Digalodon rubidgei Broom and Robinson, 1948

Kammerer, C. F., Angielczyk, K. D. & Fröbisch, J., 2015, Redescription of Digalodon rubidgei, an emydopoid dicynodont (Therapsida, Anomodontia) from the Late Permian of South Africa, Fossil Record 18 (1), pp. 43-55 : 44-49

publication ID	https://doi.org/ 10.5194/fr-18-43-2015
DOI	https://doi.org/10.5281/zenodo.11585207
persistent identifier	https://treatment.plazi.org/id/B973E50A-7F3F-FFC1-FCA0-FBFBFED8FBF4
treatment provided by	Felipe
scientific name	Digalodon rubidgei Broom and Robinson, 1948
status

Treatment

Figures 1–4 View Figure 1 View Figure 2 View Figure 3 View Figure 4 , 6–10 View Figure 6 View Figure 7 View Figure 8 View Figure 9 View Figure 10

Holotype: RC 76 , a complete, somewhat sheared cranium from Ferndale , Graaff-Reinet , Eastern Cape Province. This specimen and all referred material were found in rocks of the Beaufort Group in the Karoo Basin of South Africa.

Referred material: B 42, a worn skull and vertebra from Libertas, Nietgegund, Pearston, Eastern Cape Province; BP/1 /157, a poorly prepared skull and lower jaws from Hoeksplaas, Murraysburg , Western Cape Province ; RC 469 , a largely unprepared skull missing the snout from Rooiwal, Richmond , Northern Cape Province . There are four additional specimens that deviate slightly in morphology from the holotype and referred specimens of D. rubidgei but probably pertain to this species (see below): USNM 22941 About USNM , a somewhat flattened skull from Richmond , Northern Cape Province ; RC 303 , 304 , and 306, three small, partially preserved skulls from Glencliff, Aberdeen , Eastern Cape Province .

Diagnosis: a small dicynodont (maximum basal skull length 10 cm) that can be identified as an emydopoid by the presence of a precaniniform embayment of the palatal rim and shovel-shaped jaw symphysis. Digalodon rubidgei can be distinguished from all known emydopoids by the presence of paired anterior palatal ridges on the premaxilla (reversal to the non-emydopoid plesiomorphic state), a long “beak” sharply demarcated from the caniniform process, and an extremely tall zygomatic ramus of the squamosal, with a thickened, “folded-over” dorsal margin (convergent with geikiid cryptodonts). Distinguished from all emydopoids other than Compsodon by the presence of raised parietal “lips” along the lateral edges of the pineal foramen. Distinguished from all non-cistecephalid emydopoids by the short frontal contribution to the orbital margin. Digalodon has a broad posterolateral expansion of the parietal, excluding the postorbital from the back of the skull roof to an even greater degree than in Myosaurus and cistecephalids.

3 Description

The following redescription of Digalodon rubidgei is based on the holotype cranium, RC 76. The holotype is fairly well preserved and (compared with other Broom types of similar age) well prepared, with clear sutures visible over much of the skull ( Figs. 1–4 View Figure 1 View Figure 2 View Figure 3 View Figure 4 ). The skull is almost complete, missing only part of the left temporal arch. However, the bone surface of the skull is damaged in several places, particularly on the interorbital region, the dorsal surface of the snout, and the zygoma. The palatal surface has been somewhat overprepared, losing fine surface detail. The skull has also suffered postmortem shear, such that the right side of the skull has moved slightly anteriorly relative to the left. The skull is short (as in most emydopoids) and roughly “heart-shaped” in dorsal view. The intertemporal region is slightly broader than the interorbital in this specimen.

The premaxilla has, as Broom and Robinson (1948) noted, only a short contribution to the lateral surface of the snout, similar to other emydopoids ( Fig. 2 View Figure 2 ). The extent of the ascending process of the premaxilla is uncertain in RC 76 because this region is damaged. Because of this damage, the morphology of the external naris and septomaxilla is also unknown. Ventrally, the premaxilla forms a broad secondary palate as is typical of dicynodonts ( Fig. 3 View Figure 3 ). On the right side, the premaxilla appears to be similar to that of Emydops , with a laterally flaring portion anterior to the caniniform process separated from a short, squared-off anterior tip. This morphology is not present on the left side, however; it is uncertain whether this is the result of damage. Paired anterior palatal ridges are present on the premaxilla, although they are weak (possibly due to overpreparation). They bow slightly outwards at their posterior terminus and do not contact the posterior median palatal ridge. It is unclear whether lateral anterior palatal ridges (as are present in other emydopoids) were present. Only a faint ridge is present on the palatal surface of the left maxilla, but given general overpreparation of the palate it is probable that more defined ridges were originally present on both sides (especially given their presence in the specimen B 42, for which see below). The posterior median palatal ridge is a narrow, blade-like element partially obscured by matrix. It extends anteriorly to a point between the tusks.

The premaxilla and maxilla form a turtle-like “beak” anterior to the caniniform processes. The ventral margin of this beak is essentially horizontal for its entire length, unlike the hooked beak tips of many other dicynodonts. The length of the beak is somewhat exaggerated in left lateral view ( Fig. 2a View Figure 2 ) because of shear; in life it would have been intermediate in length between what is shown in Fig. 2a and c View Figure 2 . Even with this distortion accounted for, the beak is significantly longer than in other known emydopoids. There is a sharp (∼ 100 ◦) demarcation between the ventral margin of the beak and the caniniform process, but the alveolar margin is smooth; there is not a distinct notch as in pylaecephalids.

A well-developed maxillary caniniform process houses the tusk. Although this process is directed ventrally, the tusks are angled anteroventrally ( Fig. 2 View Figure 2 ). A series of small vascular foramina are present on the lateral surface of the caniniform process, probably associated with the tusk root. It is also possible that these foramina were associated with the keratinous beak, as is usually inferred for dicynodonts ( Kemp, 1982; King, 1988). However, if these foramina were associated with an overlying rhamphotheca, we would expect them to be broadly present across the surface of the premaxilla. Instead, dense concentrations of foramina are only present above the caniniform, strongly suggesting their association with the ever-growing tusk. In other non-mammalian therapsids, vascular foramina commonly occur on the external surface of the maxilla above the canine root and are particularly well developed in the taxa with the largest canines (i.e., gorgonopsians, anteosaurs). Lines of maxillary foramina are also associated with the tooth row in extant reptiles but for the most part are absent in living mammals, even in taxa with enlarged canines ( Van Valen, 1960). Instead, mammals typically have a single large maxillary foramen (the infraorbital foramen) through which the infraorbital nerve (associated with mechanoreception of the vibrissae) and artery run ( Muchlinski, 2008). Thus, non-mammalian therapsids appear to retain a “reptilian” style of maxillary vasculature primarily associated with the teeth. That said, this does not mean that the beak of Digalodon (and other dicynodonts) lacked a keratinous covering, only that it was not driving foraminal distribution on the skull surface.

In addition to the many small foramina on the lateral maxillary surface, a large vascular foramen is present on the posterior face of the caniniform process in both maxillae ( Figs. 2 View Figure 2 , 3 View Figure 3 ), as in Emydops ( Angielczyk et al., 2005, 2009). It is unknown whether small foramina were also present on the medial surface of the maxilla, as this region is overprepared. A labial fossa sensu Angielczyk and Kurkin (2003) is absent. In ventral view, there is a distinct embayment in the maxillary margin anterior to the caniniform process and a postcaniniform keel behind it, as are typical of emydopoids.

The surface of the nasals is damaged, and the position of their anterior contact with the premaxilla is uncertain ( Fig. 1 View Figure 1 ). It appears that a lengthy midnasal suture separated the premaxilla from the frontals. The nasals are constricted at midlength by the prefrontals. It is not clear whether nasal bosses were present. An eminence jutting laterally from the right nasal looks superficially like a boss, but is actually just an overhanging bit of bone displaced by shear. If they were present, the nasal bosses would have been small, unlike the expanded bosses typical of cryptodonts. The preserved edges of the nasofrontal suture suggest it ran transversely across the interorbital region in a straight line.

The left prefrontal is damaged ( Fig. 2b View Figure 2 ) but the right one is well preserved ( Figs. 1 View Figure 1 , 2d View Figure 2 ). The prefrontal makes up the anterodorsal edge of the orbit and makes a significant contribution to the surface of the snout. Ventrally, it has an interdigitated suture with the lacrimal and nasal. Additionally, it has a short ventral contact with a narrow ascending process of the maxilla ( Fig. 2d View Figure 2 ). The lacrimal is a small bone mostly restricted to the anterior orbital wall. It has a small facial contribution between the prefrontal and maxilla and continues ventrally as a thin strip separating the maxilla from the orbit ( Fig. 2d View Figure 2 ). The lacrimal foramen is exposed on the left side of the skull; it is located near the top of the lacrimal and does not exit onto the snout surface.

The jugal is barely visible in lateral view, and in the intact skull would have been exposed only as thin strips at the anteroventral and posteroventral corners of the orbit ( Fig. 2 View Figure 2 ). The left jugal is also exposed laterally below the postorbital bar, beneath the squamosal ( Fig. 2b View Figure 2 ), but this is probably due to displacement of the squamosal dorsally – it would have covered this part of the jugal in the undistorted skull. The jugal is more broadly visible in dorsal and ventral views, forming the lateral margin of the subtemporal fenestra.

The frontal is a large bone making up most of the interorbital and a sizable portion of the intertemporal region ( Fig. 1 View Figure 1 ). The right frontal makes a relatively short contribution to the orbital margin compared with most emydopoids, but is similar to the condition in Cistecephalus . The broader contribution of the left prefrontal to the orbital margin appears to be attributable to damage to the preceding prefrontal, which has been crushed inwards. No postfrontal is present. The frontal has a smooth, uninterrupted border with the postorbital along its lateral edge. Posteriorly, the frontals are separated by a tripartite process made up of the preparietal and paired anterior projections of the parietals. A narrow posterior process of the frontal extends to the level of the pineal foramen.

The postorbital has a narrow anterior process that forms the posterodorsal part of the orbital margin ( Figs. 1 View Figure 1 , 2 View Figure 2 ). The thick postorbital bar appears to be composed entirely of the postorbital bone, without a substantive ventral contribution by the jugal. The posterior ramus of the postorbital is strongly biplanar, with a nearly 90 ◦ angle between its exposure on the skull roof and in the temporal fenestra. Within the temporal fenestra, the postorbital extends to the posterior end of the skull, but is excluded from reaching the occipital edge of the skull roof by a lateral expansion of the parietal ( Fig. 2b View Figure 2 ). The postorbital is also excluded from the back of the skull roof by an extension of the parietal in Kombuisia , Myosaurus , and cistecephalids, but the postorbital contribution to the skull roof is significantly shorter in Digalodon than in those taxa.

The preparietal is a narrow, finger-like element extending forward from the anterior margin of the pineal foramen ( Fig. 1 View Figure 1 ). It is nearly equal in length and width to the paired processes of the parietals that flank it, but extends slightly anterior to them in the form of a very thin, attenuate anterior process. The preparietal is flush with the skull roof, following the slope of the intertemporal region posterodorsally. The pineal foramen is located at the junction between the parietals and the preparietal and is an elongate, ovoid opening. It is flanked laterally by swollen, “lip-like” eminences of the parietals ( Fig. 1a View Figure 1 ). These “lips” do not form a complete pineal boss, but are separated by shallow grooves at the posterior and anterior edges of the pineal foramen, as in the enigmatic probable emydopoid Compsodon ( Angielczyk et al., 2014) . As mentioned above, the parietal expands laterally towards its posterior end, excluding the postorbital from the dorsal skull roof.

The squamosal is the largest bone in the skull, making up most of the zygomatic arch and the lateral margins of the occiput ( Figs. 1 View Figure 1 , 2 View Figure 2 ). The squamosal is displaced anteriorly on the right side of the skull, obscuring the suborbital portions of the maxilla and jugal. It is also displaced dorsally on the left side, exposing part of the subtemporal portion of the jugal. Although the anterior tip of the left squamosal is broken, sutures around the underlying bone indicate that it contacted the maxilla below the orbit. The zygomatic ramus of the squamosal is remarkably tall for an emydopoid and, uniquely in the group, its dorsal edge is thickened and “folded over” ( Fig. 2c View Figure 2 ). This morphology is typical of geikiids (e.g., Aulacephalodon , Pelanomodon ), as noted by Cluver and King (1983). Ventral to the zygomatic arch, the squamosal forms a broad plate, overlain anteroventrally by the quadratojugal. In occipital view, there is a dorsolateral notch in the squamosal below the zygomatic arch ( Fig. 4 View Figure 4 ), a feature known only in Dicynodontoides , Kombuisia , and Compsodon among emydopoids. The squamosal is a major component of the occiput, making up the lateral borders of the interparietal, supraoccipital, and paroccipital process of the opisthotic. Contact with the tabular is obscured by breakage and matrix, but must have been present. The ventral process of the squamosal completely obscures the quadratojugal posteriorly. The squamosal makes a small contribution to the lateral margin of the post-temporal fenestra. This fenestra is ovoid, angled slightly dorsolaterally, and is at a similar height on the occiput as the foramen magnum. The rest of the fenestra margin is made up of the supraoccipital dorsally and opisthotic ventrally.

The palatine ( Fig. 3 View Figure 3 ) is similar in morphology to that of Diictodon or Emydops , and unlike the extremely reduced condition in Dicynodontoides . The expanded anterior portion of the palatine extends laterally, overhanging the internal choana. It lacks the “leaf-shaped” morphology typical of Pristerodon . As in other emydopoids, the palatine surface is relatively smooth, with only fine pitting, unlike the highly rugose palatines of bidentalians. Unlike in Myosaurus , the palatine surface is not pierced by a foramen. Both palatines are displaced in this skull: anteriorly for the right and medially for the left. The ectopterygoid is lateral to the palatine and is similar in size. It has a strongly interdigitated anterior suture with the maxilla.

The vomer is a fused midline element exposed posterior to the secondary palate ( Fig. 3 View Figure 3 ). Anteriorly, it forms a narrow rod that is confluent with the posterior median palatal ridge of the premaxilla. Posteriorly, the ventral surface of the vomer diverges into two ridges surrounding the interpterygoid vacuity. This vacuity is obscured by matrix in this specimen, but it is likely that it housed the cultriform process of the parasphenoid as in other dicynodonts.

The anterior rami of the pterygoid are angled anterolaterally ( Fig. 3 View Figure 3 ). The left ramus is bent due to distortion, but the right one is mostly straight, unlike the curved rami of Pristerodon and many dicynodontoids. No posteriorly converging ridges are visible on these rami, which is probably the result of overpreparation, as they are clearly present in USNM 22941 (see Fig. 9b View Figure 9 ). The median pterygoid plate is broad, and a well-developed crista oesophagea was clearly present, but its surface is damaged and partially covered with matrix. The quadrate rami of the pterygoid are poorly preserved. Only the left quadrate ramus is exposed; it is a thin, rod-like structure directed posterolaterally.

Both stapes are preserved in articulation, extending between the quadrate and basal tuber. They are dumbbellshaped and imperforate, as is typical of dicynodonts. Because of overlying matrix, it is uncertain whether a stapedial dorsal process was present: this process is absent in Emydops and Dicynodontoides , but present in basal dicynodonts as well as Kombuisia , Myosaurus , and cistecephalids ( Fröbisch, 2007). The quadrate and quadratojugal are plate-like elements bearing prominent ventral articular surfaces for contact with the mandible. They are of typical dicynodont morphology ( King, 1988; Angielczyk and Rubidge, 2013).

In the basicranium, the parasphenoid, basisphenoid, basioccipital, and opisthotic have fused into a single element. It is unknown whether the prootic is also part of this fused unit, as in some other dicynodonts, as its border cannot be clearly seen in this specimen. The stapedial facet of the basal tuber is angled ventrolaterally, as in other emydopoids. Oddly, the exoccipital does not seem to be completely fused to the other basicranial elements, as a suture with at least the opisthotic is present ( Fig. 4 View Figure 4 ). The occipital condyle is tripartite, with a well-developed depression between the basioccipital and two exoccipital portions. The paroccipital process of the opisthotic is transversely short and very tall, with the greatest height at its lateral margin. Near the midheight of its lateral margin, this process bears a knob-like paroccipital eminence (tympanic process of Cox, 1959). This structure is typically well developed in emydopoids, most notably in Emydops , where it forms a spike-like posterior protrusion.

The foramen magnum is roughly triangular, narrowing in height dorsally ( Fig. 4 View Figure 4 ). It is surrounded by the exoccipitals at base and supraoccipital at apex. The supraoccipital is a broad, flat element that is narrowest above the foramen magnum. The interparietal is a large, roughly trapezoidal bone making up most of the dorsal portion of the occipital plate. A weak nuchal crest is present on the interparietal midline. The tabular is poorly preserved, with only partial exposure on the left side of the occiput.