Tetrapodomorpha

Tetrapodomorphs Temporal range: Early Devonian-Recent, 406–0 Ma PreЄ Є O S D C P T J K Pg N
The basal tetrapodomorph Tiktaalik
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Class:	Sarcopterygii
Subclass:	Tetrapodomorpha Ahlberg, 1991
Subgroups
See text

Tetrapodomorpha 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, like Tiktaalik, have been referred to as "fishapods" by their discoverers, being half-fish half-tetrapods, in appearance and limb morphology. 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. Tetrapodomorpha contains several groups of related lobe-finned fishes, collectively known as the osteolepiforms. The Tetrapodamorpha minus the crown group Tetrapoda is 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).

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

Classification

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

After Benton, 2004 [1]:

• Subclass Sarcopterygii
  • Infraclass Tetrapodomorpha
    • • Order Rhizodontida
    • Superorder Osteolepidida (or Osteolepiformes)
      • Order Osteolepiformes
        • Family Tristichopteridae
      • Order Panderichthyida

Phylogeny

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

Tetrapodomorpha	Eusthenopteron Panderichthys Livoniana Elpistostege  Elginerpetidae  Elginerpeton Obruchevichthys Metaxygnathus Ventastega Tetrapoda

References

• Mikko Haaramo. "Tetrapodomorpha - Terrestrial vertebrate-like sarcopterygians". http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Deuterostoma/Chordata/sarcopterygii/tetrapodomorpha.htm. Retrieved 6 April 2006. 
• P. E. Ahlberg and Z. Johanson (1998). "Osteolepiforms and the ancestry of tetrapods". Nature 395 (6704): 792–794. Bibcode 1998Natur.395..792A. doi:10.1038/27421. 
• Michel Laurin, Marc Girondot and Armand de Ricqlès (2000). "Early tetrapod evolution". TREE 15 (3). http://www.ese.u-psud.fr/epc/conservation/Publi/abstracta/AE_TREE2000.pdf.