Key points are not available for this paper at this time.
John D. McEachran, Katherine A. Dunn, Phylogenetic Analysis of Skates, a Morphologically Conservative Clade of Elasmobranchs (Chondrichthyes: Rajidae), Copeia, Vol. 1998, No. 2 (May 1, 1998), pp. 271-290
McEachran et al. (Fri,) studied this question.