Tetrapodomorpha

Tetrapodomorpha Temporal range: Early Devonian–Present,
Life restoration of Panderichthys
Modern tetrapods
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Rhipidistia
Clade:	Tetrapodomorpha Ahlberg, 1991
Subgroups
†Kenichthys †Tungsenia Choanata †Rhizodontida †"Osteolepidida" †Tristichopteridae Eotetrapodiformes Elpistostegalia Stegocephali Tetrapoda See also below.
Synonyms
Choanata

Tetrapodomorpha (also known as Choanata) is a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) 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 (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 unit encompassing the fish to tetrapod transition.

Characteristics

Among the characteristics defining tetrapodomorphs are modifications to the fins, notably a humerus 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 onwards, and include Osteolepis, Panderichthys, Kenichthys and Tungsenia. Tetrapodomorpha evolved from ancient lobe-finned fish (sarcopterygians) around 390 million years ago in the Middle Devonian period.

Stem tetrapoda

Tiktaalik, a tetrapodomorph with wrists, straddles the fish-tetrapod divide

The stem tetrapoda are a cladistically defined group, consisting of all animals more closely related to extant four-legged vertebrates than to their closest extant relatives (the lungfish), but excluding the crown group Tetrapoda. They are thus paraphyletic, though acceptable in phylogenetic nomenclature as the group is defined by strict reference to phylogeny rather than to traits as in traditional systematics. Thus, some finned sarcopterygians are considered to be stem tetrapods.

Content of the group

See also: Labyrinthodontia

Stem tetrapods are members of tetrapodomorpha, the total group and clade that also includes their descendants, the crown tetrapods:

The stem Tetrapoda encompass three distinct grades successively closer to crown group Tetrapoda:

• Osteolepiformes, a group of lobe-finned fishes that includes the families Tristichopteridae, Canowindridae, Megalichthyidae, and Osteolepidae
• Elpistostegalia, the more advanced lobe-finned fishes (Tristichopteridae) and the "fishapods" (genera such as Panderichthys and Tiktaalik)
• Ichthyostegalia, the primarily aquatic primitive labyrinthodonts such as Acanthostega, Ichthyostega, Tulerpeton, and probably loxommatids.

Both Ichthyostegalia and Labyrinthodontia constitute paraphyletic evolutionary grades rather than clades, with amniotes and modern amphibians branching off at some point from the latter. The stem tetrapods may also include one or both of Temnospondyli and Lepospondyli, depending on author. This is due to the uncertain origin of the modern amphibians, whose position in the phylogenetic tree dictates what lineages go in the crown group Tetrapoda. Neither is there for the moment a consensus of the phylogeny of stem tetrapods, nor how Tetrapoda itself should be defined (i.e. as a crown group, or as an apomorphy-based group, using the limb with digits), making the actual content of the group uncertain.

Classification

In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish.

The exact shape of the phylogenetic tree is uncertain, with Zachelmie tetrapod tracks predating most tetrapodomorph fossils

Taxonomy

After Benton, 2004; and Swartz, 2012.

• Subclass Sarcopterygii

• Infraclass Tetrapodomorpha

• Order †Rhizodontida

• Family †Sauripteridae
• Family †Rhizodontidae

• Superorder †Osteolepidida (or Osteolepiformes)

• Family †Canowindridae
• Family †Thysanolepidae
• Family †Tristichopteridae

• Order †Osteolepiformes (or Megalichthyiformes)

• Family †Osteolepidae
• Family †Megalichthyidae

• Clade Eotetrapodiformes

• Clade Elpistostegalia (or Panderichthyida)

• Clade Stegocephalia

• Family †Elpistostegidae
• Family †Whatcheeriidae
• Family †Colosteidae
• Superfamily †Baphetoidea
• Superclass Tetrapoda

Other clades include the Eotetrapodiformes (Tinirau, Platycephalichthys, the Tristichopteridae and Elpistostegalia). Older taxa which include late stem tetrapods and early tetrapods are the Labyrinthodontia and Ichthyostegalia.

Relationships

The cladogram is based on a phylogenetic analysis of 46 taxa using 204 characters by B. Swartz in 2012.

Tetrapodomorpha

†Kenichthys †Rhizodontidae †Canowindridae †Marsdenichthys †Canowindra †Koharalepis †Beelarongia †Megalichthyiformes †Gogonasus †Gyroptychius †Osteolepis †Medoevia †Megalichthyidae Eotetrapodiformes †Tristichopteridae †Spodichthys †Tristichopterus †Eusthenopteron †Jarvikina †Cabonnichthys †Mandageria †Eusthenodon †Tinirau †Platycephalichthys Elpistostegalia †Panderichthys †Tiktaalik †Elpistostege †Elginerpeton †Ventastega †Acanthostega †Ichthyostega †Whatcheeriidae †Colosteidae †Crassigyrinus †Baphetidae Tetrapoda (crown group)

The following cladogram follows the results found by Clement et al. (2021).

	Dipnomorpha
Tetrapodomorpha	Kenichthys Tungsenia Rhizodontida Hongyu Gooloogongia Sauripterus Barameda Screbinodus Rhizodus Strepsodus Osteolepidida Gyroptychius Osteolepis Gogonasus Medoevia Megalichthyidae Canowindridae Beelarongia Canowindra Koharalepis Marsdenichthys Spodichthys Tristichopterus Eusthenopteron Jarvikina Cabonnichthys Platycephalichthys Mandageria Eusthenodon Tinirau Bruehnopteron Panderichthys Tiktaalik Elpistostege Tetrapoda Ventastega Acanthostega Ichthyostega Tulerpeton

References

• Mikko Haaramo. "Tetrapodomorpha – Terrestrial vertebrate-like sarcopterygians". Archived from the original on 12 May 2006. Retrieved 6 April 2006.
• P. E. Ahlberg & Z. Johanson (1998). "Osteolepiforms and the ancestry of tetrapods". Nature. 395 (6704): 792–794. Bibcode:1998Natur.395..792A. doi:10.1038/27421. S2CID 4430783.
• Michel Laurin, Marc Girondot & Armand de Ricqlès (2000). "Early tetrapod evolution" (PDF). TREE. 15 (3). Archived from the original (PDF) on 2009-09-22. Retrieved 2006-04-06.