| Bulbasaurus | |
|---|---|
 | |
| Photograph and diagram of holotype skull | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | †Anomodontia |
| Clade: | †Dicynodontia |
| Family: | †Geikiidae |
| Genus: | †Bulbasaurus Kammerer & Smith, 2017 |
| Type species | |
| Bulbasaurus phylloxyron Kammerer & Smith, 2017 | |
Bulbasaurus (meaning "bulbous reptile") is an extinct genus ofdicynodont that is known from theLopingian epoch of theLate Permian period of what is nowSouth Africa, containing the type and only speciesB. phylloxyron. It was formerly considered as belonging toTropidostoma; however, due to numerous differences fromTropidostoma in terms of skullmorphology and size, it has been reclassified the earliest known member of the familyGeikiidae, and the only member of the group known from theTropidostoma Assemblage Zone. Within the Geikiidae, it has been placed close toAulacephalodon, although a morebasal position is not implausible.
Bulbasaurus was ostensibly not directly named after thePokémonBulbasaur, but rather after itsnasal 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 ofBulbasaurus among the geikiids include the hook-like beak, very large tusks, and absence of bossing on theprefrontal bone.
Theholotype specimen ofBulbasaurus was found by Roger M.H. Smith in the Vredelus locality, which is located at an altitude of 1,445 metres (4,741 ft), in the district ofFraserburg,Northern Cape,South Africa. This locality is part of what is known as theTropidostoma Assemblage Zone, which belongs to theLopingian (upperPermian) Hoedemaker Member of the MiddleTeekloof Formation. TheTropidostoma AZ is named after theoudenodontidTropidostoma, 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 bothpostorbital bars. It was discovered lying right-side-up in a bed of greysiltstone with embeddedmicritenodules, and there were no associated remains from the rest of the skeleton.[1] "Head-only" preservation is common intherapsid 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.[2]
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 inBeaufort 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 theIziko Museums inCape Town or the Council for Geoscience inPraetoria. Before being referred to the new genusBulbasaurus, they were initially treated as specimens ofTropidostoma in collections.[1]
Bulbasaurus was described by Christian Kammerer and Smith in 2017. The description states that the generic name combines the Latinbulbus, referring to the very large and bulbousnasal bosses, with the common suffix-saurus. As for the specific namephylloxyron, meaning literally "leaf razor", it is derived from the Greekphyllos andxyron, and apparently refers to the keratinous covering on the premaxilla, maxilla, and palate that would have been used to shear plant material.[1] Thus, as published, the name ofBulbasaurus does not directly refer toPokémon, or specifically the similarly-namedBulbasaur. However, Kammerer noted that "if one wished to read between the lines concerning certain similarities, I wouldn't stop them",[3] and later added that "similarities between this species and certain other squat, tusked quadrupeds may not be entirely coincidental."[4]
At the front of the upper jaw ofBulbasaurus, the tips of the fusedpremaxillae are strongly hook-like, much more so thanTropidostoma and other dicynodonts[1] but not as much asDinanomodon.[5] Also unlikeTropidostoma, 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 othergeikiids but unlikeTropidostoma. At the outer extremities of the premaxillae, low and roughened ridges are located near the base of the tusks.[1]
Further back on the interior of the upper jaw, thepalatine 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 eitherTropidostoma orOudenodon, instead resemblingAulacephalodon more closely. Thepterygoid bones are robust in contrast toTropidostoma, and bear ridges that converge into a tall, blade-like process known as thecrista oesophagea. The pterygoids also project outwards in rod-like structures to meet thequadrate bones.[1]
Compared to other dicynodonts, themaxillary tusk ofBulbasaurus was massive; the holotype skull, which is 14 cm (5.5 in) long, has a 1.9 cm (0.75 in) tusk diameter. OnlyAulacephalodon has comparably large tusks proportionally, but these belong to adult specimens much larger thanBulbasaurus (in juveniles the size ofBulbasaurus, the tusks are still erupting).[6] 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.[7] Unlike other geikiids and most othercryptodontian dicynodonts, there is no ridge behind the tusk, although matureAulacephalodon also lack this ridge.[1]
The nasal bones, which form the roof the snout, bear a pair of enlarged bosses of bone as in other cryptodontians.[8] In contrast to the small, oval-shaped, relatively narrow, and smooth-textured bosses ofTropidostoma, the roughened bosses ofBulbasaurus are very large and nearly form a single continuous boss (although a narrow strip of the premaxilla extends backwards between the bosses).Aulacephalodon andPelanomodon 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 forOdontocyclops.[9] Thesuture between the nasals and thefrontal bones is slightly raised relative to the rest of the skull; the same raised suture is also seen inAulacephalodon andPelanomodon.[1]
Typical of geikiids, the interorbital region between the eyes was quite broad. Thelacrimal bones,prefrontal bones, frontal bones, andjugal 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 inTropidostoma andOudenodon, 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 inAulacephalodon. The elongate jugals form part of thezygoma, or bony cheek, and ends at thetemporal fenestra. It also forms part of the temporal and postorbital bars;Pelanomodon differs fromBulbasaurus in having small bosses on the latter portions of the jugals.[1][10]
Most of the postorbital bar is made up of thepostorbital bones, which are very robust relative to other cryptodontians as in other geikiids. However, compared to other geikiids, the postorbital bar ofBulbasaurus 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 theparietal bones to varying extents. Thesquamosal bones also contribute to the postorbital bar; its back edge along the postorbital bar is somewhat twisted inBulbasaurus, which is seen in other cryptodontians but is taken to an extreme byAulacephalodon andPelanomodon, 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, likeAulacephalodon,Pelanomodon,Oudenodon, andTropidostoma.[1][10]
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 inAulacephalodon andPelanomodon but in contrast toTropidostoma. Thepineal 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 largerhaciocephalids andEndothiodon or some large specimens belonging toAulacephalodon. On thebraincase, no sutures are visible, suggesting that the bones are very fused. Theoccipital bones are likewise very fused. The contribution of thesupraoccipital 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 theforamen 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.[1]
Themandible ofBulbasaurus was largely similar toAulacephalodon. 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 inPelanomodon andGeikia but not seen inAulacephalodon. 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 ofBulbasaurus have not been prepared in depth. Theribs are gently curved and are bicipital in that they have two heads. On thehumerus, the deltopectoral crest was robust and strongly separated.[1]
Most skulls referred toBulbasaurus 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.[1]
These differences are most likely due to growth, as similar transformations are also seen inAulacephalodon.[6][10] However, the latter (and all other geikiids where the growth sequence is known) differs fromBulbasaurus 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 asOudenodon,Tropidostoma, and other oudenodontids.[10] Overall, the relatively smallBulbasaurus provides evidence that the growth sequence of large geikiids such asAulacephalodon did not develop along with their size, but rather was already present ancestrally and was retained as geikiids grew.[1]
In 2017,Bulbasaurus was assigned to the Geikiidae clade ofdicynodonts 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 aphylogenetic analysis based on that conducted by Kammereret al. in 2011,[5][11] which found it as the closest relative ofAulacephalodon on the basis of it lacking a ridge behind its tusk (which is ontogenetically influenced). However, this assignment is somewhat questionable, and forcingBulbasaurus as a basal geikiid outside of theGeikiinae (Aulacephalodon,Geikia, andPelanomodon) 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 genusBulbasaurus 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.[6][7][10][12][13] However,Bulbasaurus matches neither of those morphologies; it differs from the genusTropidostoma as a whole in many respects (addressed above). Additionally, even the holotype ofTropidostoma (which is probably immature judging by the unerupted tusks) is larger than adult specimens ofBulbasaurus, which further warrants their separation. The same is true ofBulbasaurus andAulacephalodon, 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 juvenileAulacephalodon[6][10]) and TM 1480 (assigned once toDicynodon hartzenbergi[5]) by its larger tusks and wider interorbital region, among other characteristics. These specimens are additionally from the youngerCistecephalus assemblage zone.[1]
Although theTropidostoma Assemblage Zone, from whereBulbasaurus hails, is named after theoudenodontidTropidostoma,Tropidostoma is only the third most common dicynodont in this assemblage zone. Most common is the smallDiictodon, over 2000 specimens of which are known from theTropidostoma AZ alone. Also more common thanTropidostoma isPristerodon. Other dicynodonts present includeCistecephalus,Dicynodontoides,Emydops,Endothiodon,Oudenodon,Palemydops, andRhachiocephalus.[14] Notable in theTropidostoma AZ is the lack of geikiid anddicynodontoid dicynodonts (Dicynodontoides is adiictodont), which is unusual since they must have already diverged from their ancestral lineages by this time;Bulbasaurus happens to fill the former gap.[1]
TheTropidostoma AZ also records the gradual diversification oftherocephalians andgorgonopsians.[1] Therocephalians present includeHofmeyria,Ictidosuchoides (most common),Ictidosuchops,Ictidosuchus, andLycideops; gorgonopsids present includeAelurognathus,Aelurosaurus,Aloposaurus,Cyonosaurus,Gorgonops (most common),Lycaenops, andScymnognathus.[14] Within theKaroo Supergroup,cynodonts also first appear within theTropidostoma AZ;[1] they includeAbdalodon (formerly assigned toProcynosuchus)[15] andCharassognathus.[16] Rarer members of theTropidostoma AZ assemblage include theburnetiamorphsLobalopex[17] andLophorhinus;[18]parareptilesPareiasaurus andSaurorictus; thearchosauromorphYounginia; and thetemnospondylRhinesuchus.[14]
Bulbasaurus was probably buried onfloodplains surrounding a meandering river up to 350 metres (1,150 ft) wide and withpoint bars up to 3 kilometres (1.9 mi) in diameter.[1][19] This river flowed from the southern mountains northeast onto analluvial fan some 500 kilometres (310 mi) wide. The water flow in the rivers was seasonally dependent, but there was probably flowing water year-round. About every 30,000 years, the river banks were breached by flooding, leavingoverbank deposits and a series of small, isolated ponds.[14]
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