Bulbasaurus (meaning "bulbous reptile") is an extinct genus of dicynodont that is known from the Lopingian epoch of the Late Permian period of what is now South Africa, containing the type and only species B. phylloxyron. It was formerly considered as belonging to Tropidostoma; however, due to numerous differences from Tropidostoma in terms of skull morphology and size, it has been reclassified the earliest known member of the family Geikiidae, and the only member of the group known from the Tropidostoma Assemblage Zone. Within the Geikiidae, it has been placed close to Aulacephalodon, although a more basal position is not implausible.
Bulbasaurus was ostensibly not directly named after the Pokémon Bulbasaur, but rather after its nasal bosses, which are unusually bulbous among geikiids; however, the describers noted that the similarity in name "may not be entirely coincidental." Additionally, the specific name of the type species means "leaf razor", which is most directly a reference to its keratin-covered jaws. Other distinguishing characteristics of Bulbasaurus among the geikiids include the hook-like beak, very large tusks, and absence of bossing on the prefrontal bone.
Discovery and naming
The holotype specimen of Bulbasaurus was found by Roger M.H. Smith in the Vredelus locality, which is located at an altitude of , in the district of Fraserburg, Northern Cape, South Africa. This locality is part of what is known as the Tropidostoma Assemblage Zone, which belongs to the Lopingian (upper Permian) Hoedemaker Member of the Middle Teekloof Formation. The Tropidostoma AZ is named after the oudenodontid Tropidostoma, which occurs commonly at the site. The holotype itself, which is catalogued as SAM-PK-K11235, is a partially complete skull that is missing the left subtemporal and both postorbital bars. It was discovered lying right-side-up in a bed of grey siltstone with embedded micrite nodules, and there were no associated remains from the rest of the skeleton. "Head-only" preservation is common in therapsid fossils of the Hoedemaker Member, because the specimens were probably left out in the open and became disarticulated before being rapidly buried by flash floods.
Other referred specimens include the nearly-complete skull CGP/1/938 (from the Wilgerbosch Kloof locality in Fraserburg), the complete skull CGP/1/949 (also from Wilgerbosch Kloof), the complete skull with associated lower jaws and postcranial remains CGP/1/970 (from the Blaauwkrans locality in Beaufort West, Western Cape), the complete but crushed skull CGP/1/2263 (locality unknown), the crushed skull with lower jaws SAM-PK-K10106 (from the Paalhuisberg locality in Beaufort West), and the complete juvenile skull with lower laws SAM-PK-K10587 (from the Doornhoek locality in Beaufort West). All of these specimens are either held at the Iziko Museums in Cape Town or the Council for Geoscience in Praetoria. Before being referred to the new genus Bulbasaurus, they were initially treated as specimens of Tropidostoma in collections.
Bulbasaurus was described by Christian Kammerer and Smith in 2017. The description states that the generic name combines the Latin bulbus, referring to the very large and bulbous nasal bosses, with the common suffix -saurus. As for the specific name phylloxyron, meaning literally "leaf razor", it is derived from the Greek and , and apparently refers to the keratinous covering on the premaxilla, maxilla, and palate that would have been used to shear plant material. Thus, as published, the name of Bulbasaurus does not directly refer to Pokémon, or specifically the similarly-named Bulbasaur. However, Kammerer noted that "if one wished to read between the lines concerning certain similarities, I wouldn't stop them", and later added that "similarities between this species and certain other squat, tusked quadrupeds may not be entirely coincidental."
Description 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 but not as much as Dinanomodon. 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.
Compared to other dicynodonts, the maxillary tusk of Bulbasaurus was massive; the holotype skull, which is long, has a tusk diameter. Only Aulacephalodon has comparably large tusks proportionally, but these belong to adult specimens much larger than Bulbasaurus (in juveniles the size of Bulbasaurus, the tusks are still erupting). 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.
Nasal and orbital rim
The nasal bones, which form the roof the snout, bear a pair of enlarged bosses of bone as in other cryptodontians. 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.
Typical of geikiids, the interorbital region between the eyes was quite broad. The lacrimal bones, prefrontal bones, frontal bones, and jugal bones form the margin of the eye socket, with the portion comprised by the lacrimals having an orbital ridge that is better-developed and more raised. Unlike other cryptodontians, there is no evidence of a second set of bosses on the prefrontals, although their surfaces are somewhat thickened. A relatively deep midline depression (mildly developed in Tropidostoma and Oudenodon, but absent in other geikiids) is visible on the frontals, which are situated largely between the eyes and form roughened edges where they contributes to the rims of the eye sockets. There appears to be no separation of the postfrontal bones from the frontals, which is probably an adult characteristic as in Aulacephalodon. The elongate jugals form part of the zygoma, or bony cheek, and ends at the temporal fenestra. It also forms part of the temporal and postorbital bars; Pelanomodon differs from Bulbasaurus in having small bosses on the latter portions of the jugals.
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