Stegocephalia

Taxobox
name = Stegocephalia*
fossil_range =Devonian to Jurassic

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image_caption="Proterogyrinus", an Anthracosaur
regnum = Animalia
phylum = Chordata
unranked_classis = Amphibia "sensu lato"
ordo = Stegocephalia*

Stegocephalia is an old term for early (generally large) amphibians, comprising all pre-Jurassic and some later extinct large amphibians of more or less salamander-like build. The term comes from Greek "stego cephalia" - "roofed head", and refer to the copious amounts of dermal armour some of the larger forms evidently had. Originally, the term was used as a systematic unit at the rank of order. Later, the term Labyrinthodontia was used, devided into three order. [Romer, A. S., (1947, revised ed. 1966) Vertebrate Paleontology, University of Chicago Press, Chicago] However, bout the Stegocephalia (and the Labyrinthodontia) being paraphyletic, the name is now used in an informal way to denote the early non-piscine vertebrates, excluding amniotes (the first reptiles and their descendants) and modern lissamphibians).

Recent taxonomic work has redefined stegocephalians as all vertebrates more closely related to temnospondyli than to "Panderichthys" (the closest relative of tetrapods known to have retained paired fins, see below) [Laurin M. (1998): The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I-systematics, middle ear evolution, and jaw suspension. "Annales des Sciences Naturelles", Zoologie, Paris, 13e Series 19: pp 1-42.] Therefore, stegocephalia includes all vertebrate groups that have toes rather than fins, and a few ("Elginerpeton", "Metaxygnathus", "Ventastega" and possibly "Hynerpeton") that may retain paired fins. Contrary to the old usage of this term, the Stegocephali refers to a clade in this scheme, encompassing all presently living land vertebrates as well as their early amphibian ancestors.

ystematic overview of Stegocephalians

Systematic based on Colbert (1969) [Colbert, E. H., (1969), "Evolution of the Vertebrates", John Wiley & Sons Inc (2nd ed.): pp 102-103]

(from lobe-fined fish) `- "Eusthenopteron" (advanced lobe-fined fish) `- "Panderichthys" (lobe-fined fish with limb-like fins) `- "Tiktaalik" (transitional fish/amphibian) `- "Acanthostega" (early amphibian with fishlike gills) `- "Ichthyostega" (early amphibian) `- "Crassigyrinus" (early amphibian)
- Loxommatidae(eel-like primitive temnospondyles)
`-Temnospondyls (large, flat-headed stegocephalians) `- Anthracosaurs (reptile-like amphibians)
- Seymouriamorphs (advanced repile-like amphibians)
|- "Westlothiana" (small amphibian or possibly early reptile)
`+ Diadectomorphs (earliest reptiles or sister groups of reptiles)
`- amniotes (i.e. first reptiles) `- Lepospondyls (small stegocephalians) `?- Lissamphibia (modern amphibians)

References