Thrinaxodon is a member of the group Therapsida, which also contains the group Mammalia. Therapsida is, in turn, a member of the group Synapsida. Thrinaxodon is, thus, a close relative of the mammals, but not a mammal itself.
The following images are CAT-Scan pictures of a skull of Thrinaxodon that is housed in the University of California Museum of Paleontology. They were created by Dr. Tim Rowe of the University of Texas at Austin. Dr. Rowe received his Ph.D. from Berkeley and did his dissertation work on the relationships of other members of the group Synapsida to Mammalia. His images of Thrinaxodon are available on CD-ROM, published by the University of Texas Press. The CD includes cross-sectional views of the Thrinaxodon skull in three dimensions, movies that "scan through" the skull by linking all of the CAT-Scan images, and digital copies of all of the original publications on Thrinaxodon.
One of these, Estes, 1961 is included here. You can read it to find out more about Thrinaxodon and what you are seeing in the CAT-Scan images below.