Tetrapodomorpha (also known as Choanata) is a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s consisting of

tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...

s (four-limbed vertebrates) and their closest

sarcopterygian Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class or subclass) of vertebrate animals which includes a group of bony fish commonly referred to as lobe-finned fish. These vertebrates ar ...

relatives that are more closely related to living tetrapods than to living

lungfish Lungfish are freshwater vertebrates belonging to the class Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, inc ...

. 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 (the last common ancestor of living tetrapods and all of its descendants) 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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

unit encompassing the fish to tetrapod transition.

Characteristics

Among the characteristics defining tetrapodomorphs are modifications to the fins, notably a

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

with convex head articulating with the glenoid fossa (the socket of the shoulder joint). Another key trait is the internal nostril or choana. Most fish have two pairs of nostrils, one on either side of the head for incoming water (incurrent nostrils) and another pair for outgoing water (excurrent nostrils). In early tetrapodomorphs like '' Kenichthys'', the excurrent nostrils had shifted towards the mouth's perimeter. In later tetrapodomorphs, including tetrapods, the excurrent nostril is positioned inside the mouth, where it is known as the choana. The nearly-equivalent clade Choanata often refers to these later forms specifically.

History

Tetrapodomorph fossils are known from the early

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

onwards, and include '' Osteolepis'', '' Panderichthys'', '' Kenichthys'' and '' Tungsenia''. Tetrapodomorpha evolved from ancient lobe-finned fish (

s) around in the Middle Devonian period.

Classification

Zachelmie tracks vs selected Devonian fossils

Taxonomy

After Benton, 2004; and Swartz, 2012. * Subclass

Sarcopterygii Sarcopterygii (; )—sometimes considered synonymous with Crossopterygii ()—is a clade (traditionally a class (biology), class or subclass) of vertebrate animals which includes a group of bony fish commonly referred to as lobe-finned fish. The ...

:* Infraclass Tetrapodomorpha ::* Order † Rhizodontida ::::* Family † Sauripteridae ::::* Family † Rhizodontidae ::* Superorder † Osteolepidida (or Osteolepiformes) ::::* Family † Canowindridae ::::* Family † Thysanolepidae ::::* Family †

Tristichopteridae 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 ...

:::* Order † Osteolepiformes (or Megalichthyiformes) ::::* Family † Osteolepidae ::::* Family † Megalichthyidae ::* Clade Eotetrapodiformes :::* Clade Elpistostegalia (or Panderichthyida) ::::* Clade

Stegocephalia Stegocephali (often spelled Stegocephalia, from Greek , lit. "roofed head") is a clade of vertebrate animals containing all fully limbed tetrapodomorphs. It is equivalent to a broad definition of the superclass Tetrapoda: under this broad ...

:::::* Family † Elpistostegidae :::::* Family † Whatcheeriidae :::::* Family †

Colosteidae Colosteidae is a Family (biology), family of stegocephalians (stem-group tetrapod, tetrapods) that lived in the Carboniferous period. They possessed a variety of characteristics from different tetrapod or stem-tetrapod groups, which made them his ...

:::::* Superfamily † Baphetoidea :::::* Superclass Tetrapoda Other clades include the Eotetrapodiformes ('' Tinirau'', '' Platycephalichthys'', the

and Elpistostegalia). Older taxa which include late stem tetrapods and early

s are the Labyrinthodontia and

Ichthyostegalia Ichthyostegalia is an obsolete order of early amphibians, representing the earliest landliving vertebrates. The group is thus an evolutionary grade rather than a clade. While the group are recognized as having feet rather than fins, most, if ...

Relationships

The cladogram is based on a phylogenetic analysis of 46 taxa using 204 characters by B. Swartz in 2012. The following cladogram follows the results found by Clement ''et al''. (2021).

Citations

References

* * * {{Taxonbar, from=Q1209254 Extant Early Devonian first appearances