Stegocephalia

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Stegocephalians
Temporal range: Late Devonian–Recent
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Sarcopterygii
Order: Elpistostegalia
Clade: Stegocephalia
Cope, 1868
Subgroups

Elpistostege
Tiktaalik
Tetrapoda

Stegocephalia is an older name for the large fossil amphibians, comprising all pre-Jurassic and some later extinct large groups having more or less salamander-like build. The term was coined in 1868 by American palaentologist 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] In recent times, Canadian paleontologist Michel Laurin has used as a phylogenetic expression to encompass all vertebrates with toes rather than fins, i.e. tetrapods.[2]

History of classification[edit]

Typical 19th century depiction of a 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 cladistically 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[edit]

Main article: Labyrinthodontia

Below is a suggested evolutionary tree of tetrapods and their relatives, from Colbert 1969 and Caroll 1997.[7][8] Dashed lines indicate relationships that commonly vary between authors.

From lobe-finned fish 

Eusthenopteron Eusthenopteron BW.jpg




Panderichthys Panderichthys BW.jpg


Stegocephalia

Tiktaalik Tiktaalik BW.jpg


Tetrapoda

Acanthostega Acanthostega BW.jpg




Ichthyostega Ichthyostega BW.jpg




Crassigyrinus Crassigyrinus BW.jpg





Loxommatidae Eucritta1DB.jpg



Temnospondyls CyclotosaurusDB2 White background.jpg




Reptile-like amphibians

Seymouriamorpha Seymouria BW.jpg




Westlothiana Westlothiana BW.jpg




Diadectomorpha Diadectes1DB.jpg


Amniota

Class Reptilia Gracilisuchus BW.jpg





Batrachomorpha

Lepospondyls Hyloplesion.jpg (small Labyrinthodonts)



Lissamphibia Prosalirus BW.jpg (modern amphibians)











References[edit]

  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'Academie des sciences. Serie III, Sciences de la vie 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. ^ Colbert, E. H., (1969), Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
  8. ^ Carroll, R. L. (1997): Patterns and Processes of Vertebrate Evolution. Cambridge University Press, Cambridge. 464 pages

External links[edit]