictly aquatic animals.
Hyneria is a genus of large prehistoric predatory lobe-finned fish which lived in fresh water during the Famennian stage of the Devonian period.
Tetrapodomorpha is a clade of vertebrates consisting of tetrapods and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advanced forms transitional between fish and the early labyrinthodonts, such as Tiktaalik, have been referred to as "fishapods" by their discoverers, being half-fish, half-tetrapods, in appearance and limb morphology. The Tetrapodomorpha contains the crown group tetrapods and several groups of early stem tetrapods, which includes several groups of related lobe-finned fishes, collectively known as the osteolepiforms. The Tetrapodomorpha minus the crown group Tetrapoda are the stem Tetrapoda, a paraphyletic unit encompassing the fish to tetrapod transition.
Tristichopterids (Tristichopteridae) were a diverse and successful group of fish-like tetrapodomorphs living throughout the Middle and Late Devonian. They first appeared in the Eifelian stage of the Middle Devonian. Within the group sizes ranged from a few tens of centimeters (Tristichopterus) to several meters.
Rhizodontida is an extinct group of predatory tetrapodomorphs known from many areas of the world from the Givetian through to the Pennsylvanian - the earliest known species is about 377 million years ago (Mya), the latest around 310 Mya. Rhizodonts lived in tropical rivers and freshwater lakes and were the dominant predators of their age. They reached huge sizes - the largest known species, Rhizodus hibberti from Europe and North America, was an estimated 7 m in length, making it the largest freshwater fish known.
Mandageria fairfaxi is an extinct lobe-finned fish that lived during the Late Devonian period. It is related to the much larger Hyneria; although Mandageria was smaller, it probably hunted in the same way.
Onychodus is a genus of prehistoric lobe-finned fish which lived during the Devonian Period. It is one of the best known of the group of onychodontiform fishes. Scattered fossil teeth of Onychodus were first described from Ohio in 1857 by John Strong Newberry. Other species were found in Australia, England, Norway and Germany showing that it had a widespread range.
Eusthenodon is an extinct genus of tristichopterid tetrapodomorphs from the Late Devonian period, ranging between 383 and 359 million years ago. They are well known for being a cosmopolitan genus with remains being recovered from East Greenland, Australia, Central Russia, South Africa, Pennsylvania, and Belgium. Compared to the other closely related genera of the Tristichopteridae clade, Eusthenodon was one of the largest lobe-finned fishes and among the most derived tristichopterids alongside its close relatives Cabonnichthys and Mandageria.
Megalichthyidae is an extinct family of tetrapodomorphs which lived from the Middle–Late Devonian to the Early Permian. They are known primarily from freshwater deposits, mostly in the Northern Hemisphere, but one genus (Cladarosymblema) is known from Australia, and the possible megalichthyid Mahalalepis is from Antarctica.
Osteolepididae is a family of primitive, fish-like tetrapodomorphs that lived during the Devonian period. The family is generally thought to be paraphyletic, with the traits that characterise the family being widely distributed among basal tetrapodomorphs and other osteichthyans. Some of the genera historically placed in Osteolepididae have more recently been assigned to the family Megalichthyidae, which appears to be a monophyletic group.
Letognathus is a genus of rhizodont tetrapodomorph that lived during the Carboniferous period. Its remains come from the Blue Beach Member of the Horton Bluff Formation, near Hantsport, Nova Scotia. Like most rhizodonts, it was of relatively large size, had a large recurved fang at the symphysis of the lower jaw, and a row of three coronoid fangs along the length of the jaw in addition to its marginal dentition. Letognathus is important for rhizodont systematics because it retains a number of primitive features, such as ossified Meckel's cartilage, are not found in the genera Rhizodus and Strepsodus.
Whatcheeria is an extinct genus of early tetrapod from the Mississippian of Iowa. Fossils have been found in 340 million year old fissure fill deposits in the town of Delta. The type species, Whatcheeria deltae was named in 1995. It is classified within the family Whatcheeriidae, along with the closely related Pederpes and possibly Ossinodus.
Laccognathus embryi is an extinct species of porolepiform lobe-finned fish recovered from Ellesmere Island, Canada. It existed during the Frasnian age of the Late Devonian epoch.
Eotetrapodiformes is a clade of tetrapodomorphs including the four-limbed vertebrates and their closest finned relatives, two groups of stem tetrapods called tristichopterids and elpistostegalids.
The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. Tetrapods are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear. They are areas of active research and debate among palaeontologists at present.
Cladistic classification of Sarcopterygii is the classication of Sarcopterygii as a clade containing not only the lobe-finned fishes but also the tetrapods, which are closely related to lungfish. The taxon Sarcopterygii was traditionally classified as a paraphyletic group considered either a class or a subclass of Osteichthyes. Identification of the group is based on several characteristics, such as the presence of fleshy, lobed, paired fins, which are joined to the body by a single bone.
Andersonerpeton is an extinct genus of aïstopod from the Bashkirian of Nova Scotia, Canada. It is known from a single jaw, which shares an unusual combination of features from both other aistopods and from stem-tetrapod tetrapodomorph fish. As a result, Andersonerpeton is significant for supporting a new classification scheme which states that aistopods evolved much earlier than previously expected. The genus contains a single species, A. longidentatum, which was previously believed to have been a species of the microsaur Hylerpeton.
