Introduction
Emydopoidea is one of the three major groups of “advanced” dicynodont
therapsids (therochelonians sensu Kammerer and Angielczyk, 2009). Unlike the
other therochelonians, which are primarily large-bodied (> 1 m
body length) animals, emydopoids are typically small, with only a single
genus (Dicynodontoides) attaining a skull length greater than 15 cm (Angielczyk et
al., 2009). Four families of emydopoids are known: the basal Emydopidae
(containing Emydops) and the three clades united in Kistecephalia – Kingoriidae
(Dicynodontoides and Kombuisia), Myosauridae (Myosaurus), and the fossorial Cistecephalidae (Cistecephalus,
Cistecephaloides, and Kawingasaurus). Emydopoid taxonomy and anatomy have been extensively studied in the
past few decades and their phylogenetic relationships are generally well
understood (Cox, 1959, 1972; Cluver, 1978; Hammer and Cosgriff, 1981;
Angielczyk et al., 2005, 2009; Fröbisch, 2007; Fröbisch and Reisz,
2008). Emydopoids are common in the early Late Permian of South Africa, with
hundreds of specimens recovered from the Tropidostoma and Cistecephalus assemblage zones (AZs) of the
Karoo Basin (Smith et al., 2012). In the latest Permian (Dicynodon AZ), however,
emydopoids have traditionally been considered a rare faunal component, part
of a general pattern of extinction of small dicynodonts (e.g., Emydops, but also
including Diictodon and Pristerodon) well before the terminal Permian extinction (Smith
and Botha-Brink, 2014). Despite the extinction of these well-known genera, there
are various enigmatic, emydopoid-like small dicynodonts from the Late
Permian that have been historically understudied and may influence this
supposed pattern.
Broom and Robinson (1948) described Digalodon rubidgei based on a single skull (RC 76)
collected in the Dicynodon AZ of Ferndale, Graaff-Reinet. The new taxon was diagnosed
primarily by characters of the intertemporal region. For example, the
lengthy intertemporal portion of the frontals in RC 76 was contrasted with
the condition in “all typical Dicynodonts” [sic] in which “the frontals
lie mainly between the orbits” (Broom and Robinson, 1948:402). It should be
noted that the frontal morphology of RC 76 is actually very similar to that
of the common Permian dicynodont Pristerodon, and a long contribution of the frontal to
the intertemporal region is also present in myosaurids and cistecephalids.
In their defense, Broom and Robinson (1948:404) considered at least
Myosaurus and Cistecephalus to be atypical dicynodonts, “much specialized and degenerate types”,
and even mentioned that they “may have evolved from a dicynodont like
Digalodon”. Broom and Robinson (1948) also distinguished Digalodon from Dicynodon (sensu lato,
including Diictodon, Oudenodon, and various other taxa – see Kammerer et al., 2011) by its
broad parietal exposure in the intertemporal region. They noted (correctly)
that this represents a primitive character common in small-bodied
dicynodonts like Pristerodon and most emydopoids.
Following its initial description, Digalodon was largely ignored by subsequent
workers, appearing primarily in comprehensive lists of Karoo vertebrates or
therapsid taxa (e.g., Haughton and Brink, 1954; Romer, 1956; Lehman, 1961). Two
attempts were made to refer the mysterious type specimen of D. rubidgei to a
better-known dicynodont taxon. Cluver and King (1983) noted a few
similarities between Digalodon and Aulacephalodon, leading King (1988) to consider them tentative
synonyms (presumably considering the much smaller Digalodon a juvenile of the latter
genus). Brink (1986) listed Digalodon rubidgei as a junior synonym of
Dicynodontoides parringtoni (i.e., Dicynodontoides recurvidens)
without comment. Most recently, Angielczyk et al. (2009) reconsidered
the synonymy of Digalodon and Dicynodontoides, and concluded that Digalodon represents a distinct taxon
possibly related to Emydops. Furthermore, they referred two additional specimens to
D. rubidgei (RC 469 and USNM 22941), but noted that these referrals were tentative
pending a full redescription of the species.
Photograph (a) and interpretive drawing (b) of RC 76,
the holotype of Digalodon rubidgei, in dorsal view. Gray indicates matrix, hatching indicates
damaged bone surface, and cross-hatching indicates plaster. Scale bar equals
1 cm. Abbreviations: f, frontal; ip, interparietal;
j, jugal; la, lacrimal; mx, maxilla; na, nasal; pa,
parietal; pf, pineal foramen; pmx, premaxilla; po,
postorbital; pp, preparietal; pr, prootic; prf,
prefrontal; sq, squamosal; ta, tabular.
Here, we completely redescribe the cranial morphology of Digalodon rubidgei based on the
holotype. Additionally, we address the status of additional skulls that may
be referable to Digalodon and consider the geographic and stratigraphic range of this
taxon and its phylogenetic position.
Institutional Abbreviations – B, Bremner Collection, Graaff-Reinet Museum, Graaff-Reinet, South
Africa; BP, Evolutionary Studies Institute (formerly Bernard Price
Institute for Palaeontological Research), University of the Witwatersrand,
Johannesburg, South Africa; RC, Rubidge Collection, Wellwood,
Graaff-Reinet District, South Africa; USNM, National Museum of Natural
History, Smithsonian Institution, Washington, D.C., USA.
Systematic paleontology
Synapsida Broom, 1905
Anomodontia Owen, 1860a
Dicynodontia Owen, 1860a
Superfamily Emydopoidea van Hoepen, 1934
Revised definition: Emydops arctatus (Owen, 1876) and all taxa more closely related to it
than to Oudenodon bainii Owen, 1860b, Dicynodon lacerticeps
Owen, 1845, Diictodon feliceps (Owen, 1876), or Endothiodon bathystoma Owen, 1876 (modified
from Kammerer and Angielczyk, 2009). Kammerer and Angielczyk's (2009)
definition of Emydopoidea used only Oudenodon and Dicynodon as external specifiers, based on
existing phylogenies (e.g., Angielczyk, 2001, 2007; Angielczyk and Kurkin
2003; Fröbisch, 2007) that reconstructed pylaecephalids (Diictodon and allies) and
endothiodontids near the base of Dicynodontia. More recent analyses
(Kammerer et al., 2011, 2013) have recovered a sister-group relationship
between pylaecephalids and the traditional emydopoids (i.e., Emydops +
Kistecephalia). Furthermore, recent discoveries (Castanhinha et al., 2013;
Cox and Angielczyk, 2015) have revealed a distribution of character
states that blurs the traditional morphological distinctions between
“endothiodonts” (sensu Kammerer and Angielczyk, 2009) and emydopoids (e.g.,
similar palatine pad morphologies and premaxillary teeth in Niassodon, a taxon
Castanhinha et al. recovered as a kingoriid emydopoid). We consider it in
the best interests of taxonomic stability to refine the definition of the
robustly diagnosed group Emydopoidea to better ensure its traditional
composition is maintained.
Digalodon Broom and Robinson, 1948
Type species: Digalodon rubidgei Broom and Robinson, 1948.
Diagnosis: as for type and only species.
Digalodon rubidgei Broom and Robinson, 1948
Figures 1–4, 6–10
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, 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.
RC 76, the holotype of Digalodon rubidgei, in left lateral (a,
photograph; b, interpretive drawing) and right lateral (c,
photograph; d, interpretive drawing) views. Gray indicates matrix, hatching
indicates damaged bone surface, and cross-hatching indicates plaster. Scale
bar equals 1 cm. Abbreviations: cp, caniniform process;
ec, ectopterygoid; f, frontal; j, jugal; la, lacrimal;
lcf, lacrimal foramen; mx, maxilla; na, nasal; pa,
parietal; pmx, premaxilla; po, postorbital; prf,
prefrontal; pt, pterygoid; q, quadrate; qj, quadratojugal;
sq, squamosal; t, tusk; vf, vascular foramen.
Photograph (a) and interpretive drawing (b) of RC 76,
the holotype of Digalodon rubidgei, in ventral view. Gray indicates matrix, hatching indicates
damaged bone surface, and cross-hatching indicates plaster. Scale bar equals
1 cm. Abbreviations: apr, anterior palatal ridge; apt,
anterior pterygoid ramus; bo, basioccipital; bt, basal tuber;
co, crista oesophagea; cp, caniniform process; ec,
ectopterygoid; ipv, interpterygoid vacuity; j, jugal; mpr,
posterior median palatal ridge; mx, maxilla; op, opisthotic;
pl, palatine; pmx, premaxilla; ps, parasphenoid; q, quadrate; qpt, quadrate pterygoid ramus; sq,
squamosal; st, stapes; t, tusk; v, vomer; vf,
vascular foramen.
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). 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). 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). 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.
Photograph (a) and interpretive drawing (b) of RC 76,
the holotype of Digalodon rubidgei, in occipital view. Gray indicates matrix, hatching
indicates damaged bone surface, and cross-hatching indicates plaster. Scale
bar equals 1 cm. Abbreviations: bo, basioccipital; dn,
dorsolateral notch in squamosal; eo, exoccipital; fm, foramen
magnum; ip, interparietal; op, opisthotic; pa, parietal;
pe, paroccipital eminence; ptf, post-temporal fenestra; q,
quadrate; so, supraoccipital; sq, squamosal; st, stapes;
ta, tabular.
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) because of shear; in life it would have been intermediate in
length between what is shown in Fig. 2a and c. 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).
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, 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). 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) but the right one is well preserved
(Figs. 1, 2d). 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). 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). 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). The left jugal is also exposed
laterally below the postorbital bar, beneath the squamosal (Fig. 2b), 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). 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, 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). 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). 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). 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, 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). 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), 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) 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). 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). 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). 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 dumbbell-shaped 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). 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.
Phylogenetic position of Digalodon rubidgei
within Dicynodontia based on the results
of the phylogenetic analysis. Eo. = Eodicynodon.
Photographs of B 42, a referred specimen of Digalodon rubidgei, in dorsal (a),
palatal (b), right lateral (c), left lateral (d),
occipital (e), and anterior (f) views. Scale bar equals 1 cm.
Abbreviations: ae, anterior emargination of palatal rim;
apr, anterior palatal ridge; co, crista oesophagea; ip,
interparietal; lar, lateral anterior palatal ridge; lpf, lateral
palatal foramen; pa, parietal; pla, pila antotica; ve,
vertebra; vf, vascular foramen.
Photographs of BP/1/157, a referred specimen of Digalodon rubidgei,
in dorsal (a) and left lateral (b) views. Abbreviations: cp,
caniniform process; ds, tip of dentary symphysis; nb, nasal
boss. Scale bar equals 1 cm.
The foramen magnum is roughly triangular, narrowing in height dorsally (Fig. 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.
Discussion
Status of other “large emydopoid” material from Graaff-Reinet
Although RC 76 was long considered unique, Angielczyk et al. (2009)
tentatively referred two additional specimens from the area around
Graaff-Reinet (RC 469 and USNM 22941) to Digalodon rubidgei. In our examination of dicynodont
material from this area, we have found further emydopoid material that is
not referable to Emydops, Dicynodontoides, myosaurids, or cistecephalids. We discuss the status of
these specimens below.
There are three specimens we consider definitely referable to Digalodon rubidgei: B 42,
BP/1/157, and RC 469. B 42 is a mostly complete but heavily worn skull
missing part of the right temporal arch (Fig. 6). The snout is worn off,
causing it to appear shorter than it would have been in life. This specimen
can be identified as an emydopoid on the basis of an embayment in the
palatal rim anterior to the caniniform process. Additionally, it can be
confidently referred to D. rubidgei by the relatively short contribution of the
postorbital to the skull roof (Fig. 6a), presence of paired anterior palatal
ridges (Fig. 6b), and very tall, thick zygomatic arch (Fig. 6c, d). For the
most part, the heavy wear on this skull makes it less morphologically
informative than the holotype. However, there are a few areas where it
clarifies the morphology of damaged regions of the holotype. Other than a
bent basicranial girder (Fig. 6b), B 42 is largely undistorted. Unlike in
the holotype, in which an irregular portion of the interparietal is exposed
dorsally at the back of the intertemporal bar (Fig. 1), the interparietal of
B 42 is restricted to the occiput (Fig. 6a). This indicates that the broad
contribution of the interparietal to the skull roof in the holotype is not
natural, but is the result of the right side of the skull being sheared
forward.
Photograph of RC 469, a specimen referable to Digalodon rubidgei, in dorsal view.
Scale bar equals 1 cm.
The palate of B 42 is better preserved than in the holotype, showing the
palatines in their natural orientation and the vomer in three dimensions
(Fig. 6b). Unlike the holotype, this specimen preserves clear lateral
anterior palatal ridges, which are typical of pylaecephalids and emydopoids.
In B 42, they are restricted to the maxilla, but it is likely they
originally extended on to the (now worn off) premaxillary tip. An elongate
lateral palatal foramen is present between the expanded anterior portion of
the palatine and maxilla/ectopterygoid. These foramina are typical of
dicynodonts and were probably obscured by crushing in the holotype. Finally,
it shows that the crista oesophagea was tall and sharp.
Breakage of the right temporal arch in B 42 has exposed the lateral wall of
the braincase (Fig. 6c), a region that is poorly exposed in the holotype.
The posterior wall of the temporal fenestra is a flattened plate made up of
the squamosal (dorsally), the internal portion of the supraoccipital, and
the prootic. It is still uncertain whether the prootic is fused with the
opisthotic, as sutures are poorly preserved in this specimen. The prootic
bears a distinct, anteriorly directed pila antotica, as is primitive for
therapsids. Part of the epipterygoid footplate is preserved ventrolateral to
the prootic, but the ascending process (columella) is either broken off or
obscured by matrix.
An isolated vertebra is preserved lodged in the right temporal fenestra of B
42 (Fig. 6b, c). It is an unremarkable dicynodont dorsal vertebra. This is
the only postcranial material associated with a Digalodon specimen.
BP/1/157 is a distorted and very poorly prepared skull, but importantly and
uniquely among Digalodon specimens it preserves the mandible (Fig. 7b). This specimen
can be identified as D. rubidgei on the basis of its relatively long “beak” that is
sharply demarcated from the caniniform process, tall zygomatic arch, and
exclusion of the postorbital from the posterior skull roof by a lateral
expansion of the parietal. The mandibular symphysis is typically emydopoid,
with a tall, shovel-like beak tip. A mandibular fenestra overlain by a
lateral dentary shelf is present, although the morphology of this shelf
cannot be determined due to damage. This specimen also preserves the nasal
bosses, and demonstrates that only a small median boss was present, as in
nearly all non-bidentalian dicynodonts.
USNM 22941, a “large emydopoid” from Richmond considered a
possible sexually dimorphic female of Digalodon rubidgei in dorsal
(a), palatal (b), left lateral (c), and occipital (d) views. Scale bar equals
1 cm.
Specimens tentatively considered juvenile representatives of
Digalodon rubidgei. RC 303 in dorsal (a) and right lateral (b) views. RC 306 in
dorsal (c) and right lateral (d) views. Scale bars equal 1 cm.
RC 469 is a very incompletely prepared partial skull (Fig. 8) with its snout
broken off at the level of the tusks (a section through the tusk roots can
be seen in anterior view). It can be identified as Digalodon rubidgei on the basis of the
lateral parietal expansion excluding the postorbital from the back of the
skull roof. The morphologies of the preparietal and flanking anterior
parietal processes in this specimen are nearly identical to that of the
holotype.
There are several other specimens that we have identified that may also
represent Digalodon rubidgei, but they show some differences that prevent us from definitively
referring them to the genus at this time. USNM 22941 is a somewhat
dorsoventrally flattened specimen with a worn-off ventral margin to the
snout (Fig. 9). This specimen is clearly an emydopoid (precaniniform
embayment present), but general skull proportions preclude a myosaurid or
cistecephalid identification. The broad intertemporal region is unlike
kingoriids, and it is larger than any known individual of Emydops. USNM 22941
exhibits the posterior expansion of the parietal (excluding the postorbital
from the back of the skull roof) here considered characteristic of
Digalodon rubidgei among emydopoids. However, it differs from the previously discussed
specimens of D. rubidgei in several striking features. Although a weak raised edge is
present around the pineal foramen, no swollen, “lip-like” structure is
present – the intertemporal region is fairly flat (although angled upwards
posteriorly, as in RC 76). Also, the zygomatic ramus of the squamosal is
very dorsoventrally thin compared to the other specimens of D. rubidgei. It could be
possible to explain this difference by taphonomic distortion, with the
apparently “taller” zygoma being the result of crushing. However, although
this could be invoked for RC 76 and BP/1/157, it cannot explain the presence
of this morphology in the nearly undistorted B 42. Furthermore, the
thickened dorsal margin of the zygoma in RC 76 cannot be explained
taphonomically, considering that its edge is downturned perpendicular to the
direction of shear in the skull. As such, we consider the differences in
morphology between the zygoma of RC 76 and USNM 22941 to be real.
Photographs of “large emydopoid” material from the Graaff-Reinet
area referable to Compsodon helmoedi. RC 641, a specimen referable
to Compsodon helmoedi in dorsal (a)
view. RC 736, a specimen referable to Compsodon helmoedi in dorsal (b) and right lateral
(c) views. Scale bars equal 1 cm.
Unlike all previously discussed specimens, USNM 22941 clearly lacks tusks.
Sexually dimorphic absence in tusks is known in the pylaecephalid Diictodon feliceps (Sullivan
et al., 2003; Sullivan and Reisz, 2005). Intriguingly, Sullivan and Reisz
(2005) also argued that pineal boss development is sexually dimorphic in
Diictodon. Development of the thickened, rugose zygoma in Aulacephalodon has also been considered a
sexually dimorphic feature (Tollman et al., 1980). It is possible, then,
that Digalodon rubidgei was an extremely sexually dimorphic taxon, with males (represented by
RC 76, RC 469, B 42, and BP/1/157) bearing tusks and having a swollen pineal
region and tall, thickened zygoma, whereas females (represented by USNM
22941) lacked tusks and had a flat pineal region and narrow zygoma. More
material is required to address this issue.
Several additional small, incompletely prepared specimens (RC 303, 304, and
306) closely resemble USNM 22941 and may also represent Digalodon rubidgei (Fig. 10). These
specimens all have a thin zygomatic arch; given their significantly smaller
size than the specimens discussed above, their lack of what we interpret as
a secondary sexual feature in Digalodon could be attributable to immaturity rather
than sex.
Two other “large emydopoid” specimens from Graaff-Reinet with swollen
intertemporal regions are worth considering. RC 641 is a partially prepared
skull from Ferndale, Graaff-Reinet (Fig. 11a). The smaller of the two
skulls, it has suffered anterior shear of the skull roof. RC 736 is a
damaged skull from Boskraal, Graaff-Reinet (Fig. 11b, c). Like Digalodon, these have
a broad intertemporal bar at its anterior end (unlike kingoriids). However,
instead of having a swollen region formed by the parietal, as in
Digalodon, the swollen portion in these specimens represents expansion of the
postorbitals, greatly constricting the exposure of the parietals posterior
to the pineal foramen. This morphology is typical of Compsodon helmoedi (Angielczyk et
al., 2014), and we suggest that these specimens represent additional South
African specimens of this poorly known taxon. Unfortunately, poor
preservation of these specimens obscures whether they had a distinct
postorbital (present in Compsodon, but absent in Digalodon). This identification will be
considered in further detail in a complete description of the Zambian
material of C. helmoedi, currently under preparation.
Stratigraphic and geographic range of Digalodon rubidgei
Even if the tentatively referred “female” and “subadult” specimens are taken
into consideration, Digalodon specimens are restricted to a very narrow portion of
the Karoo Basin, being found only in the central basin near the junction
between the Western, Eastern, and Northern Cape provinces, primarily in the
Camdeboo Local Municipality surrounding Graaff-Reinet. These strata
represent a limited temporal range, covering the upper
Cistecephalus AZ and Dicynodon AZ, and
yield a unique, localized dicynodont fauna. In addition to Digalodon, all known
specimens of Pelanomodon (Broom, 1938), Kitchinganomodon (Maisch, 2002),
Keyseria, and Basilodon (Kammerer et al., 2011) have been found in this area. Intriguingly,
the rare dicynodont Compsodon is also part of this fauna; this taxon has recently
been found to be a component of the therapsid fauna of the Upper Madumabisa
Mudstone Formation of Zambia (Angielczyk et al., 2014), potentially allowing
for finer correlation of the Zambian fauna with this narrow section of the
Beaufort Group.