The 16 species and subspecies of the Ambystomidae of the United States are placed into six groups according to their throat morphology as follows: The Dicamptodon group, the Rhyacotriton group, the A. Annulatum–A. cingulatum group, the A. gracile-A. jeffersonianum-A. mabeei-A. macrodactylum-A. maculatum group, the A. talpoideum group, the A. opacum-A. texanum group, and the A. rosaceum-A. t. tigrinum-A. t. melanostictum-A. t. nebulosum group. Dicamptodon appears to be the most primitive group and Rhyacotriton the most specialized. Based on the morphology of the throat region it is suggested that A. rosaceum be placed in the tiger salamander group as a subspecies of A. tigrinum.
The results of this investigation support the hypothesis that Ambystomidae is primitive to Salamandridae and to Plethodontidae. The findings which have led to this conclusion are based upon the protrusibility of the tongue, and upon the structure and presence of the epibranchial, otoglossal, second radial, and second basibranchial cartilages of the three families.
Krogh, John E. and Tanner, Wilmer W.
"The hyobranchium and throat myology of the adult Ambystomidae of the United States and northern Mexico,"
Brigham Young University Science Bulletin, Biological Series: Vol. 16
, Article 1.
Available at: http://scholarsarchive.byu.edu/byuscib/vol16/iss1/1