Reexamination of the holotype and the description of new material of Edaphosaurus novomexicanus from the Permo-Pennsylvanian Cutler Formation of north-central New Mexico reveals that this species is the most primitive member of the Edaphosaurus clade. This assignment, based previously upon supposed plesiomorphies of the postcrania, is supported here by two primitive character states that occur in a derived form in the Texan species of Edaphosaurus. The presence of two autapomorphies in E. novomexicanus suggests that this species is probably not a morphological relict, as previously supposed, but may be part of a lineage that did not exhibit the level of diversification seen in its probable sister group, the clade of Texan edaphosaurids (E. boanerges, E. cruciger, and E. pogonias).

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