Ctenorhachis is an extinct genus of the family Sphenacodontidae

Sphenacodontidae

Sphenacodontidae is a family of small to large, advanced, carnivorous, Late Pennsylvanian to middle Permian pelycosaurs. Primitive forms were generally small in size , but during the later part of the early Permian these animals grew progressively larger , to become the top predators of their...

. Ctenorhachis was related to Dimetrodon

Dimetrodon

Dimetrodon was a predatory synapsid genus that flourished during the Permian period, living between 280–265 million years ago ....

, but did not belong to the same subfamily as Dimetrodon and Sphenacodon

Sphenacodon

Sphenacodon was a pelycosaur that was about in length. Sphenacodon belongs to the family Sphenacodontidae, a lineage that was related to the therapsids...

, being a more basal

Basal (phylogenetics)

In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 member of Sphenacodontidae. Ctenorhachis lived in the Early Permian epoch. Two specimens are known that have been found from the Wichita Group outcropping in Baylor

Baylor County, Texas

As of the census of 2000, there were 4,093 people, 1,791 households, and 1,156 families residing in the county. The population density was 5 people per square mile . There were 2,820 housing units at an average density of 3 per square mile...

 and Archer

Archer County, Texas

Archer County is a county located in the US state of Texas, and was formed in 1858 from Fannin County. It is part of the Wichita Falls, Texas, Metropolitan Statistical Area. As of 2005, the population estimate is 9,095, up from 8,854 in 2000. Its county seat is Archer City. Archer is named for...

 counties, north-central Texas. Only the vertebrae and pelvis

Pelvis

In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...

 are known. Articulated vertebrae from the holotype specimen possess blade like neural spines that are greatly enlarged, although not nearly to the extent that can be seen in more derived sphenacodontds such as Dimetrodon and Secodontosaurus

Secodontosaurus

Secodontosaurus was a pelycosaur that lived in Texas during the Early Permian age. Though it had the same body style as other sphenacodontids, it had an unusually low and narrow skull. It had a long neural spines, like Dimetrodon, which Secodontosaurus was related to...

, in which they form a large sail. The pelvis is nearly identical to that of Dimetrodon. As suggested in the original description of the genus, Ctenorhachis may represent a short-spined sexual dimorph, although the authors find this unlikely.