In 2017, Bulbasaurus was assigned to the Geikiidae clade of dicynodonts on account of its prominent nasal-frontal ridge, its relatively wide interorbital region, and its twisted squamosal on the postorbital bar.
This assignment was supported by a phylogenetic analysis based on that conducted by Kammerer et al.
in 2011, which found it as the closest relative of Aulacephalodon on the basis of it lacking a ridge behind its tusk (which is ontogenetically influenced).
However, this assignment is somewhat questionable, and forcing Bulbasaurus as a basal geikiid outside of the Geikiinae (Aulacephalodon, Geikia, and Pelanomodon) only requires one additional evolutionary step.
Overall, the Cryptodontia (including the Geikiidae) were very unstable, suggesting that current datasets may not be able to sufficiently evaluate their relationships.
An excerpt from the consensus of two phylogenetic trees, illustrating the relationships between cryptodontians, is shown below.

Tropidostoma, the genus Bulbasaurus was originally assigned to, exhibits two distinct morphologies - a robust morph with short snout and large tusks, and a gracile morph with long snout and small tusks, which probably represents sexual dimorphism as in other dicynodonts.
However, Bulbasaurus matches neither of those morphologies; it differs from the genus Tropidostoma as a whole in many respects (addressed above).
Additionally, even the holotype of Tropidostoma (which is probably immature judging by the unerupted tusks) is larger than adult specimens of Bulbasaurus, which further warrants their separation.
The same is true of Bulbasaurus and Aulacephalodon, in addition to their differing boss morphologies and different interorbital widths.
Bulbasaurus also differs from the problematic specimens BP/1/763 (assigned once to its own genus, Proaulacocephalodon, or to a juvenile Aulacephalodon) and TM 1480 (assigned once to Dicynodon hartzenbergi) by its larger tusks and wider interorbital region, among other characteristics.
These specimens are additionally from the younger Cistecephalus assemblage zone.
In 2017, Bulbasaurus was assigned to the Geikiidae clade of dicynodonts on account of its prominent nasal-frontal ridge, its relatively wide interorbital region, and its twisted squamosal on the postorbital bar. This assignment was supported by a phylogenetic analysis based on that conducted by Kammerer et al. in 2011, which found it as the closest relative of Aulacephalodon on the basis of it lacking a ridge behind its tusk (which is ontogenetically influenced). However, this assignment is somewhat questionable, and forcing Bulbasaurus as a basal geikiid outside of the Geikiinae (Aulacephalodon, Geikia, and Pelanomodon) only requires one additional evolutionary step. Overall, the Cryptodontia (including the Geikiidae) were very unstable, suggesting that current datasets may not be able to sufficiently evaluate their relationships. An excerpt from the consensus of two phylogenetic trees, illustrating the relationships between cryptodontians, is shown below.
Tropidostoma, the genus Bulbasaurus was originally assigned to, exhibits two distinct morphologies - a robust morph with short snout and large tusks, and a gracile morph with long snout and small tusks, which probably represents sexual dimorphism as in other dicynodonts. However, Bulbasaurus matches neither of those morphologies; it differs from the genus Tropidostoma as a whole in many respects (addressed above). Additionally, even the holotype of Tropidostoma (which is probably immature judging by the unerupted tusks) is larger than adult specimens of Bulbasaurus, which further warrants their separation. The same is true of Bulbasaurus and Aulacephalodon, in addition to their differing boss morphologies and different interorbital widths. Bulbasaurus also differs from the problematic specimens BP/1/763 (assigned once to its own genus, Proaulacocephalodon, or to a juvenile Aulacephalodon) and TM 1480 (assigned once to Dicynodon hartzenbergi) by its larger tusks and wider interorbital region, among other characteristics. These specimens are additionally from the younger Cistecephalus assemblage zone.
Bulbasaurus