Premaxilla, maxilla, and palate At the front of the upper jaw of
Bulbasaurus, the tips of the fused
premaxillae are strongly hook-like, much more so than
Tropidostoma and other dicynodonts Also unlike
Tropidostoma, the flattened front face of the premaxillae bear a tall, narrow, prominent ridge;
Aulacephalodon also has a similar ridge, albeit broader and not as sharp. The back of the premaxillae narrow and extend between the roughened bosses on the nasal bones. Viewed from underneath, the bone is roughly pentagonal; the bottom surface bears two ridges near the front, as well as an additional ridge extending backward from where the two forward ridges end, which gradually becomes taller and wider. These ridges are separated by depressions in the bone of roughly equal depth, which is like other
geikiids but unlike
Tropidostoma. At the outer extremities of the premaxillae, low and roughened ridges are located near the base of the tusks. Further back on the interior of the upper jaw, the
palatine bones are exposed as a palatine pad, which is very roughened and would have been covered in keratin, although the portion where they contact the premaxillae is smooth and sloping. The back portion of the palatines are thinner than the rest of the bone, but it is still thicker than that of either
Tropidostoma or
Oudenodon, instead resembling
Aulacephalodon more closely. The
pterygoid bones are robust in contrast to
Tropidostoma, and bear ridges that converge into a tall, blade-like process known as the . The pterygoids also project outwards in rod-like structures to meet the
quadrate bones. The root of the tusk bulges outwards from the surface of the maxilla due to its large size. Extensive pitting on the surface of the maxilla is suggestive of some kind of keratinous covering, which has also been inferred for other dicynodonts. Unlike other geikiids and most other
cryptodontian dicynodonts, there is no ridge behind the tusk, although mature
Aulacephalodon also lack this ridge. In contrast to the small, oval-shaped, relatively narrow, and smooth-textured bosses of
Tropidostoma, the roughened bosses of
Bulbasaurus are very large and nearly form a single continuous boss (although a narrow strip of the premaxilla extends backwards between the bosses).
Aulacephalodon and
Pelanomodon also have large and roughened bosses, but they are separated in part by the nasals. At the back of the bosses, a slight indentation wrapping around the top and sides of the skull separates them from the eye socket, typical of cryptodontians except for
Odontocyclops. The
suture between the nasals and the
frontal bones is slightly raised relative to the rest of the skull; the same raised suture is also seen in
Aulacephalodon and
Pelanomodon.
Postorbital skull Most of the postorbital bar is made up of the
postorbital bones, which are very robust relative to other cryptodontians as in other geikiids. However, compared to other geikiids, the postorbital bar of
Bulbasaurus is relatively smooth and free of bosses. The sides of the postorbitals, which would have anchored jaw musculature, are very concave. Near the back, the postorbitals curve and converge to form a somewhat pinched intertemporal bar that overlaps the
parietal bones to varying extents. The
squamosal bones also contribute to the postorbital bar; its back edge along the postorbital bar is somewhat twisted in
Bulbasaurus, which is seen in other cryptodontians but is taken to an extreme by
Aulacephalodon and
Pelanomodon, where the bone has become entirely twisted such that the interior faces outwards. Projections of the squamosal bones partially surround the posttemporal fenestrae on the rear of the skull, like
Aulacephalodon,
Pelanomodon,
Oudenodon, and
Tropidostoma. As for the underlying parietals themselves, they are slightly concave. In front of the parietals are the small midline preparietal bones, which are relatively broad and have a rounded tip, as in
Aulacephalodon and
Pelanomodon but in contrast to
Tropidostoma. The
pineal foramen is bordered by the preparietals and parietals, and it is surrounded by a simple ridge instead of being on a raised boss like either the large
rhaciocephalids and
Endothiodon or some large specimens belonging to
Aulacephalodon. On the
braincase, no sutures are visible, suggesting that the bones are very fused. The
occipital bones are likewise very fused. The contribution of the
supraoccipital bones to the back of the skull is unusually extensive and occupies much of the area not part of the squamosals above the level of the
foramen magnum. Also unusual are the smaller elements at the back of the skull, namely the postparietals and tabulars. The postparietals are not part of the continuous flat surface at the back of the skull, instead forming a sharp divot; additionally, a strong midline crest is present on the postparietals and do not extend onto other bones. Meanwhile, the tabulars are wider than they are long.
Mandible and postcrania The
mandible of
Bulbasaurus was largely similar to
Aulacephalodon. At the front of the mandible, the two toothless dentary bones fuse at the front to form a continuous beak with a sharp, pointed tip. The somewhat convex front surface of this junction, or the dentary symphysis, is separated from the sides of the dentaries by sharp ridges, a condition also seen in
Pelanomodon and
Geikia but not seen in
Aulacephalodon. Overall, the dentary was tall and robust, the symphysis more so than the rest of the bone. Located at the mid-height of the dentaries are the mandibular fenestrae, which are small and oval, and bordered on top by a dentary shelf that expands into a boss. Asides from the skull, the other portions of
Bulbasaurus have not been prepared in depth. The
ribs are gently curved and are bicipital in that they have two heads. On the
humerus, the deltopectoral crest was robust and strongly separated.
Ontogeny Most skulls referred to
Bulbasaurus are long, with two skulls (CGP/1/2263 and SAM-PK-K10587) being smaller at and 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. ==Classification==