| Stegocephali | |
|---|---|
 | |
| Examples of stegocephalians (clockwise from top left):Litoria phyllochroa,Acanthostega gunnari,Vulpes vulpes, andTyto alba | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sarcopterygii |
| Clade: | Tetrapodomorpha |
| Clade: | Elpistostegalia |
| Clade: | Stegocephali Cope, 1868 |
| Subgroups | |
See text. | |
Stegocephali (often spelledStegocephalia, fromGreekστεγοκεφαλια, lit. "roofed head") is aclade ofvertebrateanimals containing all fullylimbedtetrapodomorphs. It is equivalent to a broad definition of thesuperclassTetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first vertebrate with four limbs each withdigits in the extremity (pentadactyly), rather thanfins of theirsarcopterygian relatives.
Stegocephalians include both the modern lineage of limbed vertebrates (thecrown group tetrapods, including modernamphibians,reptiles,birds andmammals) as well as a portion of thestem group, the earliest limbed tetrapodomorphs such asIchthyostega andAcanthostega, which evolved in theDevonian period long before the origin of the crown group. Manypaleontologists prefer a stricter definition of Tetrapoda which applies solely to the crown group, excluding earlier types of limbed tetrapodomorphs. Stegocephali was re-established to replace the broad definition of Tetrapoda, resolving the usage of two conflicting definitions in discussions of tetrapod evolution.
Stegocephali was coined in 1868 by the American paleontologistEdward Drinker Cope, who used it as a general category of prehistoric amphibians. This name was in reference to the skull form of many early tetrapods, with a low, solid shape combining numerous strongly-textureddermal bones.[1] In its original usage, the term quickly became obsolete. In 1998, Canadian paleontologistMichel Laurin repopularized the term and provided a formal phylogenetic definition as amonophyleticclade containing both crown-group and stem-group tetrapods. Laurin's Stegocephali is roughly defined as including all vertebrates closer to modern tetrapods than toPanderichthys. This definition was intended to include taxa with digits rather than fins, except wheresecondarily lost.[2][3] Another definition, published inPhylonyms, defines the group as including all taxa closer toEryops than toTiktaalik,Panderichthys, orEusthenopteron.[4] The discovery of theZachelmie trackways in 2010 suggests that stegocephalians possibly emerged 395 Ma or earlier.[5]
Originally, the term was used as asystematic unit at the rank oforder. The term rose to prominence in American and British science in the 19th century, though the largely equivalent termLabyrinthodontia had been coined 18 years earlier byHermann Burmeister in reference to the tooth structure.[6] The terms were used interchangeably during the early 20th century, usually divided into three orders.[7] Cope originally spelled the term as "Stegocephali", thoughA.S. Woodward introduced a popular alternative spelling, "Stegocephalia", in 1898.[4] In their original usage, Stegocephali (and the Labyrinthodontia) areparaphyletic, the name is now used in an informal way to denote the early non-piscinevertebrates, excludingamniotes (the firstreptiles and their descendants) and modernlissamphibians.
PaleontologistMichel Laurin took up the older term and defined stegocephaliansphylogenetically as allvertebrates more closely related toTemnospondyli than toPanderichthys (the closest relative oftetrapods known to have retained paired fins, see below).[2] Therefore, Stegocephali includes all vertebrate groups that have toes rather than fins, and a few (Elginerpeton,Metaxygnathus,Ventastega and possiblyHynerpeton) that may have retained paired fins. Contrary to the old usage of this term, the Stegocephali refers to aclade in this scheme. This concept of the clade Stegocephali was chosen to substitute for the nameTetrapoda by those who sought to restrict Tetrapoda to thecrown group.[8] As such, it encompasses all presently living land vertebrates as well as their early amphibious ancestors.
Below is an evolutionary tree of stegocephalians, as recovered from an analysis by Swartz in 2012.[9]
| Elpistostegalia |
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As recovered by Clacket al. 2016:[10]
| Elpistostegalia |
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