Most skulls referred to Bulbasaurus are 13–16 centimetres (5.1–6.3 in) long, with two skulls (CGP/1/2263 and SAM-PK-K10587) being smaller at 10.9 centimetres (4.3 in) and 10.4 centimetres (4.1 in) respectively.
The larger skulls generally belong to mature specimens.
While the CGP/1/2263's size is largely due to compression, SAM-PK-K10587 seems to be a genuinely immature individual.
Notably, it differs from other specimens in having a shorter and less hooked snout; relatively smaller but still completely erupted tusks; less developed nasal bosses that are more separated by the premaxilla; a narrower interorbital region between the eyes; a wider intertemporal region at the back of the skull; relatively weak depressions in the interorbital and intertemporal regions; no overlap of the parietals by the postorbitals; and minimal twisting of the squamosal on the postorbital bar.

These differences are most likely due to growth, as similar transformations are also seen in Aulacephalodon.
However, the latter (and all other geikiids where the growth sequence is known) differs from Bulbasaurus in that the degree of overlap of the parietals by the postorbitals does not change; instead, the parietals themselves simply become wider.
In this respect, Bulbasaurus retains the ancestral cryptodontian condition, which is also seen in rhachiocephalids as well as Oudenodon, Tropidostoma, and other oudenodontids.
Overall, the relatively small Bulbasaurus provides evidence that the growth sequence of large geikiids such as Aulacephalodon did not develop along with their size, but rather was already present ancestrally and was retained as geikiids grew.
Most skulls referred to Bulbasaurus are 13–16 centimetres (5.1–6.3 in) long, with two skulls (CGP/1/2263 and SAM-PK-K10587) being smaller at 10.9 centimetres (4.3 in) and 10.4 centimetres (4.1 in) respectively. The larger skulls generally belong to mature specimens. While the CGP/1/2263's size is largely due to compression, SAM-PK-K10587 seems to be a genuinely immature individual. Notably, it differs from other specimens in having a shorter and less hooked snout; relatively smaller but still completely erupted tusks; less developed nasal bosses that are more separated by the premaxilla; a narrower interorbital region between the eyes; a wider intertemporal region at the back of the skull; relatively weak depressions in the interorbital and intertemporal regions; no overlap of the parietals by the postorbitals; and minimal twisting of the squamosal on the postorbital bar.
These differences are most likely due to growth, as similar transformations are also seen in Aulacephalodon. However, the latter (and all other geikiids where the growth sequence is known) differs from Bulbasaurus in that the degree of overlap of the parietals by the postorbitals does not change; instead, the parietals themselves simply become wider. In this respect, Bulbasaurus retains the ancestral cryptodontian condition, which is also seen in rhachiocephalids as well as Oudenodon, Tropidostoma, and other oudenodontids. Overall, the relatively small Bulbasaurus provides evidence that the growth sequence of large geikiids such as Aulacephalodon did not develop along with their size, but rather was already present ancestrally and was retained as geikiids grew.
Bulbasaurus