Cistecephalidae BROOM 1903
Creators: name: Macungo, Zanildo
name: Benoit, Julien
name: Fernandez, Vincent
name: Araújo, Ricardo M N
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Therapsida
Cistecephalidae

description: CISTECEPHALIDAE BROOM, 1903
Specimen: DMMM-PK-16-1.
Orbitosphenoid: Specimen DMMM-PK-16-1 does not have the orbitosphenoid preserved.
Pterygoid: The pterygoid of the Malawian cistecephalid DMMM-PK-16-1 (pt) has both the palatal and quadrate rami well preserved but they are riddled by several small breaks and cracks (Figs 2, 8). Its morphology is the typical dicynodont X-shape in dorsal and ventral views. The palatal rami (aptr) are parallel on their posterior half and diverge laterally in the anterior direction. Although most similar to Cistecephaloides, this condition is different from all cistecephalids in which the anterior rami are divergent along their entire length. However, it is important to note that the pterygoids are typically much exposed to taphonomic damage in almost all dicynodonts, so this comparison should be taken with caution. In DMMM-PK-16-1, the quadrate rami (ptqr) diverge at an angle of 117°, whereas the palatal rami form an angle of ~29° with the anteroposterior median axis of the skull. Both palatal and quadrate rami of the pterygoid diverge at an angle of ~83°, measured on the lateral surface of the pterygoid. The Malawian cistecephalid has no pterygoid anteromedial process dorsally. In dorsal aspect, the palatal rami of the pterygoid are shorter and mediolaterally wider than the quadrate rami (Fig. 8A). The quadrate ramus of the pterygoid reaches the quadrate posteriorly and curves slightly medially in ventral view (Fig. 8C).
In lateral view, the palatal ramus is dorsoventrally taller than the quadrate ramus in the Malawian cistecephalid DMMM-PK-16-1, but the quadrate ramus is mediolaterally narrow. The lateral aspect of the quadrate ramus of the pterygoid is excavated by a shallow anteroposterior trench (pt ltr) that accommodates the epipterygoid footplate (Fig. 8B).
In DMMM-PK-16-1, the two pterygoids are separated medially by the parabasisphenoid to which is co-ossified posteriorly. The pterygoid of the Malawian cistecephalid does not form a distinct median plate per se, as the rami do not meet on the midline. There is considerable variation on how the pterygoids meet at the median pterygoid plate in cistecephalids [see Angielczyk et al. (2019) for further discussion of this variation]. However, in the Malawian cistecephalid DMMM-PK-16-1, the two pterygoids are separated by a thin crest emerging from the parasphenoid, a condition shared with Cistecephaloides (Cluver, 1974a), but contrasting with the condition seen in Cistecephalus, Kaaeingasaurus, Kembaaeacela and Sauroscaptor (Cox, 1972; Keyser, 1973; Kammerer et al., 2016; Angielczyk et al., 2019).
In lateral view (Fig. 8B), the basioccipital tubera slope almost vertically and their anterior wall is thicker than the posterior one. The basioccipital tubera form the medial wall of the fenestra ovalis. The fenestra ovalis is elliptical and is oriented obliquely, and its largest diameter is 2.3 mm. The basioccipital makes a small contribution to the medial border of the jugular foramen in the Malawian cistecephalid. Most of the ventral anatomy of the basioccipital is damaged in DMMM-PK-16-1. The remaining features are the wide medial depression (bod) that separates the basioccipital tubera anteriorly, and delineates the basioccipital condyle posteriorly (Fig. 8C). The ventral depression of the basioccipital reaches the S-like suture with the basisphenoid anteriorly. It appears slightly deeper than the dorsal one, despite this region being damaged. In ventral view, the basioccipital condyle is subtriangular with a wide base forming the posterior margin (Fig. 8C).
Exoccipital: The exoccipital forms the dorsal portion of the occipital condyle and extends dorsally to form the foramen magnum lateral walls (Fig. 8). As it is damaged, the exoccipital appears knob-like in shape in both anterior and posterior views. Anteriorly, the exoccipital condyle bears a subvertical tuberosity (eoat) that laterally flanks the jugular foramen (jf) and forms the anterior articular surface of the condyle (Fig. 8D). The exoccipital condyle has a flat anterior surface caused by erosion. Little can be said about the anterior face of the dorsal component due to damage, except that it has a lobate outline.
The exoccipital condyles are severely damaged in this specimen such that their midline contact has been broken. However, the sulcus on the posterior surface of the basioccipital condyle suggests that the left and right exoccipitals met medially as in other cistecephalids (Keyser, 1973; Cluver, 1974a; Angielczyk et al., 2019). The left exoccipital is better preserved dorsally and its dorsal portion forms a D-shaped structure (eodc, Fig. 8E) in posterior view. The exoccipital dorsal component forms a convex suture with the opisthotic and supraoccipital laterally. This component is connected to the exoccipital condyle by a shallow exoccipital sulcus (eopsu) in posterior view (Fig. 8E).
Medially, the right exoccipital condyle is pierced by two conspicuous foramina for the hypoglossal nerve and maybe an accompanying vessel (hf), which are separated ventrally by a thin crest (Fig. 8F). The two foramina are oriented obliquely, and the lateral one is the largest. The lateral foramen is oval and its major diameter is 0.3 mm, whereas the medial foramen is circular with a diameter of 0.1 mm.
Supraoccipital: The supraoccipital (su) shows the typical anteroposterior shortening and transverse widening of the skull in cistecephalid dicynodonts (Keyser, 1973; Cluver, 1974a; Kammerer et al., 2016; Angielczyk et al., 2019) and has a saddle shape in posterior view (Fig. 8O). In specimen DMMM-PK-16-1, the supraoccipital is eroded around the medial lobe and the dorsal portion of the left ala. The supraoccipital surrounds the dorsal half of the foramen magnum, but the dorsal margin of the foramen could not be delimited in this specimen because of damage. The supraoccipital contacts the exoccipital ventromedially along an oblique suture and the opisthotic ventrally along a horizontal suture. It contacts the prootic anteriorly along a suture parallel to the coronal plane observed in lateral view.
In anterior view, the anterodorsal portion of the medial lobe has two anteroposteriorly and dorsoventrally short descending processes (sap) that probably delimited the hindbrain in life as in Kaaeingasaurus (Fig. 8M). The right process is deflected at its base and forms a notch with the anterior face of the supraoccipital ala, and also forms the anterodorsal border of the foramen magnum. The supraoccipital ala has a bulge anteriorly which makes up the posterodorsal wall of the large and deep floccular fossa (flo) in DMMM-PK-16-1. This bulge is also penetrated by the bony enclosure of the anterior semicircular canal (ascc) just dorsal to the floccular fossa (flo) and by the crus communis (cc) on the posterolateral edge of the floccular fossa (Fig. 8M). Laterally, the supraoccipital ala is excavated by the lateral recess (slr, Fig. 8P), which is flanked by the supraoccipital posteriorly and leads to the posttemporal fenestra. In posterior view, the medial lobe of the supraoccipital expands dorsally toward the top of the skull and has a semicircular depression on its median posterodorsal region (ssd, Fig. 8O). The lateral alae of the Malawian cistecephalid DMMM-PK-16-1 supraoccipital are more extended and pointed laterally, and form the entire dorsal border of the posttemporal fenestra in posterior view (Fig. 8O). The suture with the exoccipital is located anteriorly in the recess for the posterior semicircular canal (pscc).
Prootic: The prootic (pr) is well preserved in DMMM-PK-16- 1 comprising the alar process posteriorly and the anterodorsally directed pila antotica (pa) (Fig. 8 G-L). As is typical in dicynodonts, the prootic of DMMM-PK-16-1 forms the anterolateral wall of the braincase as it borders the foramen magnum anteriorly. Contrary to the situation in Kaaeingasaurus, the prootic in DMMM-PK-16-1 is strongly co-ossified to only the supraoccipital and opisthotic, but a gap of 0.2 mm separates it from the basioccipital and basisphenoid (gap, Fig. 8G).
In anterior view, the prootic is gently twisted medially in the Malawian cistecephalid. Its alar process (ap) expands laterally in anterior view, resulting in a concave lateral margin of the bone (Fig. 8H). The dorsal edge of the alar process of the prootic is slightly pointed and extends dorsally in anterior view.The right pila antotica is best preserved in DMMM-PK-16-1. In anterior view, the pila antotica is dorsoventrally thin and mediolaterally compressed on the dorsal side when compared to the remaining taxa described here. The dorsalmost part of the pila antotica is bent laterally in anterior view (Fig. 8H). Its anteroventral part is broad as it overlaps the parabasisphenoid. This overlay is not sutural as the two bones are separated by a ~ 0.5 mm gap. As observed laterally, the pila antotica is obliquely oriented, tall and tapers dorsally. The lateral face of the pila antotica ventral to the trigeminal notch is depressed.
In lateral view, the prootic appears U-shaped. The alar process and the pila antotica are separated by the trigeminal notch (tgn, Fig. 8I), which is wider in the Malawian cistecephalid and Kaaeingasaurus than in Myosaurus and Pristerodon. The alar process of the prootic tapers dorsally in lateral view, and it is widest at mid-height. Whereas the lateral margin of the prootic forms an oblique suture with the opisthotic, its ventral margin forms a horizontal contact with the basioccipital in lateral view (Fig. 8I). Dorsal to the suture with the opisthotic, the prootic forms the anterior border of the posttemporal fenestra (ptf). The fenestra ovalis (fo) is formed at the intersection of the prootic-opisthotic-basioccipital, and the prootic forms its anterior border. Ventrally, the prootic is perforated by the facial foramen which is located dorsal to the fenestra ovalis (Fig. 8I).
In medial view, the floccular fossa (flo) excavates the internal aspect of the prootic horizontally (Fig. 8J). The prootic forms about 60% of its anterior margin. The floccular fossa is 1.6 mm deep in DMMM-PK-16-1. The portion of the prootic that forms the floor of the floccular fossa is blade like (pbl) in medial view (Fig. 8J). This blade-like process roofs the vestibular cavity and separates it from the floccular fossa. In DMMM-PK-16-1, the vestibule (ve) is large, inflated and vertically oriented as in other cistecephalids (Laass & Schillinger, 2015; Laass & Kaestner, 2017; Angielczyk et al., 2019). The prootic has a thicker aspect dorsal to the floccular fossa, where it is excavated by a small circular cavity for the anterior semicircular canal (ascc, Fig. 8J).
In ventral view, the prootic forms a concave contact with the basioccipital and parabasisphenoid. The facial foramen in DMMM-PK-16-1 is small, circular and excavates the prootic vertically in ventral view (Fig. 8K). The vestibule of the Malawian cistecephalid is conical when observed ventrally.
In posterior view, the prootic has a convex lateral margin. Its dorsomedial margin is knob-like but the ventromedial one is nearly oblique (Fig. 8L). At its mid-height, the prootic bears a horizontal blade-like process (pbl) that sutures with the supraoccipital and opisthotic. Dorsal to the blade, the prootic sutures with the supraoccipital, and ventral to the blade it sutures with the opisthotic. Also, the vestibule of the inner ear is visible ventral to the blade in posterior view (Fig. 8L). The lateral semicircular canal (lscc) perforates the prootic and opisthotic immediately ventral to the horizontal blade-like process.
Opisthotic: The opisthotic of the Malawian cistecephalid (op) is hourglass shaped in anterior and posterior views and its dorsal and ventral margins are concave (Fig. 8 M-P). It can be divided into two subunits: the ventromedial process and the main body medially that sends two crests laterally.
The opisthotic bears an anterior projection that houses the vestibule (Fig. 8M, N). This bulge (obg) serves as a sutural part with the prootic anteriorly. On its dorsal margin, the opisthotic bulge is perforated by a cavity for the lateral semicircular canal (lscc). This cavity connects to the vestibule (ve) on the anteromedial surface of the opisthotic. The vestibule is spherical and inflated as is typical for cistecephalids and excavates a shallow concavity on the anterior face of the ventromedial process of the opisthotic (Fig. 8M). The vestibule is anteroventrally bordered by a V-like low crest (oac). The opisthotic seems to have housed only 40% of the vestibule leaving the other 60% to the prootic. Lateral to the bulge, the face of the opisthotic bears a vertical depression (oad) for articulation with the medial side of the quadrate-quadratojugal complex. The recess for the lateral semicircular canal (lssc) of DMMM-PK-16-1 is small and oval in internal view (Fig. 8N). At its ventralmost margin, the vestibule opens into a large elliptical fenestra ovalis, observed in ventral view.
In posterior view, the main body is a horizontal bulge-like process (opp, Fig. 8O) located medially, which flattens dorsally to form the floor of the posttemporal fenestra (ptf) laterally and sutures with the supraoccipital dorsally. The posttemporal fenestra appears elliptical in posterior view, with the long axis oriented horizontally (Fig. 8O). Whereas the suture with the exoccipital appears oblique, the suture with the supraoccipital is horizontal in posterior view. The ventromedial part of the opisthotic forms a tuber-like subvertical ventromedial process (ovmp, Fig. 8O) that curves slightly posteroventrally to form the anterior wall of the jugular foramen and the posterior border of the fenestra ovalis. This gentle curvature of the process excludes the basioccipital from the jugular foramen in DMMM-PK-16-1 (Fig. 2L). The posterior aspect of the opisthotic in DMMM-PK-16-1 bears two divergent crests (opc, Fig. 8O) that flank a shallow horizontal depression (od, Fig. 8O). The lower crest is shaped like a long rod that projects laterally in the oblique direction to suture with the squamosal and quadrate. This crest terminates laterally forming the knob-like ‘tympanic process’ of Cox (1959), see tmp (Fig. 8O, P). In contrast, the upper crest is short, broader and horizontal.
description: Published as part of Macungo, Zanildo, Benoit, Julien, Fernandez, Vincent & Araújo, Ricardo M N, 2023, X-ray microcomputed and synchrotron tomographic analysis of the basicranial axis of emydopoid dicynodonts: implications for fossoriality and phylogeny, pp. 1-46 in Zoological Journal of the Linnean Society 198 (1) on pages 20-24, DOI: 10.1093/zoolinnean/zlac033, http://zenodo.org/record/7922143