Stegocephalia
Encyclopedia
Stegocephalia is an old term for early (generally large) amphibian
s, comprising all pre-Jurassic and some later extinct large amphibians of 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 forms evidently had.
Originally, the term was used as a systematic
unit at the rank of order
. Later, the term Labyrinthodontia
was used, divided into three orders. 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 lissamphibia
ns).
has taken up the old 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) 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
. As such, it encompasses all presently living land vertebrates as well as their early amphibious ancestors.
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s, comprising all pre-Jurassic and some later extinct large amphibians of more or less salamander-like build. The term was coined in 1868 by American palaentologist Edward Drinker Cope
Edward Drinker Cope
Edward Drinker Cope was an American paleontologist and comparative anatomist, as well as a noted herpetologist and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested in science; he published his first scientific paper at the age of nineteen...
and comes from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
στεγοκεφαλια - "roofed head", and refer to the copious amounts of dermal armour
Armour (zoology)
Armour in animals is external or superficial protection against attack by predators, formed as part of the body , usually through the hardening of body tissues, outgrowths or secretions. It has therefore mostly developed in 'prey' species...
some of the larger forms evidently had.
Originally, the term was used as a systematic
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
unit at the rank of order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
. Later, the term Labyrinthodontia
Labyrinthodontia
Labyrinthodontia is an older term for any member of the extinct subclass of amphibians, which constituted some of the dominant animals of Late Paleozoic and Early Mesozoic times . The group is ancestral to all extant landliving vertebrates, and as such constitutes an evolutionary grade rather...
was used, divided into three orders. However, the Stegocephalia (and the Labyrinthodontia) being paraphyletic
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
, 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 lissamphibia
Lissamphibia
The subclass Lissamphibia includes all recent amphibians and means smooth amphibia.Extant amphibians fall into one of three orders — the Anura , the Caudata or Urodela , and the Gymnophiona or Apoda .Although the ancestry of each group is still unclear, all share certain common characteristics,...
ns).
Systematic overview of Stegocephalians
Below is a suggested evolutionary tree of Labyrinthodontia, from Colbert 1969 and Caroll 1997. Dashed lines indicate relationships that commonly vary between authors.Cladistic use of Stegocephalia
Paleontologist Michel LaurinMichel Laurin
Michel Laurin is a Canadian vertebrate paleontologist specialising in the origin and phylogeny of tetrapods, comparative biology and paleobiology. As an undergraduate he worked in the laboratory of Robert L. Carroll, and took his doctor thesis on the osteology of seymouriamorphs under Robert R....
has taken up the old term and defined stegocephalians cladistically
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
as all vertebrates more closely related to temnospondyli
Temnospondyli
Temnospondyli is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carboniferous, Permian, and Triassic periods. A few species continued into the Cretaceous. Fossils have been found on every continent...
than to Panderichthys
Panderichthys
Panderichthys is a 90–130 cm long fish from the Devonian period 397 million years ago, of Latvia. It is named after the german-baltic palaeontologist Christian Heinrich Pander. It has a large tetrapod-like head...
(the closest relative of tetrapods known to have retained paired fins, see below) Therefore, stegocephalia includes all vertebrate groups that have toes rather than fins, and a few (Elginerpeton
Elginerpeton
Elginerpeton is a monotypic genus of early tetrapod, the fossils of which were recovered from Scat Craig, Scotland, in rocks dating to the late Devonian Period...
, Metaxygnathus
Metaxygnathus
Metaxygnathus is an extinct genus of tetrapodomorph lobe-finned fishs found in Late Devonian deposits of New South Wales, Australia .It is only known from a lower jawbone.-References:...
, Ventastega
Ventastega
Ventastega was a basal tetrapod that lived during the Famennian subdivision of the Late Devonian period approximately 374.5 to 359.2 million years ago, though Ventastega origins as a tetrapod lineage are probably seated in the preceding Frasnian period of the Late Devonian when a surge of...
and possibly Hynerpeton
Hynerpeton
Hynerpeton was a basal carnivorous tetrapod that lived in the lakes and estuaries of the Late Devonian period around 360 million years ago. Like many primitive tetrapods, it is sometimes referred to as an "amphibian", though it is not a true member of the modern Lissamphibia...
) that may have retained paired fins. Contrary to the old usage of this term, the Stegocephali refers to a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
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
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
. As such, it encompasses all presently living land vertebrates as well as their early amphibious ancestors.