The Rachitomi were a group of extinct Palaeozoic labyrinthodont amphibian

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, according to an earlier classification system. They are defined by the structure of the vertebrae, having large, semi-circular intercentra below the notochord

Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...

 and smaller paired, though prominent pleurocentra on each side above and behind, forming anchoring pints for the rib

Rib

In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...

s.

This form of complex backbone was found in some crossopterygian fish, the Ichthyostegalia, most 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...

 and some Reptiliomorpha

Reptiliomorpha

Reptiliomorpha refers to an order or subclass of reptile-like amphibians, which gave rise to the amniotes in the Carboniferous. Under phylogenetic nomenclature, the Reptiliomorpha includes their amniote descendants though, even in phylogenetic nomenclature, the name is mostly used when referring to...

. Primitive reptiles retained the complex rachiotomous vertebrae, though with the pleurocentra being the more dominant structures. As a phylogentic unit, the Rachitomi thus form a paraphyletic unit

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...

.