Rhachitomi is a group of

temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order (biology), order of small to giant tetrapods—often considered Labyrinthodontia, primitive amphi ...

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

s that includes all temnospondyls except

edopoids Edopoidea is a clade of primitive temnospondyl amphibians including the genus ''Edops'' and the family Cochleosauridae. Edopoids are known from the Late Carboniferous and Early Permian of North America and Europe, and the Late Permian of Afric ...

and dendrerpetontids. It was established as a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

name by German paleontologist Rainer R. Schoch in 2013, although the name had first been established in 1919 by British paleontologist

D. M. S. Watson Prof David Meredith Seares Watson FRS FGS HFRSE LLD (18 June 1886 – 23 July 1973) was the Jodrell Professor of Zoology and Comparative Anatomy at University College, London from 1921 to 1951. Biography Early life Watson was born in the Highe ...

to encompass an

evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Definition An evolutionary grade is a group of ...

of temnospondyls leading to the group

Stereospondyli The Stereospondyli are a group of extinct temnospondyl amphibians that existed primarily during the Mesozoic period. They are known from all seven continents and were common components of many Triassic ecosystems, likely filling a similar ecologi ...

. American paleontologist

Alfred Romer Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) was an American paleontologist and biologist and a specialist in vertebrate evolution. Biography Alfred Romer was born in White Plains, New York, the son of Harry Houston Romer an ...

used the term in a similar sense, grouping most Permian and Triassic temnospondyls under Rhachitomi. A similar name that appeared earlier in the scientific literature is Rachitomi, which was named by American paleontologist

Edward Drinker Cope Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontologist, comparative anatomist, herpetologist, and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested ...

in 1882. Rachitomi was commonly used in the late nineteenth and early twentieth century to include early amphibians such as ''

Eryops ''Eryops'' (; from Greek , , 'drawn-out' + , , 'face', because most of its skull was in front of its eyes) is a genus of extinct, amphibious temnospondyls. It contains the single species , the fossils of which are found mainly in early Permian (a ...

'' and ''

Archegosaurus ''Archegosaurus'' is a genus of temnospondyl amphibian which lived during the Asselian to Wuchiapingian stages of the Permian, around 299-253 million years ago. The remains of this animal, consisting of at least 90 partial skeletons (mostly skull ...

'' that had rhachitomous vertebrae. Many early tetrapods have vertebrae that are split into two parts below the

notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consis ...

: a pleurocentrum and an intercentrum. In rhachitomous vertebrae, the intercentrum is large and semicircular, while the pleurocentrum divided into two smaller paired elements. Schoch defined Rhachitomi as a

node-based taxon Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...

to include four major and well-supported clades of temnospondyls:

Dvinosauria Dvinosaurs are one of several new clades of Temnospondyl amphibians named in the phylogenetic review of the group by Yates and Warren 2000. They represent a group of primitive semi-aquatic to completely aquatic amphibians, and are known from t ...

Eryopidae Eryopidae were a group of medium to large amphibious temnospondyli, known from North America and Europe. They are defined as all eryopoids with interpterygoid vacuities (spaces in the interpterygoid bone) that are rounded at the front; and larg ...

and a clade formed by

Zatracheidae Zatracheidae (sometimes mistakenly spelled Zatrachydidae or Zatrachysdidae) is a family of Late Carboniferous and Early Permian temnospondyli, temnospondyl amphibians known from North America and Europe. Zatracheidids are distinguished by lateral ...

and

Dissorophoidea Dissorophoideans are a clade of medium-sized, temnospondyl amphibians that appeared during the Moscovian in Euramerica, and continued through to the Late Permian and the Early Triassic of Gondwana. They are distinguished by various details of th ...

. Not all members of Rhachitomi have rhachitomous vertebrae; the largest subgroup, Stereospondyli, lacks pleurocentra. Below is a

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...

from Schoch's analysis showing the placement of Rhachitomi within Temnospondyli:

References

{{Taxonbar, from=Q7320024 Eutemnospondyls Carboniferous temnospondyls Permian temnospondyls Triassic temnospondyls Jurassic temnospondyls Cretaceous temnospondyls Mississippian first appearances Early Cretaceous extinctions