Stegocephalia

Stegocephalia is a name used for four-limbed stem-tetrapods, and their amphibian-grade descendants, and in phylogenetic nomenclature for all tetrapods. The term was coined in 1868 by American paleontologist Edward Drinker Cope and comes from Greek στεγοκεφαλια - "roofed head", and refer to the copious amounts of dermal armour some of the larger primitive forms evidently had.[1] Canadian paleontologist Michel Laurin gave the group its first formal phylogenetic definition, roughly including all vertebrates with digits rather than fins, except where secondarily lost.[2]

Stegocephalia Temporal range: Late Devonian - Present, 375–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Examples of stegocephalians: Litoria phyllochroa, Tyto alba, Vulpes vulpes and Tiktaalik roseae.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Elpistostegalia
Clade:	Stegocephalia Cope, 1868
Subgroups
†Tiktaalik †Elpistostege †Ichthyostegalia †Whatcheeriidae †Colosteidae †Baphetoidea Tetrapoda

History of classification

Typical 19th century depiction of a primitive stegocephalian, based on a modern toad

Originally, the term was used as a systematic unit at the rank of order. The term rose to prominence in American and British science in the 19th century, though the largely equivalent term Labyrinthodontia had been coined 18 years earlier by Hermann Burmeister in reference to the tooth structure.[3] The terms were used interchangeably during the early 20th century, usually divided into three orders.[4] However, 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).

Paleontologist Michel Laurin took up the older term and defined stegocephalians phylogenetically as all vertebrates more closely related to Temnospondyli than to Panderichthys (the closest relative of tetrapods known to have retained paired fins, see below) [5] Therefore, Stegocephalia includes all vertebrate groups that have toes rather than fins, and a few (Elginerpeton, Metaxygnathus, Ventastega and possibly Hynerpeton) that may have retained paired fins. Contrary to the old usage of this term, the Stegocephali refers to a clade in this scheme. This concept of the clade Stegocephalia was chosen to substitute for the name Tetrapoda by those who sought to restrict Tetrapoda to the crown group.[6] As such, it encompasses all presently living land vertebrates as well as their early amphibious ancestors.

Phylogeny

Below is an evolutionary tree of stegocephalians, as recovered from an analysis by Swartz in 2012.[7]

Elpistostegalia	Panderichthys   Stegocephalia     Tiktaalik     Elpistostege         Elginerpeton       Ventastega       Acanthostega       Ichthyostega       Whatcheeriidae       Colosteidae       Crassigyrinus       Baphetidae     Crown group Tetrapoda

References

1. Cope E. D. 1868. Synopsis of the extinct Batrachia of North America. Proceedings of the Academy of Natural Sciences of Philadelphia: pp 208-221
2. Laurin, M; Soler-Gijón, R (May 2001). "The oldest stegocephalian from the Iberian peninsula: evidence that temnospondyls were euryhaline". Comptes Rendus de l'Académie des Sciences, Série III. 324 (5): 495–501. PMID 11411292.
3. Burmeister, H. (1850): Die Labyrinthodonten aus dem Saarbrücker Steinkohlengebirge, Dritte Abtheilung: der Geschichte der Deutschen Labyrinthodonten Archegosaurus. Berlin: G. Reimer, 74 pp.
4. Romer, A. S., (1947, revised ed. 1966) Vertebrate Paleontology, University of Chicago Press, Chicago
5. 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.
6. Laurin & al. Terrestrial Vertebrates. Stegocephalians: Tetrapods and other digit-bearing vertebrates. Tree of life
7. 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.

External links

• Tree of Life: Terrestrial Vertebrates