Nectrideans are a Carboniferous and Permian group of tetrapods, typically regarded as 'amphibians' and classified alongside microsaurs and lysorophians within the group Lepospondyli. However, close relationships with the long-bodied aïstopods have been supported by some: indeed, nectrideans have been found to be paraphyletic to aïstopods in some studies (Ruta et al. 2003). Thomson & Bossy (1970) gave the name Holospondyli to a hypothesised nectridean + aïstopod clade. Diplocaulus - the most famous nectridean - is typically portrayed as here. However, fossils show that these 'horned' nectrideans actually had skin webs connecting the tips of their 'horns' to their bodies.
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Ruta, M., Coates, M. I. & Quicke, D. L. J. 2003. Early tetrapod relationships revisited. Biological Reviews 78, 251-345.
Thomson, K. S. & Bossy, K. H. 1970. Adaptive trends and relationships in early Amphibia. Forma et Functio 3, 7–31.