Pristerodon mackayi, HUXLEY, 1868
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publication ID |
https://doi.org/ 10.1093/zoolinnean/zlac033 |
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https://doi.org/10.5281/zenodo.7922155 |
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persistent identifier |
https://treatment.plazi.org/id/3F6A87AB-FFAC-FFF4-7281-F88FFCF6FD96 |
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Plazi |
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scientific name |
Pristerodon mackayi |
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PRISTERODON MACKAYI HUXLEY, 1868
Specimen: BP/1/2642.
In the present study, Pristerodon is used as the outgroup taxon for comparison with the emydopoids because it displays the more generalized anatomy of basal dicynodonts (Barry, 1967; Cluver & King, 1983; Keyser, 1993; King & Rubidge, 1993; Ray, 2001; Modesto, 2003; Angielczyk, 2007; Laass, 2015).
Orbitosphenoid: The orbitosphenoid is a verticallyoriented bone located at the skull midline ( Figs 1 View Figure 1 , 2B View Figure 2 , 3 View Figure 3 ). It contacts the frontal and parietal dorsally and the parabasisphenoid complex ventrally and supports the olfactory bulbs in life ( Hopson, 1979). Its exact anatomical constituents have been heavily debated ( Olson, 1938, 1944; Barry, 1967; Kemp, 1969; Cluver, 1971), but it is generally agreed that the orbitosphenoid consists of two dorsally projecting wings and a medial vertical crest (the mesethmoid) in non-mammalian synapsids ( Araújo et al., 2017; Angielczyk et al., 2019). However, the homology between the orbitosphenoid of therapsids and the sphenethmoid of other amniotes remains uncertain. The orbitosphenoid of Pristerodon conforms to the typical synapsid pattern, displaying a characteristic Y-shaped cross-section (Laass, 2015).
In anterior view, the wings of the orbitosphenoid (obw) are curved, which results in a slightly convex outer wall, making an angle of 48° with the sagittal plane ( Fig. 3A View Figure 3 ). Anterodorsally, the median vertical process (obvp) projects dorsally and divides the olfactory cavity medially (ofc). The median vertical process is slender at its midheight but has expanded dorsal and ventral edges in anterior view ( Fig. 3A View Figure 3 ). At its dorsalmost part, the median vertical process forms a broad horizontal surface (hpl) that contacts the frontal anterodorsally. This articulation surface is oval in dorsal view ( Fig. 3B View Figure 3 ). The wings of the orbitosphenoid are thin anteriorly but are thicker posteriorly ( Fig. 3B View Figure 3 ). In dorsal view, the wings of the orbitosphenoid form a long anteroposterior gutter that host the olfactory bulbs ( Fig. 3B View Figure 3 ).
In lateral view, the orbitosphenoid has a subtrapezoidal outline. The dorsal margin of the wings is horizontal in lateral view, except for a notch that excavates the wings at midlength (obdn, Fig. 3C View Figure 3 ). The ventral margin of the mesethmoid articulates with the dorsal groove of the parasphenoid rostrum. In lateral view, the mesethmoid extends posteriorly as a wall (mpw) that supports the dorsal wings ( Fig. 3C View Figure 3 ).
Pterygoid: In Pristerodon , the pterygoids (pt) form an important part of the palate and display the typical dicynodont X-shaped morphology in ventral view ( Figs 2D View Figure 2 , 3 View Figure 3 ). The median plate sends out divergent anterior palatal and posterior quadrate rami that form an angle of 115° between the two rami, measured on the lateral surface of the pterygoid. The palatal and quadrate rami of the pterygoid form an angle of 18° and ~56° with the anteroposterior median axis of the skull, respectively. The median plate of the pterygoid forms an interdigitated suture with the parabasisphenoid internally, but this suture has an M-like outline in ventral view ( Fig. 3F View Figure 3 ). The pterygoid bounds the narrow and long interpterygoid vacuity just anterior to the median plate, and in this taxon the vacuity is bounded anteriorly by the vomer (Barry, 1967; Cluver & King, 1983; Keyser, 1993). The pterygoid in BP/1/2642 has several cracks which hampers the description of the complete morphology of the quadrate ramus ( Fig. 3E, F View Figure 3 ). However, as illustrated by other specimens of Pristerodon (e.g. BP/1/7206, BP/1/3024), the quadrate ramus of BP/1/2642 is thin and laterally compressed, similar to most dicynodonts.
In dorsal view, there is a small anteromedial process (pt amp, Fig. 3E View Figure 3 ) that develops on the dorsal edge of the palatal ramus of the pterygoid in BP/1/2642, similar to that described as the anterior pterygoidal process in SAM-PK-10153 by Cluver & King (1983) [but it is not the ‘anterior pterygoid blade’ of Cluver (1974b), which referred to the anterior tip of the palatal ramus more generally]. The two processes nearly touch each other at the midline and contact the vomer medially ( Cluver & King, 1983). The anteromedial pterygoid process is separated from the rest of the anterior ramus by an anteroposterior notch (pt nt, Fig. 3E View Figure 3 ). The pterygoid median plate is overlapped dorsally by the laterally expanded basisphenoid.
In lateral view, the palatal ramus is dorsoventrally taller anteriorly than it is posteriorly. This ramus presents a lateral process (ptlp) on its anteroventral region ( Fig. 3H View Figure 3 ). The divergence of the anteromedial process gives a Y-shape to the palatal ramus in lateral view. The median plate expands laterally, forming the short and stout pterygoid lateral lamina (ptll). The pterygoid lamina is bordered by a sulcus dorsally (pt dsu, Fig. 3H View Figure 3 ). The quadrate ramus is considerably narrower than the palatal ramus. It projects posteriorly and reaches the medial side of the quadrate. A small triangular portion of the epipterygoid footplate (epi) is discernible on the left quadrate ramus ( Fig. 3H View Figure 3 ). Due to damage, we could not determine how much of the epipterygoid overlaps the quadrate ramus of the pterygoid.
The anterior margin of the palatal ramus is U-shaped in ventral view. Because a lamina is present dorsolaterally (ptll), the pterygoid median plate is wider posteriorly than the anterior ramus in Pristerodon . There is a trough medial to the pterygoid lamina that tapers posteriorly and anteriorly in ventral view (ptvt, Fig. 3F View Figure 3 ). This excavation is delimited medially by the prominent crista oesophagea (co), which is composed of a pair of anteroposteriorly oriented ridges in BP/1/2642. A deep, elongate anteroposterior furrow (fu) is present posteriorly between these ridges ( Fig. 3F View Figure 3 ). This furrow is not visible in better-preserved specimens of Pristerodon (e.g. BP/1/3024, BP/1/7206, SAM-PK-10153 and USNM 23580), where the two sides of the crista oesophagea are fused into a single, thin median ridge. Exposure of the furrow and demarcation of the component ridges in BP/1/2642 can be attributed to damage incurred through acid preparation of this specimen (C.F. Kammerer, pers. comm.).
Parabasisphenoid: The parabasisphenoid complex (pbs) forms the anterior floor of the braincase ( Figs 2 View Figure 2 , 3 View Figure 3 ). The parabasisphenoid in BP/1/2642 has many minute cracks, but a small portion of the parasphenoid rostrum (ps) is present together with the basipresphenoidal region (anterior to the pituitary fossa) and its basipostsphenoid portion (posterior to the pituitary fossa). The basipostsphenoid portion in Pristerodon protrudes posteroventrally and overlaps the basioccipital forming the basal tubera, and encloses the carotid foramina more anteriorly as in other dicynodonts ( Modesto et al., 2003; Castanhinha et al., 2013). Along its dorsal aspect, the basipostsphenoid hosts the pituitary gland posterior to the sella turcica. In Pristerodon , the complex formed by the parasphenoid rostrum and basipresphenoid (PRB complex) is slender, tapers posteriorly and in dorsal view has an anteroposterior groove (psgr) on its anterodorsal portion ( Fig. 3I View Figure 3 ). The posterior half of the PRB bears a tall process of triangular outline in lateral view ( Fig. 3J View Figure 3 ).
In dorsal view, the Pristerodon basipostsphenoid flares laterally from the posterior end of the parasphenoid rostrum ( Fig. 3I View Figure 3 ). It is excavated by a subvertical concavity (bpdc), which is delineated ventromedially by the tuberculum sellae (tse). The left and right tuberculum sellae meet on the midline anterior to the sella turcica and continue towards the border with the parasphenoid rostrum.The tuberculum sellae marks the anterior border of the sella turcica (stu, Fig. 3I View Figure 3 ). The base of the pituitary fossa (sella turcica) is perforated by the carotid foramina (ic) and is bordered laterally by a discrete clinoid process (clp). The clinoid processes ( Fig. 3I, J View Figure 3 ) consist of thin crests that give a triangular outline to the pituitary fossa in dorsal view. Posterior to the pituitary fossa, the dorsum sellae (ds) forms a low tuberosity. The carotid foramina do not join dorsally into a single orifice. The carotid foramina in Pristerodon are oval with the longest diameter extending anteroposteriorly in ventral view ( Fig. 3K View Figure 3 ). The distance between the two internal carotid foramina in ventral view is narrow in Pristerodon (1.2 mm).
In ventral view, the basipostsphenoid contacts the basioccipital posteriorly along a sigmoidal suture and, the pterygoid anteriorly along an interdigitated M-shape suture. There is an anteroposterior sulcus with a lobate shape on the posterior median end of the basipostsphenoid, delimited by the basisphenoidal tubera laterally (bpvsu, Fig. 3K View Figure 3 ). This sulcus is 3.5 mm wide and extends posteriorly onto the basioccipital. The posteroventrally projected basisphenoidal tubera (bpt) form buttress-like structures that broaden ventrally, where they form a wall being excluded from the fenestra ovalis posteriorly.
Basioccipital: The basioccipital (bo) in Pristerodon forms the posterior portion of the braincase floor, posterior to the parabasisphenoid complex ( Figs 2 View Figure 2 , 4 View Figure 4 ). The basioccipital in Pristerodon is expanded mediolaterally but tapers anteriorly and posteriorly, resulting in a subhexagonal shape in both ventral and dorsal views. The basioccipital can be divided into two main anatomical subunits: the basioccipital portion of the occipital condyle posteriorly and the basioccipital tubera anterolaterally.
In posterior view, the basioccipital displays an inverted U-shape with the tubera pointing ventrally. As in other dicynodonts, the basioccipital in Pristerodon forms the ventral lobe of the occipital condyle and extends laterally to border the vestibule (fenestra ovalis) and the jugular foramen medially. In posterior view, the basioccipital condyle (boc) is less prominent than the exoccipital condyles ( Figs 3G View Figure 3 , 4A View Figure 4 ), and its dorsal portion flares out laterally to form two symmetrical concavities (bocc) in which the exoccipital condyles are hosted ( Fig. 4A View Figure 4 ). The two concavities are separated by a median ridge (bocr) on the dorsal face of the basioccipital condyle. There is an occipital pit at the intersection between the basioccipital and exoccipital condyles in posterior view.
In dorsal view, the basioccipital tubera portion is outlined by two anteriorly conjoined sigmoid ridges (bosr) that terminate posteriorly, contacting the opisthotic and bordering the medial wall of the fenestra ovalis (fo, Fig. 4B View Figure 4 ). These ridges meet anteriorly to form the anteroposteriorly oriented median ridge of the basioccipital (bomr). The basioccipital median ridge is thinner than the sigmoid ridges and is present only on the anteriormost region of the basioccipital. In dorsal view, the dorsal suture of the basioccipital condyle in Pristerodon is marked by a stretched M-shaped flat edge (ms) that smoothly develops ventrally to accommodate the exoccipital condyles ( Fig. 4B View Figure 4 ). The jugular foramen (jf) opens a hemicylindrical sulcus between the M-shaped suture and the sigmoid ridge as the basioccipital forms its floor in dorsal view. In Pristerodon , only a small portion of the basioccipital contributes to the jugular foramen laterally compared to the exoccipital and opisthotic.
In ventral view, the basioccipital tubera have a smoothly excavated anteroventral surface that contacts the basisphenoidal tubera. They are separated by a deeper and wider anteroposteriorly oriented midline trough (bovtr) thatopensposteriorly ( Fig.4C View Figure 4 ).Thistrough is interrupted posteriorly by a low and blunt eminence, in a similar position to the intertuberal ridge (itr) of some other dicynodonts. Previous works on Pristerodon ( Huxley, 1868; Broom, 1915; Barry, 1967; Cluver & King, 1983) do not comment on the presence of this ridge on the basioccipital, and this structure is so weakly developed in BP/1/2642 that is should not be considered a true intertuberal ridge of the sort seen in Lystrosaurus (e.g. NMQR 3593; NMQR 815; NM C299). In Lystrosaurus and most other taxa where the intertuberal ridge is coded as present (see, e.g. Angielczyk et al., 2021), the intertuberal ridge is a tall, clearly demarcated structure comparable in height to the basal tubera. Posterior to the low, intertuberal eminence in BP/1/2642, there is a deeper, collar-like basioccipital excavation (bocex) that separates the basioccipital tubera from the basioccipital condyle. The basioccipital condyle in BP/1/2642 is spherical and is pierced by minute foramina (bocf) in ventral view. The fenestra ovalis (fo) is oval and its long axis extends mediolaterally.
In lateral view, the basioccipital tuber projects obliquely ventral to the horizontal plane, and its walls are thicker anteriorly than posteriorly ( Fig. 4D View Figure 4 ). In lateral view, the vestibule of the inner ear (ve) excavates a conical cavity on the basioccipital lateral wall (fo, Fig. 4D View Figure 4 ).
Exoccipital: The exoccipitals (eo) form the posterolateral portion of the braincase and constitute most of the ventral half of the foramen magnum and the dorsal two-thirds of the occipital condyle in most dicynodonts, including Pristerodon ( Figs 3 View Figure 3 , 4 View Figure 4 ; Cox, 1965; Modesto et al., 2003; Fröbisch, 2007; Castanhinha et al., 2013). Each exoccipital forms the medial border of the jugular foramen. Typically, the exoccipital is traversed by the canal for the hypoglossal cranial nerve (CN XII) in dicynodonts ( Castanhinha et al., 2013), but this could not be segmented in the specimen BP/1/2642 due to damage. The exoccipital contacts the supraoccipital dorsally, the opisthotic laterally and the basioccipital ventrally. The exoccipital is co-ossified to the supraoccipital and opisthotic anteriorly. The ventral suture with the basioccipital is W-shaped. The exoccipital in Pristerodon can be divided into two main components: the exoccipital condyle (eoc) and the dorsal component (eodc).
In anterior view, each exoccipital is knob-like and is excavated by the jugular foramen ( Fig. 4E View Figure 4 ). The exoccipital condyle is robust and sends a short tuber-like process (eoat) that overlaps the M-like posterodorsal edge of the basioccipital ( Fig. 4E View Figure 4 ). The base of the exoccipital condyle shows a sigmoidal outline in anterior view. The articular surface of the exoccipital condyle is convex. The dorsal component of the exoccipital has an expanded dorsal end at the contact with the supraoccipital in anterior view. The exoccipital forms the posterodorsal border of the jugular foramen (jf), which is located between the dorsal edge of the dorsal component and the exoccipital condyle in anterior view ( Fig. 4E View Figure 4 ). The foramen excavates a semicylindrical cavity in the exoccipital.
In posterior view, the foramen magnum (fm) is vertically elliptical and the exoccipital covers 40% of its ventral portion ( Figs 2 View Figure 2 , 3 View Figure 3 ). The exoccipital is dorsoventrally short in posterior view ( Figs 3G View Figure 3 , 4F View Figure 4 ). The dorsal end of the exoccipital in posterior view coincides with the greater width of the foramen magnum, which is 6 mm wide. The exoccipital condyle is typically reniform in dicynodonts but it is damaged in BP/1/2642. Therefore, the exoccipital condyle of BP/1/2642 is formed by an oblique bulge (eobg) that is overlapped by the subvertical buttress (eobt) of the dorsal component. Although the proatlas is not preserved, it is possible that this buttress could have served as an articular surface for this portion of the atlantal arch. The dorsal component forms a subvertical and hourglass-shaped buttress (eobt) that sutures with the supraoccipital dorsally and descends to overlap the exoccipital condyle. The dorsal component extends dorsally to just above the level of the dorsal margin of the jugular foramen. The two exoccipitals are co-ossified at the midline above the basioccipital ( Fig. 3G View Figure 3 ).
Supraoccipital: The supraoccipital (su) is the largest element of the occiput, forming the posterodorsal part of the braincase ( Figs 2 View Figure 2 , 3 View Figure 3 ). The supraoccipital in Pristerodon is subvertically oriented with a slight anterior tilt ( Fig. 4I View Figure 4 ). Pristerodon shows the typical anatomy of the dicynodont supraoccipital, which is divided into three main anatomical subunits (e.g. Castanhinha et al., 2013; Macungo et al., 2020): one medial lobe and two lateral alae. The supraoccipital bounds the dorsal half of the foramen magnum. Its lateral wing borders the posttemporal fenestra dorsally, and hosts the posterior half of the floccular fossa and the vestibular organ medially. Anteriorly, the supraoccipital contacts the prootic and ventrally the opisthotic and exoccipital.
In anterior view, the supraoccipital in Pristerodon is characterized by two anteriorly projected crests (sap, Fig. 4G, I View Figure 4 ) that lengthen ventrally in contact with the dorsal process of the prootic. These crests are oblique and meet dorsally at the midline. They border an anteromedial triangular recess (sar) that is bounded ventrally by an incipient horizontal ridge (sor). Although Laass (2015) and Laass et al. (2017) identified the ‘unossified zone’, this structure is dorsal to the supraoccipital. The anteromedial triangular recess is located on the anterior surface of the supraoccipital, therefore we cannot elaborate on the ‘unossified zone’. The anterior face of the supraoccipital lateral alae is perforated by small circular oblique cavities that accommodate the anterior semicircular canal (ascc) in anterior view. These cavities communicate internally to the large vertical one that accommodates the crus communis (cca, Fig. 4G, H View Figure 4 ). This region accommodating the crus communis does not form a distinct buttress in Pristerodon , unlike in emydopoids. Mediolaterally, the supraoccipital forms the posterior wall of the floccular fossa (flo), which has a circular outline and is 2.65 mm deep ( Fig. 4G, H View Figure 4 ).
The anterior suture with the prootic is located at the top of a dorsoventrally elongated recess (slr, Fig. 4I, J View Figure 4 ) in lateral view. This suture is dorsoventrally oriented and appears completely co-ossified on CT images. The dorsoventral recess is also known as the venous groove in Dicynodontoides and other dicynodonts ( Olson, 1944; Cox, 1959; Cluver, 1971; King, 1981; Surkov & Benton, 2004) or as the anterior posttemporal fenestra groove in Emydops ( Fröbisch & Reisz, 2008) . This recess is dorsoventrally oblique and as tall as the supraoccipital.
In posterior view, the supraoccipital of Pristerodon is tall as it forms 60% of the dorsal margin of the elliptical foramen magnum ( Fig. 3G View Figure 3 ). The supraoccipital medial lobe is robust and has a rounded dorsal margin in occipital view ( Fig. 3G View Figure 3 ). The supraoccipital alae of Pristerodon are laterally pointed and possess a horizontal process (spp), which expands laterally to border the dorsal margin of the posttemporal fenestra (ptf, Fig. 3G View Figure 3 ). The suture between the supraoccipital and the exoccipital is oblique in posterior view and is horizontal between the supraoccipital and the opisthotic ( Fig. 3G View Figure 3 ).
Prootic: The prootic (pr) forms the anterior part of the braincase lateral wall and borders the foramen magnum anterolaterally ( Figs 2 View Figure 2 , 5 View Figure 5 ). Its medial wall is excavated by the floccular fossa and is pierced by the anterior semicircular canal. Due to its complex
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