Nectridea is the name of 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 close relatives of modern amphibians.^[1] They were characterized by long, flattened tails to aid in swimming, as well as numerous features of the vertebrae.

Quick Facts Scientific classification, Subgroups ...

Nectridea Temporal range: Late Carboniferous–Lopingian PreꞒ Ꞓ O S D C P T J K Pg N
Skeleton and model of Diplocaulus.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Sarcopterygii
Clade:	Tetrapodomorpha
Clade:	Stegocephali
Order:	†Nectridea Miall, 1875
Subgroups
Arizonerpeton Diplocaulidae Scincosauridae Urocordylidae

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Nectrideans are a diverse group of tetrapods, including the aquatic Urocordylidae, the presumably terrestrial Scincosauridae, and the bizarre horned members of Diplocaulidae (also known as Keraterpetonidae), which includes the "boomerang-headed" Diplocaulus, one of the most famous genera of prehistoric amphibians (in the traditional sense of the word). By the time the earliest known nectrideans appeared in the Late Carboniferous fossil record, they had already diversified into these families, indicating that basal nectrideans are unknown. These different families are united primarily by features of the spinal column rather than the skull.

Both diplocaulids and urocordylids are considered to be completely aquatic, although only diplocaulids possess lateral line canals on their skulls. However, this does not mean that urocordylids lacked lateral lines, as several modern amphibians possess that organ without any skeletal indication. Scincosaurids have much more well-ossified limbs and thinner tails than other nectrideans, and are usually considered to be terrestrial. However, some have been reported to possess lines of pits on the skull, which may be an indication that they were amphibious rather than purely terrestrial.^[4]

A trace fossil known as Hermundurichnus fornicatus of a tetrapod resting on a lake bed may have been attributed to Diplocaulus or an animal like it. This trace indicates that the underside of nectrideans was covered in small, diamond-shaped scales and that the "horns" of the skull were connected with the body by flaps of skin.^[5]

There has been some controversy over the precise classification of Nectridea over the past century, and even whether it constitutes a valid monophyletic clade. A few older studies consider nectrideans to be very basal tetrapods, related to Ichthyostega or colosteids. However, most studies generally place nectrideans inside the subclass Lepospondyli. Aistopoda, a group of legless lepospondyls, have often been allied with particular families of nectrideans, but not the order as a whole. For example, Anderson (2001) positioned diplocaulids as close relatives of Aistopods with urocordylids and scincosaurids as progressively more primitive members of the group. On the other hand, Ruta et al. (2003) claimed that urocordylids were closer to aistopods. As aistopods do not possess most nectridean features, they have traditionally been excluded from the group. However, this would make Nectridea an invalid paraphyletic group as the aistopods, may have been descended from nectrideans, are not considered nectrideans themselves.^[6]

The following is a cladogram from the Ruta et al. (2003):