The holotype skull was found in situ with no skeletal elements in association and was not perimineralized with calcareous nodular material. This ‘head-only’ taphonomic style is typical of therapsid occurrences in proximal floodplain deposits of the Hoedemaker Member ( Smith, 1993 ) and is attributed to extended periods of non-deposition, allowing for sub-aerial disarticulation of skeletons, then short transportation and rapid burial by silt-laden overbank floods.

The holotype specimen of the new taxon (SAM-PK-K11235) was collected by one of the authors (RMHS) in Tropidostoma AZ exposures of the locality Vredelus (Fraserburg District, Western Cape Province). The Tropidostoma AZ strata in the southern Karoo Basin mainly coincide with the argillaceous Hoedemaker Member (Teekloof Formation), which represents the distal portion of a large distributary fluvial system. Three dimensional outcrops of exhumed paleomeanderbelts (the Reiersvlei Sandstone of Smith (1987) ) in the vicinity of the type locality of the new dicynodont show that the former rivers had channel widths of up to 350 m, and point bar diameters of ∼3 km.

All specimens mentioned in the text were examined personally by the lead author. For information on the methodology of the phylogenetic analysis, refer to the Discussion. The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/ . The LSID for this publication is: urn:lsid:zoobank.org:pub:EB1EE1F8-C512-4272-BFA3-C8938E0806E5. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.

Diagnosis : Dicynodont distinguished from all taxa other than geikiids by the combination of a transverse nasofrontal ridge, extremely broad interorbital region, and twisted squamosal contribution to subtemporal bar. Distinguished from the geikiids Aulacephalodon , Pelanomodon , and Geikia by the near-confluence of the nasal bosses (covering the majority of the nasal surface), absence of prefrontal bosses, sharply curved, ‘hook’-like beak tip, tall, highly discrete premaxillary ridge, presence of massive tusks at relatively small skull size, absence of bosses or swellings on the zygomatic arch, relatively weakly twisted subtemporal bar, relatively narrow intertemporal region, and a postparietal out-of-plane with both the occipital plate and skull roof. Further distinguished from Pelanomodon and Geikia by the presence of tusks and absence of a postcaniniform crest.

Specimens: Tropidostoma dubium (SAM-PK-K11238) in (A) dorsal, (D) ventral, (G) right lateral, and (J) anterior views. Bulbasaurus phylloxyron (CGP/1/938) in (B) dorsal, (E) ventral, (H) right lateral, and (K) anterior views. Aulacephalodon bainii (BP/1/3744) in (C) dorsal, (F) ventral, (I) right lateral, and (L) anterior views. Specimen of Bulbasaurus largely undistorted; specimens of Tropidostoma and Aulacephalodon have suffered some lateral compression. Scale bars equal 5 cm; all figures of an individual specimen to scale with one another. Characters: 1, intertemporal exposure of parietal: elongate, narrow channel in Tropidostoma , ‘pinched’ but anteriorly broad in Bulbasaurus , and broad throughout but transversely expanded posteriorly in Aulacephalodon . 2, postfrontal: large and triangular in Tropidostoma , absent in Bulbasaurus and large specimens of Aulacephalodon . 3, interorbital width: narrow in Tropidostoma , relatively broad in Bulbasaurus and Aulacephalodon . 4, nasal bosses: discrete and widely-separated, transversely narrow, ovoid, smooth bosses in Tropidostoma , large, rugose bosses covering almost all of nasal surface and nearly meeting on midline in Bulbasaurus , and large, rugose, but discrete and widely-separated bosses in Aulacephalodon . 5, quadrate ramus of pterygoid: relatively thin in Tropidostoma , relatively thick in Bulbasaurus and Aulacephalodon . 6, anterior ramus of pterygoid: relatively thin in Tropidostoma , relatively thick in Bulbasaurus and Aulacephalodon . 7, postcanine teeth: present in Tropidostoma , absent in Bulbasaurus and Aulacephalodon . 8, maxillary tusk: relatively small in Tropidostoma , massive in Bulbasaurus , also massive in Aulacephalodon but only at large skull size—specimen figured here is at minimal size for large tusks in the taxon; smaller individuals have thin or just-erupting tusks. 9, depression between anterior palatal ridges: deeper than depressions lateral to ridges in Tropidostoma , of equal depth as lateral depressions in Bulbasaurus and Aulacephalodon . 10, subtemporal bar: with only a slight deflection of the dorsal edge in Tropidostoma , ‘twisted’ in Bulbasaurus , such that the medial surface of the bar becomes the dorsal surface posteriorly, and strongly ‘twisted’ in Aulacephalodon , such that the medial surface of the bar becomes the lateral surface posteriorly. 11, ‘beak’: with only weak curvature in Tropidostoma and Aulacephalodon , only extreme anterior edge with hooked tip, versus strongly ventrally-curved in Bulbasaurus , with most of premaxillary ventral margin making up hooked tip. 12, snout depth: very deep in Aulacephalodon , comparatively shallow in Tropidostoma and Bulbasaurus . 13, anterior premaxillary ridge: absent or weakly-diffuse in Tropidostoma , very discrete and well-developed in Bulbasaurus , and absent in smaller individuals of Aulacephalodon such as the one figured here (and even in the largest adults, the premaxillary ridge of Aulacephalodon is proportionally lower and more diffuse than that of Bulbasaurus ).

Description

The following description is based primarily on the specimens SAM-PK-K11235 (the holotype), CGP/1/938, and CGP/1/949, all of which are isolated crania of similar size (Table 1). SAM-PK-K11235 is the least complete of the three and has large cracks across the intertemporal bar and snout, but was chosen as the holotype because it is the specimen that best exhibits cranial sutures. Description of the mandible and postcranium is based primarily on CGP/1/970 (one of only two Bulbasaurus specimens preserving the mandible and the only specimen preserving any postcrania). All of these specimens were previously identified as the oudenodontid Tropidostoma in collections; for this reason contrasts with Tropidostoma especially will be emphasized throughout the description, among broader comparisons with known dicynodont taxa. Comparisons with Tropidostoma dubium are based primarily on the specimens NHMUK R1662 (a topotype specimen making up part of Seeley’s (1889) original hypodigm for Tropidostoma), CGP/1/939, CGP/1/968, SAM-PK-K8603, and SAM-PK-K11238.

Figure 8: Stereopair of CGP/1/949, referred specimen of Bulbasaurus phylloxyron gen. et sp. nov., in dorsal view. Scale bars equal 5 cm.

Figure 9: Stereopair of CGP/1/949, referred specimen of Bulbasaurus phylloxyron gen. et sp. nov., in ventral view. Scale bars equal 5 cm.

Figure 10: Stereopair of CGP/1/949, referred specimen of Bulbasaurus phylloxyron gen. et sp. nov., in right lateral view. Scale bars equal 5 cm.

Figure 11: Stereopair of CGP/1/949, referred specimen of Bulbasaurus phylloxyron gen. et sp. nov., in occipital view. Scale bars equal 5 cm.

Figure 12: CGP/1/970, referred specimen of Bulbasaurus phylloxyron gen. et sp. nov., in (A) dorsal and (B) left lateral views. Postcranial elements edited out to highlight cranial morphology; see Figs. 13 and 14 for postcranium. Scale bar equals 5 cm.

Figure 13: Stereopair of CGP/1/970, referred specimen of Bulbasaurus phylloxyron gen. et sp. nov., in semi-dorsal view. Scale bars equal 5 cm.

Figure 14: Stereopair of CGP/1/970, referred specimen of Bulbasaurus phylloxyron gen. et sp. nov., in ventral view. Scale bars equal 5 cm.

Figure 15: CGP/1/2263, referred specimen of Bulbasaurus phylloxyron gen. et sp. nov., in (A) dorsal, (B) right lateral, (C) occipital, (D) ventral, and (E) anterior views. Scale bar equals 5 cm.

Figure 16: SAM-PK-K10587, referred specimen of Bulbasaurus phylloxyron gen. et sp. nov., in (A) dorsal, (B) right lateral, (C) occipital, (D) ventral, (E) left lateral, and (F) anterior views. Scale bar equals 5 cm.

The premaxillae of Bulbasaurus phylloxyron are fused to form a single median element, as in most dicynodonts (King, 1988; Kammerer & Angielczyk, 2009). On the dorsal skull surface, the premaxilla forms the anterior tip of the ‘beak’; palatally it forms a broad plate making up most of the secondary palate (Figs. 2–5). The tip of the ‘beak’ is sharply deflected in Bulbasaurus (Figs. 3, 7H); although not to the extreme degree of the dicynodontoid Dinanomodon (Kammerer, Angielczyk & Fröbisch, 2011), it is substantially more ‘hook’-like than in Tropidostoma and other cryptodonts (Figs. 7G and 7I). The anterior face of the premaxilla is flattened and bears a highly discrete median ridge with weak depressions on either side (Figs. 2A and 7K). In Tropidostoma, although there is usually a median ridge, it is relatively weak and diffuse (Fig. 7J). A well-developed premaxillary ridge is also present in large specimens of Aulacephalodon, although it is generally broader and more diffuse at edge than the tall, sharp ridge of Bulbasaurus. Posterodorsal to the median ridge, the premaxilla becomes a sharply tapering structure (the ascending process) narrowly extending between the nasal bosses. The posterior extent of the ascending process varies between specimens: it is remarkably long in the holotype, nearly reaching the level of the nasofrontal suture (Fig. 4B), but is relatively short in CGP/1/2263 (Fig. 15A) (although it also nearly reaches the nasofrontal suture in that specimen because of an elongate anterior process of the frontal, which is absent in the holotype). Laterally, the premaxilla contacts the maxilla below the septomaxillary footplate of the external naris (Figs. 2A and 3B). The premaxilla is roughly pentagonal in outline in palatal view (Fig. 5B). Well-developed, paired anterior palatal ridges and a single posterior median ridge are present. The paired anterior palatal ridges are relatively close together, as in Aulacephalodon and Pelanomodon (Kammerer, Angielczyk & Fröbisch, 2015a). Elongate depressions are present lateral to these ridges, and an additional, median depression is present between them. In Bulbasaurus, these three depressions are all of equal depth, as in other geikiids (Fig. 7F) but unlike Tropidostoma in which the median depression is substantially deeper than the lateral ones (Fig. 7D). The median ridge is both taller and transversely broader than the anterior ridges. It originates as a low, narrow structure at the posterior edge of the paired anterior ridges, but is not confluent with them (Figs. 5 and 9). It increases in height and breadth posteriorly, with maximal size achieved immediately anterior to the palatine pads. This ridge continues into the interpterygoid region, where it becomes formed by the vomer. Immediately lateral to the median palatal ridge the premaxillary surface is only weakly sloping; there is not a distinct, elongate depression extending along the side of the ridge. However, there are paired premaxillary depressions further laterally, in the region around the anterior terminus of the median ridge (Fig. 5B). Lateral to these depressions are low, rugose, somewhat arcuate ridges near the bases of the caniniform processes. Posteriorly, each of these ridges terminates at a confluence with the anteromedial margin of the palatine pad. Immediately lateral to this confluence is a small but discrete circular fossa at the anterior midpoint of the palatine pad, representing the lateral palatal foramen.

The septomaxilla is a small element largely confined within the external naris (Fig. 2A), but also making up the ventral rim of the naris in the form of a footplate. It is similar in morphology to that of other cryptodonts.

The maxilla is a large bone making up the ventrolateral surface of the snout (Fig. 3B). It bears a well-developed caniniform process housing a massive tusk. The tusks of Bulbasaurus are enormous proportional to skull size; in SAM-PK-K11235 the tusk diameter is 1.9 cm on a skull of 14.0 cm dorsal length (see also Table 2). Tusks in Tropidostoma are consistently proportionally smaller than those of Bulbasaurus, and absolutely smaller in all but the largest known Tropidostoma specimens (Table 3). The mean relative tusk diameter (RTD; measured as the ratio of tusk diameter to dorsal skull length) of Bulbasaurus is 0.13 (Table 2); by comparison, the mean RTD of Tropidostoma is only 0.06 (Table 3). Tusk proportions similar to those of Bulbasaurus are otherwise known only in Aulacephalodon among cryptodonts. However, Bulbasaurus is remarkable in having such large tusks at relatively small skull size. Aulacephalodon skulls of comparable size to specimens of Bulbasaurus are usually considered juveniles (Tollman, Grine & Hahn, 1980), and have the small, often still-erupting tusks typical of young dicynodonts. Aulacephalodon specimens only exhibit adult tusk proportions (i.e., those with RTD >0.10) in skulls greater than 20 cm in length (Table 4), and even these adults tend to have proportionally smaller (mean RTD 0.11) tusks than Bulbasaurus (although some individuals attain equivalent proportions). Because of the large size of the tusk root in Bulbasaurus, the caniniform process prominently bulges outwards along its lateral surface. The lateral surface of the maxilla is covered in tiny foramina and scattered larger pits. The rugose surface of the entire snout is suggestive of a keratinous covering, as has been argued to be broadly present in dicynodonts (King, 1988; Kammerer, Angielczyk & Fröbisch, 2015b). Dorsally, the maxilla extends between the lacrimal and external naris, before contacting the nasal and anteroventral tip of the prefrontal at its dorsal terminus (Fig. 3B). Posteriorly, it forms an elongate process contributing to the zygomatic arch, underlying first the jugal and then the squamosal before terminating beneath the postorbital bar. The posteroventral face of the caniniform process has a noticeably smoother bone texture than the lateral surface of the maxilla, broken only by two large foramina. No postcaniniform crest is present, unlike all cryptodonts other than Aulacephalodon (although in juvenile Aulacephalodon of comparable size to Bulbasaurus, e.g., BSPG 1934-VIII-516, a postcaniniform crest is present). Posteriorly, there is a straight suture with the jugal and, more medially, an interdigitated suture with the ectopterygoid and palatine (Fig. 5B). A labial fossa is present in this region (Fig. 15D), but the maxilla does not contribute to its border, which is formed by the jugal, ectopterygoid, and palatine.

The nasal forms the roof of the snout and bears a large boss. Paired nasal bosses are present in all known cryptodonts (Kammerer & Angielczyk, 2009) and come in various shapes and sizes within the clade. The nasal bosses of Tropidostoma are ovoid and proportionally transversely narrow (Fig. 7A), similar to those of Oudenodon and to a lesser extent Rhachiocephalus. Additionally, they are clearly separate structures, with unornamented nasal surfaces and a broad, triangular dorsal process of the premaxilla intervening between them. In Bulbasaurus, by contrast, the nasal bosses are massive and nearly confluent, with only a narrow strip of premaxilla separating them for most of their length (Figs. 2, 4, 7B and 8). Furthermore the nasal bosses of Bulbasaurus are notably rugose, whereas those of Tropidostoma, although bearing some pits, have a smoother bone surface. The nasal bosses of Aulacephalodon and Pelanomodon are also massive and frequently rugose, but unlike Bulbasaurus are also separated by broad spans of unornamented nasal (Fig. 7C). A distinct dorsoventral depression extending across the nasal, prefrontal, and maxilla separates the nasal boss from the orbital rim (Fig. 10), as in almost all other cryptodonts. Only in Odontocyclops is the nasal boss completely confluent with a prefrontal boss (Angielczyk, 2002). Posterior to the terminus of the ascending process of the premaxilla, the nasals have a short midline suture (Figs. 4B and 15A). This suture is situated in a narrow depression, in the same position where some other dicynodonts have fontanelles during development or neomorphic snout bones (Jasinoski et al., 2014; Kammerer, Angielczyk & Fröbisch, 2015a). Posteriorly, the nasal has a nearly straight but highly interdigitated suture with the frontal. In the holotype this interdigitation is dense but the individual processes are relatively short (Fig. 4), whereas in CGP/1/2263 there is a combination of dense interdigitation and a few large processes (notably an anterior process of the frontals that nearly extends between the nasal bosses; Fig. 15A). This suture is weakly but distinctly raised relative to the surrounding bone. Although not as well-developed as the tall nasofrontal ridge of Aulacephalodon and Pelanomodon, this raised suture is quite unlike the condition in Tropidostoma, in which there is no difference in skull height across the nasofrontal border.

The lacrimal is a narrow, curved bone that on the snout surface only forms the middle part of the anterior orbital margin (Fig. 3B). The lacrimal contribution to the orbital margin bears the best-developed portion of the orbital ridge, which continues onto the anterior portions of the jugal and prefrontal, but is absent around the rest of the orbital rim. Along its anteroventral margin, the lacrimal is excluded from contacting the naris by the dorsal process of the maxilla. Along its anterodorsal margin it has a variable extent between individuals, sometimes contacting the nasal (Fig. 15B) and sometimes being excluded from the nasal by a thin strip of prefrontal (Fig. 3B). Within the orbit, the lacrimal is perforated by a single, large lacrimal foramen.

The prefrontal is also a small bone that is largely limited to the anterior orbital margin (Figs. 3B–4B). Although its anterodorsal surface is somewhat thickened and convex, no distinct prefrontal boss is present (Figs. 4 and 8), unlike the majority of cryptodonts. Indeed, the lateral surface of the prefrontal in front of the orbit is less protruding than that of the lacrimal, as mentioned above.

Specimens Basal skull length (cm) Dorsal skull length (cm) Snout length (cm) Interorbital least width (cm) Anterior intertemporal width (cm) Posterior intertemporal width (cm) Temporal fenestra length (cm) CGP/1/938 15.1 13.4 4.2 4.8 3.9 1.6 7.9 CGP/1/949 16.4 13.1 4.5 4.2 4.4 3.9 7.0 CGP/1/970 N/A 16.0 6.6 5.2 5.3 0.6 9.0 CGP/1/2263 11.1 10.9 3.5 2.4 3.5 4.0 6.7 SAM-PK-K10106 N/A 13.4 5.0 4.8 4.2 1.1 8.7 SAM-PK-K10587 11.1 10.4 2.7 2.2 3.6 3.5 6.1 SAM-PK-K11235 16.9 14.0 5.2 4.4 4.0 0.6 9.6 Mean (all specimens) 14.1 13.0 4.5 4.0 4.1 2.2 7.9 Mean (excluding CGP/1/2263 and SAM-PK-K10587) 16.1 14.0 5.1 4.7 4.4 1.6 8.4 Standard deviation (all specimens) 2.8 1.9 1.3 1.2 0.6 1.6 1.3 Specimens Interpterygoid vacuity length (cm) Median pterygoid plate width (cm) Angle of quadrate ramus (left) Angle of quadrate ramus (right) Angle between squamosal rami Dentary symphysis height (cm) Dentary ramus height (cm) CGP/1/938 0.8 1.7 53.0 63.0 150.0 N/A N/A CGP/1/949 0.9 1.7 N/A 40.0 149.0 N/A N/A CGP/1/970 N/A N/A N/A N/A N/A 9.8 6.3 CGP/1/2263 N/A N/A 69.0 66.0 144.0 N/A N/A SAM-PK-K10106 N/A N/A N/A N/A N/A N/A N/A SAM-PK-K10587 0.6 1.4 44.0 52.0 150.0 N/A N/A SAM-PK-K11235 0.8 1.8 58.0 56.0 146.0 N/A N/A Mean (all specimens) 0.8 1.7 56.0 55.4 147.8 9.8 6.3 Mean (excluding CGP/1/2263 and SAM-PK-K10587) 0.8 1.7 55.5 53.0 148.3 9.8 6.3 Standard deviation (all specimens) 0.1 0.2 10.4 10.2 2.7 N/A N/A DOI: 10.7717/peerj.2913/table-1

Specimens Dorsal skull length (DSL) (cm) Left tusk diameter (cm) Right tusk diameter (cm) Tusk diameter mean (TDM) (cm) Relative tusk diameter (TDM/DSL) CGP/1/938 13.4 1.9 2.2 2.1 0.15 CGP/1/949 13.1 1.4 1.7 1.6 0.12 CGP/1/970 16.0 2.2 N/A 2.2 0.14 CGP/1/2263 10.9 1.2 1.3 1.3 0.12 SAM-PK-K10106 13.4 1.8 1.7 1.8 0.13 SAM-PK-K10587 10.4 1.2 1.3 1.3 0.12 SAM-PK-K11235 14.0 2.0 1.8 1.9 0.13 Mean 13.0 1.7 1.7 1.7 0.13 Standard Deviation 1.9 0.4 0.3 0.4 0.01 DOI: 10.7717/peerj.2913/table-2

Specimens Dorsal skull length (DSL) (cm) Left tusk diameter (cm) Right tusk diameter (cm) Tusk diameter mean (TDM) (cm) Relative tusk diameter (TDM/DSL) CGP/1/930 13.4 1.1 1.1 1.1 0.08 CGP/1/2173 12.1 0.5 0.6 0.5 0.04 CGS CM86-573 17.0 0.9 0.9 0.9 0.05 CGS F7 19.1 1.1 0.7 0.9 0.05 CGS F11 19.9 0.5 1.4 1.0 0.05 CGS R98 13.1 0.9 0.9 0.9 0.07 CGS RMS155 14.9 0.9 1.3 1.1 0.07 CGS RMS213 20.9 1.6 1.8 1.7 0.08 CGS RMS244 14.0 0.5 N/A 0.5 0.03 CGS RMS631 18.8 0.9 0.8 0.8 0.04 CGS RS327 22.4 1.9 1.5 1.7 0.08 CGS RS538 13.9 0.6 0.8 0.7 0.05 NHMUK R1662 25.3 2.1 1.5 1.8 0.07 NHMUK R4048 19.1 1.7 1.7 1.7 0.09 SAM-PK-K6742 23.4 N/A 1.4 1.4 0.06 SAM-PK-K6808 18.0 0.5 N/A 0.5 0.03 SAM-PK-K6940 14.1 0.5 0.3 0.4 0.03 SAM-PK-K8603 17.7 1.1 1.0 1.1 0.06 SAM-PK-K8639 16.2 0.9 0.4 0.7 0.04 SAM-PK-K10681 16.8 1.7 1.5 1.6 0.09 SAM-PK-K11015 13.0 1.1 1.2 1.1 0.09 SAM-PK-K11183 22.3 1.5 1.3 1.4 0.06 SAM-PK-K11238 13.6 1.0 1.0 1.0 0.07 SAM-PK-K11255 21.8 2.3 1.0 1.6 0.08 Mean 17.5 1.1 1.1 1.1 0.06 Standard deviation 3.8 0.5 0.4 0.4 0.02 DOI: 10.7717/peerj.2913/table-3

Specimens Dorsal skull length (DSL) (cm) Left tusk diameter (cm) Right tusk diameter (cm) Left/right tusk diameter mean (TDM) (cm) Relative tusk diameter (TDM/DSL) BP/1/300 18.2 0.6 0.8 0.7 0.04 BP/1/304 40.0 4.4 3.9 4.2 0.10 BP/1/2460 34.2 3.8 3.0 3.4 0.10 BP/1/2983 36.9 4.8 3.9 4.3 0.12 BP/1/4087 25.3 3.5 3.1 3.3 0.13 BSPG 1934-VIII-516 14.5 0.9 0.9 0.9 0.06 CGP/1/268 22.6 2.5 2.4 2.5 0.11 CGP/1/657 26.4 3.0 3.0 3.0 0.11 CGP/1/740 28.7 3.4 2.8 3.1 0.11 CGS S6 24.5 2.8 3.3 3.1 0.12 SAM-PK-3423 22.7 2.7 2.8 2.7 0.12 SAM-PK-5862 15.2 0.9 0.8 0.9 0.06 SAM-PK-10021 33.9 3.9 3.8 3.8 0.11 SAM-PK-10048 18.6 1.3 1.3 1.3 0.07 SAM-PK-10673 25.1 2.7 2.4 2.6 0.10 USNM 24621 37.0 3.2 4.3 3.7 0.10 Mean (all specimens) 26.5 2.8 2.7 2.7 0.10 Mean (specimens with >20 cm skull length) 29.8 3.4 3.2 3.3 0.11 Standard deviation 8.0 1.3 1.1 1.2 0.03 DOI: 10.7717/peerj.2913/table-4

The jugal is an elongate bone that is primarily part of the zygoma (Fig. 4B). It has a short facial exposure above the posterior portion of the caniniform process and makes up the ventral wall of the orbit (Fig. 3B). As previously described, it also borders the ventral surface of the maxilla, behind the caniniform process (Fig. 5B), where it partially surrounds the labial fossa. From here, the jugal curves posterolaterally, forming the ventromedial edge of the zygoma until reaching the end of the temporal fenestra. It also has a short contribution to the dorsal surface of the subtemporal bar, immediately behind the postorbital bar, and also forms part of the posteromedial face of the postorbital bar. It is generally similar in morphology to that of other cryptodonts (although Pelanomodon is aberrant in having a jugal boss as part of its contribution to the postorbital bar; Kammerer, Angielczyk & Fröbisch, 2015a).

The zygomatic ramus of the squamosal tapers anteriorly, separating the maxilla and jugal below the postorbital bar before terminating below the orbital midlength (Fig. 3B). The squamosal is prominently twisted in the subtemporal bar, such that what was its lateral surface anteriorly becomes ventral surface posteriorly. Some deflection of the edge of the squamosal in the posterior part of the subtemporal bar is typical in cryptodonts (including Tropidostoma; Fig. 7G), but in the large geikiids this is taken to an extreme, with complete reversal of the lateral and medial faces of the zygoma along its length (Fig. 7I). Bulbasaurus exhibits an intermediate condition, with some twisting of the zygoma (Fig. 7H) but not to the degree seen in Aulacephalodon and Pelanomodon. The squamosal flares broadly posterolaterally around the temporal fenestra before curving inwards to form a short intertemporal ramus immediately behind the postorbital (Fig. 4B). Posteriorly, the squamosal is a major contributor to the occipital plate, bordering the tabular and fused supraoccipital-periotic element laterally (Fig. 6B). Medial extensions of the squamosal surround the lateral half of the post-temporal fenestra, as in several other cryptodont taxa (Aulacephalodon, Pelanomodon, Oudenodon (Kammerer, Angielczyk & Fröbisch, 2015a), and Tropidostoma (C Kammerer, pers. obs., 2016)) A mediolaterally deep, dorsoventrally narrow notch is present between the zygomatic and quadrate rami of the squamosal (Fig. 11).

The frontal is largely restricted to the interorbital region in Bulbasaurus (Fig. 4B). It has a rugose edge where it forms part of the dorsal margin of the orbit, and several deep pits on its dorsal surface. The frontal curves downwards posteromedially, forming a deep median depression with the preparietal between the postorbital bars, anterior to the pineal foramen (Figs. 4 and 8). A depression in this part of the skull is sometimes weakly developed in Tropidostoma and Oudenodon, but is absent in other geikiids. This is probably related to the general broadening and flattening of the intertemporal region in these taxa relative to most other bidentalians. Posterolaterally, the border between the frontal and postorbital forms a sharp ridge. A distinct postfrontal appears to be absent in Bulbasaurus(Fig. 4B). The postfrontal is a large, triangular element at the posterodorsal margin of the orbit in Tropidostoma (Fig. 7A) and is usually narrower but still distinct in Oudenodon. The postfrontal is not discernible in adult geikiids such as Aulacephalodon, although it appears to be present in some juvenile specimens (e.g., BSPG 1934-VIII-516), suggesting that it fuses with the postorbital during growth. The absence of a distinct postfrontal in Bulbasaurus is further evidence of ‘adult’ geikiid characters being present at small skull size in this taxon.

The postorbital makes up the majority of the postorbital bar and the medial edge of the temporal fenestra (Figs. 3B, 4B and 8). The postorbital bar is typically thick in cryptodonts, but is particularly robust in geikiids, Bulbasaurus included. Unlike other geikiids, however, the postorbital bar of Bulbasaurus does not bear any bosses or dorsoventral ridges. The posterior ramus of the postorbital has a very steep lateral face, with a sharp break in slope between the postorbital laterally and the skull roof (frontal and parietal) medially. The lateral face of the postorbital is distinctly concave and would have served as the attachment site for jaw musculature. The posterior rami of the postorbitals converge posteriorly, creating a ‘pinched’ intertemporal bar. The degree of this convergence varies between specimens; in SAM-PK-K11235, the postorbitals almost completely overlap the parietals posteriorly, with only a narrow strip of parietal exposed between them (Fig. 4), whereas in CGP/1/938 and CGP/1/949, a trough-like median exposure of the skull roof is retained in the posterior intertemporal bar (Figs. 7B and 8). This style of variability is common in Tropidostoma and Oudenodon; in adult specimens of Aulacephalodon and Pelanomodon the parietal always remains broadly exposed in the intertemporal bar. Generally, however, postorbital-parietal overlap in Tropidostoma, when present, occurs throughout the length of the intertemporal bar (Fig. 7A) instead of in a ‘pinched’ posterior span. The posterior ramus of the postorbital terminates just beyond the point of the occipital plate (Fig. 4B).

The preparietal is a small, median element anterior to the pineal foramen (Fig. 4B). It is weakly depressed relative to the surrounding bone, taking into account that the entire surrounding region is strongly depressed (as discussed in the description of the frontal above). Preparietal depression is common in cryptodonts, and is also present in Tropidostoma. However, in Tropidostoma this bone is typically a narrow, anteroposteriorly elongate element with a pointed anterior tip (Fig. 7A). In Bulbasaurus, the preparietal is relatively wide, with a broadly rounded anterior tip (Fig. 4B), as in Aulacephalodon and Pelanomodon. The pineal foramen is a small, subcircular opening situated between the parietals (posteriorly and laterally) and preparietal (anteriorly). It is not elevated on a mound-like boss (as in rhachiocephalids, Endothiodon, and some large specimens of Aulacephalodon), but does have a raised, collar-like rim (Fig. 8). As noted above, the parietals are largely obscured by the postorbitals within the intertemporal bar in several specimens of Bulbasaurus. When exposed, their surface is weakly concave and otherwise unornamented.

As is usual in dicynodonts, the vomer is exposed only within the interpterygoid vacuity in ventral view (Fig. 5B). Anteriorly it is a narrow, rod-like element confluent with the median palatal ridge of the premaxilla. Posteriorly is slopes dorsally and bifurcates, developing a median trough before terminating in paired, flattened rami pressed against the medial walls of the anterior pterygoid rami.

The palatine is exposed ventrally in the form of a palatine pad, a roughly teardrop-shaped structure that would have formed part of the masticatory surface of the palate, and a shelf-like portion posterolateral to the pad that braces the medial edge of the anterior pterygoid ramus (Fig. 5B). The transversely expanded portion of the palatine pad is highly rugose, indicative of keratinous covering. Anterior to this is a short, sloping stretch of palatine contacting the premaxilla, with smooth bone texture. The palatine shelf attenuates posteriorly, extending along the medial wall of the pterygoid; this portion of the bone also has smooth bone texture. Although attenuate, this posterior portion is relatively thick in Bulbasaurus (Figs. 5B, 7E) compared to that of Tropidostoma (Fig. 7D) or Oudenodon, but is comparable to that of Aulacephalodon (Fig. 7F). In addition to its palatal contribution, the palatine is also exposed posterolaterally, behind the jugal and above the anterior pterygoid ramus along the rim of the subtemporal fenestra. Here, it contributes to the posterior rim of the labial fossa.

The ectopterygoid is a small, laminar element bordering the lateral wall of the anterior pterygoid ramus (Figs. 3B and 5B). It curves slightly laterally anteriorly, where it forms part of the border of the labial fossa. The anterior ramus of the pterygoid is a robust structure in Bulbasaurus, as in other geikiids. In Tropidostoma, the anterior pterygoid ramus is relatively narrower, with weaker lateral splay, and is generally more elongate (Fig. 7D). Thin crests on the ventral edges of the anterior pterygoid rami converge into the crista oesophagea posteriorly, on the median pterygoid plate (Fig. 9). The crista oesophagea is very well developed in Bulbasaurus, forming a tall, blade-like structure. Finally, the quadrate ramus of the pterygoid is a rod-like process, highly splayed outwards (57° relative to the long axis of the skull in the holotype) to contact the quadrate posterolaterally.

Sutures in the braincase of Bulbasaurus are not evident in any of the known specimens; it is likely that the braincase elements are extensively fused. Posterior to the median pterygoid plate, diverging ridges on what should be the parabasisphenoid (by comparison with other dicynodonts; King, 1988) extend towards the basal tubera (Fig. 5B). Each tuber is a thick, crescentic structure surrounding a ventrolateral exposure of the fenestra ovalis. The stapes is preserved in the specimen CGP/1/2263 (Fig. 15D). It is a stout, dumbbell-shaped element similar to those of other cryptodonts. A deep depression separates the basal tubera, and no intertuberal ridge is present. Posterior to this depression is the occipital condyle, which has the typical tripartite morphology of dicynodonts (presumably composed of a ventral basioccipital portion and paired dorsal exoccipital portions, although these elements are completely fused in Bulbasaurus). Circular jugular foramina are present ventrolateral to the occipital condyle, as is also typical of dicynodonts (Fig. 5B). Dorsal to the occipital condyle, the foramen magnum is ovoid, taller than wide (Figs. 6B and 11).

The occiput of SAM-PK-K11235 is well-prepared and fairly well-preserved (other than the broken edges of the squamosals), and clearly shows that most of the occipital elements are fused (Fig. 6B). Fusion between the opisthotic and prootic to form a periotic bone is common in dicynodonts, and incorporation of additional braincase bones is also observed in many taxa (Kammerer, Angielczyk & Fröbisch, 2015a). In Bulbasaurus, the occipital/braincase element is formed from fusion of the supraoccipital, exoccipitals, basioccipital, opisthotic, and prootic. The supraoccipital portion of this fused element is unusually expansive, making up the majority of the non-squamosal area of the occiput above the foramen magnum. The supraoccipital is typically a large element in dicynodonts, but usually the postparietal is nearly equal in size and the occiput has substantial contributions by the tabulars (this is the condition in Tropidostoma, for instance). In Bulbasaurus, the postparietal and tabulars are substantially smaller than the supraoccipital. The tabular is an arcuate, paired element between the squamosal, supraoccipital, and postparietal. Unusually for a therapsid, its long axis is horizontal rather than vertical. The most unusual occipital element is the postparietal, a median element at the top of the occipital plate. In most dicynodonts the postparietal is a flat, laminar element in-plane with the rest of the occiput, although it may have a dorsal process extending onto the skull roof (prominently in oudenodontids and especially so in cistecephalids). In Bulbasaurus, however, the entire postparietal is out-of-plane with both the skull roof and occipital plate, instead forming a sharply-angled ‘divot’ in the back of the skull. A very strongly developed nuchal crest is present along the postparietal midline and restricted entirely to that element.

The mandible is only preserved in two specimens of B. phylloxyron: CGP/1/970 (Figs. 12–14) and SAM-PK-K10106, and is largely unprepared in the latter. In CGP/1/970, the mandible is partially occluded to the cranium, but is somewhat dislocated (Fig. 14), revealing details of its dorsal surface (Fig. 13). In general, the mandible is similar to that of Aulacephalodon. The dentaries are fused to form an edentulous beak. The anterior face of the jaw symphysis is smoothly convex, without an anterior median ridge (Fig. 14). A sharp ridge delimits the edge between the anterior face of the symphysis and the lateral face of the jaw ramus (Fig. 12B), which is also present in Pelanomodon and Geikia but not Aulacephalodon. The anterodorsal terminus of the jaw comes to a sharp, pointed tip with a concave posterior surface. Although lower in dorsoventral height than the symphysis, the lateral portion of the jaw ramus is still tall and robust. The mandibular fenestra is a narrow, oval opening at mid-height on the dentary ramus; it is directly overhung by a small lateral dentary shelf that expands anterodorsally to form a broadened, diffuse boss. Unfortunately, the postdentary bones are not well-preserved in CGP/1/970; what is present is indistinguishable from that of other geikiids. Ventrally, the right articular is partially exposed, and has a well-developed, bulbous retroarticular process (Fig. 14).

The postcranial elements preserved in CGP/1/970 consist of a partially disarticulated set of cervical vertebrae, ribs, pectoral girdle, and forelimb elements (Figs. 13 and 14). Of these elements only the ribs (Fig. 13) and a humerus (Fig. 14) are reasonably exposed. The proximal ends of two left and four right ribs are preserved. They are bicipital and gently curved along the shaft, as in other dicynodonts. The humerus appears to represent a right humerus (visible as the large element at the bottom of Fig. 14). Its distal tip is broken off but it is otherwise intact, albeit extensively obscured by matrix and other bones. The deltopectoral crest is robust and strongly separated from the humeral shaft at a perpendicular angle.