Rhipidistia, also known as Dipnotetrapodomorpha, is a clade of

lobe-finned fish 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 ...

es which includes the

tetrapods A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...

and lungfishes. Rhipidistia formerly referred to a subgroup of

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 ...

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, and the

lymph Lymph () is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated. At the ori ...

pumping " lymph heart" (later lost in

mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...

and flying

birds Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

), are unique to this group. Another feature shared by lungfish and tetrapods is the divided atrium. The precise time at which the choana of tetrapods evolved is debated, with some considering early rhipidistians as the first choanates. The feature is also present in modern lungfish but is probably a case of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

. The basal stem-lungfish '' Diabolepis'' did not possess it. Instead, it had four nostrils (two anterior and two posterior) like most fish. However, its posterior nares are very close to the lip, meaning a ventral 'displacement' of the posterior nostril can be considered a synapomorphy of the lungfish-tetrapod clade. The complete choana then seems to have developed independently in the two surviving clades.

Etymology

The word "Rhipidistia" is from . The word "Dipnotetrapodomorpha" is from (from meaning "twice", and meaning "breathing, breath"); from , the

combining form Neoclassical compounds are compound words composed from combining forms (which act as affixes or stems) derived from classical languages (classical Latin or ancient Greek) roots. Neo-Latin comprises many such words and is a substantial componen ...

of the numeral (''tettares''); from , the combining form of (''pous'') meaning "foot"; and from , the combining form of (''morph'') meaning "physical shape".

Relationships

The cladogram presented below is based on studies compiled by Philippe Janvier and others for the ''Tree of Life Web Project'',Janvier, Philippe. 1997. Vertebrata. Animals with backbones. Version 01 January 1997 (under construction). http://tolweb.org/Vertebrata/14829/1997.01.01 in The Tree of Life Web Project, http://tolweb.org/ and Swartz 2012.

References

External links

palaeos.com page

Taxonomicon page
Rhipidistia Early Devonian first appearances Vertebrate subclasses Evolution of tetrapods {{paleo-lobefinned-fish-stub