A new sphenacodontid pelycosaur (Synapsida) from the Wichita Group, Lower Permian of north-central Texas

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Abstract
Two specimens collected recently from the middle Wichita Group (Wolfcampian–Leonardian Series, Early Permian) of Baylor and Archer Counties, Texas, represent a new sphenacodontid pelycosaur, Ctenorhachis jacksoni, gen. et sp. nov. The type includes of an articulated column of 31 vertebrae (presumably numbers 4 through 34) and an attached, nearly complete pelvis; the second specimen consists primarily of dorsal vertebrae. The vertebrae are typically sphenacodontid, with well-developed keels in the cervical and anterior dorsal series and pronounced pits on the lateral surface of the neural arch above elongated transverse processes. The pelvis is very similar to, if not indistinguishable from, Dimetrodon. Ctenorhachis is distinguished from all other sphenacodontids by neural spines that are only modestly heightened and uniformly blade-like antero-posteriorly. These spines are more primitive than those of Dimetrodon and Secodotosaurus and possibly more primitive than those of Ctenospondylus. A survey of sphenacodontid taxa from the Wichita Group of North–Central Texas indicates that existing classifications are handicapped by fragmentary type specimens that may be generically indeterminate and by a reliance upon size differences that are largely unquantified and upon stratigraphie data of questionable significance.