Tetrapodomorpha

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Tetrapodomorpha
Temporal range: Early Devonian-Recent, 409–0 Ma
Tiktaalik BW.jpg
The advanced tetrapodomorph Tiktaalik
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Sarcopterygii
Clade: Tetrapodomorpha
Ahlberg, 1991
Subgroups

See below

The Tetrapodomorpha are 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, and several groups of related lobe-finned fishes, collectively known as the osteolepiforms. The Tetrapodamorpha minus the crown group Tetrapoda are the Stem Tetrapoda, a paraphyletic unit encompassing the fish to tetrapod transition.

Among the characters 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). Early tetrapodomorphs such as Kenichthys had excurrent nostrils that had migrated to the edge of the mouth. In later tetrapodomorphs, including tetrapods, the excurrent nostril is positioned inside the mouth, where it is known as the choana.[1]

Tetrapodomorph fossils are known from the early Devonian onwards, and include Osteolepis, Panderichthys, Kenichthys and Tungsenia.[2]

Classification[edit]

In Late Devonian vertebrate speciation, descendants of pelagic lobe-finned fish – like Eusthenopteron – exhibited a sequence of adaptations: * Panderichthys, suited to muddy shallows; * Tiktaalik with limb-like fins that could take it onto land; * Early tetrapods in weed-filled swamps, such as: ** Acanthostega which had feet with eight digits, ** Ichthyostega with limbs. Descendants also included pelagic lobe-finned fish such as coelacanth species.

Taxonomy[edit]

After Benton, 2004 [1]:

Phylogeny[edit]

Cladogram modified after Ruta, Jeffery, & Coates (2003):[3]

Tetrapodomorpha 

Eusthenopteron




Panderichthys




Livoniana




Elpistostege



 Elginerpetidae 

Elginerpeton



Obruchevichthys






Metaxygnathus



Ventastega




Tetrapoda








Cladogram from Swartz, 2012:[4]

Tetrapodomorpha 

Kenichthys




Rhizodontidae



Canowindridae

Marsdenichthys




Canowindra




Koharalepis



Beelarongia






Megalichthyiformes

Gogonasus




Gyroptychius




Osteolepis




Medoevia



Megalichthyidae






Eotetrapodiformes
Tristichopteridae

Spodichthys




Tristichopterus




Eusthenopteron




Jarvikina




Cabbonichthys




Mandageria



Eusthenodon










Tinirau




Platycephalichthys


Elpistostegalia

Panderichthys





Tiktaalik



Elpistostege



Tetrapoda

Elginerpeton




Ventastega




Acanthostega




Ichthyostega




Whatcheeriidae




Colosteidae




Crassigyrinus




Baphetidae




Total-group Lissamphibia




Embolomeri



stem amniotes





















References[edit]

  1. ^ Clack, Jennifer A. (2012). Gaining Ground: The Origin and Evolution of Tetrapods. Indiana University Press. p. 74. ISBN 0-253-35675-X. Retrieved 8 June 2015. 
  2. ^ Jing Lu, Min Zhu, John A. Long, Wenjin Zhao, Tim J. Senden, Liantao Jia and Tuo Qiao (2012). "The earliest known stem-tetrapod from the Lower Devonian of China". Nature Communications 3: 1160. Bibcode:2012NatCo...3E1160L. doi:10.1038/ncomms2170. PMID 23093197. 
  3. ^ Ruta, M.; Jeffery, J. E.; Coates, M. I. (2003). "A supertree of early tetrapods". Proceedings of the Royal Society B 270 (1532): 2507–2516. doi:10.1098/rspb.2003.2524. PMC 1691537. PMID 14667343. 
  4. ^ Swartz, B. (2012). "A marine stem-tetrapod from the Devonian of Western North America". PLoS ONE 7 (3): e33683. doi:10.1371/journal.pone.0033683. PMC 3308997. PMID 22448265.  edit