''Arizonerpeton'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

nectridea Nectridea is an extinct order of lepospondyl tetrapods from the Carboniferous and Permian periods, including animals such as '' Diplocaulus''. In appearance, they would have resembled modern newts or aquatic salamanders, although they are not cl ...

tetrapodomorph Tetrapodomorpha (also known as Choanata) is a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advanced for ...

s. It contains a single species, ''Arizonerpeton wellsi''. It lived in what is now the Swisshelm Mountains of modern-day

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

, United States. This locality belongs to the Black Prince Limestone Formation, which is dated to the middle Pennsylvanian sub-period of the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

period.

Description

''Arizonerpeton'' is known exclusively from a collection of unusual

vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...

e collected by Robert Wells, the namesake of its

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

. These vertebrae can be designated as belonging to the order Nectridea, a collection of long-tailed limbed vertebrates that also includes the famous "boomerang-headed" '' Diplocaulus''. Like other nectrideans, ''Arizonerpeton's'' vertebrae had a single main body (a pleurocentrum) fused to a plate-like neural spine jutting out of the top. The flared rims of the front and rear surfaces of the pleurocentum possesses small structures which would have formed tongue-and-groove articulations with other vertebrae. The "tongue" projected forward from the front rim while the "groove" was present between two projections on the rear rim. The only definitive feature classifying this genus as a nectridean lies in the fact that there are also two pairs of joint plates ( zygapophyses) on each end of the vertebrae. The vertebrae are amphicoelous (hourglass-shaped) from the side, but not strongly so, and are covered in small pits. On the side of each vertebra, an extension known as a transverse process juts out perpendicularly, although also occasionally with a very slight upward angle. The most unusual feature of these vertebrae lie in the fact that the neural spines are bifurcated. This means that a tall gap is present between the front and rear portions of each neural spine, effectively making it seem like there are two neural spines joined at the base. Vertebrae which are believed to have been part of the sacral (hip) region have shorter neural spines than those of the back, and are sightly titled backwards. Less extreme neural spine bifurcation is also known in the early diplocaulid '' Diceratosaurus'' and some specimens of the urocordylid '' Ctenerpeton'', two other nectrideans. In addition, the pitted vertebrae are somewhat similar to those of diplocaulids. However, the deep neural spine bifurcation and "tongue-and-groove" articulations possessed by ''Arizonerpeton'' are unknown in any other nectridean groups. As a result, ''Arizonerpeton'' is usually treated as a member of Nectridea ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

''.

References

{{Taxonbar, from=Q4791513 Nectridea Pennsylvanian sarcopterygians of North America Fossil taxa described in 1985