A detailed study of the anterior osteology and myology of Callisaurus, Cophosaurus, Holbrookia, and Uma reveals the phylogenetic relationships among the sand lizards. An SPSS discriminant analysis of osteological characters combined with myological characters indicates that Callisaurus is most primitive, Cophosaurus and Holbrookia are most closely related, and Uma is the most distinct of the sand lizard genera. Because of close relationships between Cophosaurus and Holbrookia, it is postulated that earlessness evolved once, and Cophosaurus is returned to synonymy under Holbrookia.
Cox, Douglas C. and Tanner, Wilmer W.
"Osteology and myology of the head and neck regions of Callisaurus, Cophosaurus, Holbrookia, and Uma (Reptilia: Iguanidae),"
Great Basin Naturalist: Vol. 37
, Article 3.
Available at: http://scholarsarchive.byu.edu/gbn/vol37/iss1/3