To find out about important characters in the systematics of Pterygoplichthys disjunctivus, you may wish to consider the original description by Weber 1991:
Weber, C. 1991: Nouveaux taxa dans Pterygoplichthys sensu lato (Pisces, Siluriformes, Loricariidae). Revue Suisse de Zoologie v. 98 (no. 3): 637-643.
There is also a revision of the genus Pterygoplichthys by Weber 1992 which provides a key to the species of the genus:
Weber, C. 1992: Révision du genre Pterygoplichthys sensu lato (Pisces, Siluriformes, Loricariidae). Revue française d'Aquariologie Herpétologie v. 19 (no. 1-2): 1-36.
Finally, there is the description of an additional species of the genus:
Armbruster, J. W. and L. M. Page 2006: Redescription of Pterygoplichthys punctatus and description of a new species of Pterygoplichthys (Siluriformes: Loricariidae). Neotropical Ichthyology v. 4 (no. 4): 401-409.