Tetrapodomorpha
Clade of vertebrates
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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.
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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
The stem Tetrapoda are a grade containing all animals more closely related to extant four-legged vertebrates than to their closest extant relatives (the lungfish), but excluding the crown group Tetrapoda themselves (i.e. stem tetrapods are paraphyletic and thus do not form a natural group). Thus, the transition from "fish-like" finned sarcopterygians to tetrapods with fully developed limbs and digits occurred among stem tetrapods.
Content of the group
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
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.
The following cladogram follows the results found by Clement et al. (2021).