Myosaurus gracilis Haughton 1917
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
Emydopidae
Myosaurus
Myosaurus gracilis

description: MYOSAURUS GRACILIS HAUGHTON, 1917
Specimens: BP/1 /2690, BP/1 /2701a (the description is a composite of the two specimens).
Orbitosphenoid: The orbitosphenoid of Myosaurus displays the typical Y-shape in anterior view (Figs 2, 6). It is generally thicker than in Pristerodon, shorter dorsoventrally but mediolaterally wider (height-to-width ratio 0.8 in Myosaurus vs. 1.6 in Pristerodon). These different proportions may be a by-product of the allometric effect determining the lower limit of the olfactory bulbs, which then may be proportionally larger in Myosaurus. As in other dicynodonts, the orbitosphenoid comprises a vertical median process (obvp, the mesethmoid) anteriorly and the dorsally projecting wings (obw) posteriorly (Castanhinha et al., 2013). The gutter (obgu) that hosts the olfactory bulbs dorsally is wider than in Pristerodon. In anterior view, the orbitosphenoid has tall wings that open at a greater angle from the sagittal plane, which results in a broad olfactory cavity. The wings are less curved than in Pristerodon (Fig. 6A). The median vertical process (obvp) has a prominent vertical crest (obacr) on its anteriormost edge that is flanked by two lateral sulci (obasu) visible in anterior view. The dorsal aspect of the median process of the orbitosphenoid thickens dorsally in Myosaurus and bears a distinct horizontal fossa (obdfo, Fig. 6B). Anterior to this fossa, an approximately 1.4 mm long spine (obsp) projects anteriorly in the prolongation of the medial wall.
In lateral view, the orbitosphenoid is subrectangular in Myosaurus. It is short anteroposteriorly compared to Kaaeingasaurus (length-to-width ratio 0.9 in Myosaurus vs. 1.9 in Kaaeingasaurus). The dorsal margin of the wings is not horizontal as in Pristerodon, but slightly curved (Fig. 6C). The plate-like vertical wall, which supports the wings of the orbitosphenoid, is dorsoventrally short in Myosaurus and it is traversed by two oblique crests (ltcr) in lateral view (Fig. 6C). The ventralmost crest is long, and the dorsal one is much shorter.
Pterygoid: The pterygoid (pt) is incomplete in both studied specimens of Myosaurus, missing the left quadrate ramus in specimen BP/1/2701a, and the left palatal ramus and both quadrate rami in specimen BP/1/2690 (Figs 2H, 6). It has the typical X-shaped dicynodont morphology in ventral view. The palatal rami (aptr) are more horizontal than in Pristerodon, forming an angle of 22° with the anteroposterior median axis of the skull. The quadrate ramus (ptqr) angles ~61° from the sagittal axis of the skull and ~102° from the palatal ramus. The palatal ramus of the pterygoid in Myosaurus is longer than the quadrate ramus (Fig. 6G). In dorsal view, the palatal ramus of BP/1/2701a is mediolaterally thinner in Myosaurus than in Pristerodon. On the anterior half of the palatal ramus, the pterygoid of Myosaurus is excavated by a narrow anteroposterior groove (pt gr) dorsally (Fig. 6D, G). Posterior to this groove, there is an anteromedial process (pt amp) projecting over the palatal ramus and terminating posteriorly at the anterior end of the pterygoid median plate in specimen BP/1/2701a (Fig. 6G). This process is damaged in specimen BP/1/2690. The quadrate ramus is co-ossified to a small portion of the epipterygoid dorsally.
In lateral view, the palatal ramus in Myosaurus is taller than the quadrate ramus. The specimens of Myosaurus described here do not have a ventral keel (Fig. 6E, H), which is consistent with previous work on Myosaurus in which this structure is not described (Haughton, 1917; Hammer & Cosgriff, 1981), but Cluver (1974b) reports the presence of this structure on the anterior palatal ramus of the pterygoid. The anteromedial process is triangular and gives a Y-shape to the pterygoid in lateral view in specimen BP/1/2701a (Fig. 6H). This process is longer and more developed dorsally than in Pristerodon. It forms an angle of 52° from the rest of the palatal ramus in Myosaurus specimen BP/1/2701a. The quadrate ramus of the pterygoid attaches to the median pterygoid plate on the short posterior process of the median pterygoid plate. The quadrate ramus has a dorsoventrally flattened and pointed end in lateral view in BP/1/2701a (Fig. 6H).
In ventral view, the palatal ramus of the pterygoid has a constant width (Fig. 6F). The median plate of the pterygoid (ptmp) is narrow in Myosaurus. In ventral view, it contacts the basisphenoid along an interdigitating suture posteriorly and sends a short process posteriorly to meet the clinoid process of the basisphenoid (Fig. 6F). Myosaurus has a V-shaped crista oesophagea (co) that converges posteriorly on the midline between the two carotid foramina and bifurcates anteriorly onto the palatal rami. In BP/1/2690, the crista oesophagea is flat, but in the better-preserved specimen SAM-PK-K10974, it forms a thin median ridge. This difference between the two specimens may be the result of taphonomy or preparation. The suture between the two pterygoids is visible and marked by a weakly pronounced anteroposterior furrow (apf) at the midline (Fig. 6F). The interpterygoid vacuity is narrower and more elongated in Myosaurus than in Pristerodon. In ventral view, the quadrate ramus is expanded anteriorly and has a robust posterior edge.
Epipterygoid: The epipterygoid is a vertically elongated braincase sidewall element that usually rests on the quadrate ramus ventrally and ascends to contact the descending flanges of the parietal dorsally in dicynodonts (Keyser, 1973, 1974; Angielczyk & Sullivan, 2008; Castanhinha et al., 2013; Angielczyk & Kammerer, 2017; Laass & Kaestner, 2017; Angielczyk et al., 2019). The epipterygoid is preserved in specimen BP/1/2701a (Fig. 6 I-K) and comprises a subhorizontal footplate ventrally (epft) and a rod-like ascending ramus dorsally (epar). The footplate consists of a short anterior ramus and a posterior quadrate ramus that is broken off close to its mid-length (Fig. 6J, K). The preserved portion of the quadrate ramus is taller and longer than the anterior ramus in lateral view (Fig. 6J). The ventromedial surface of the footplate rests entirely on the dorsal side of the quadrate ramus of the pterygoid as in other emydopoids (Keyser, 1973; Fourie, 1991, 1993), such that the suture between the two bones (the footplate of the epipterygoid and the quadrate ramus of the pterygoid) follows the longitudinal plane of the skull. The epipterygoid footplate does not contact the parabasisphenoid in Myosaurus. As observed in lateral view, the lateral surface of the epipterygoid is nearly flat but its medial surface is concave (Fig. 6J, K). In dorsal view, the quadrate ramus of the epipterygoid footplate is mediolaterally wider than the anterior ramus.
In Myosaurus, the ascending ramus of the epipterygoid is longer and straighter than in Kaaeingasaurus, and forms a right angle with the footplate (Fig. 6J). The ascending ramus has an anteroposteriorly wide base. Dorsally, it tapers considerably to become a rod-like bone at its mid-height and broadens dorsally to form an inverted triangular dorsal process (epdp, Fig. 6J, K). The dorsal process of the ascending ramus is slightly concave medially and articulates with the parietal laterally.
Parabasisphenoid: The parabasisphenoid complex of Myosaurus (pbs) is well-preserved and is subtriangular in both dorsal and ventral views (Figs 2, 6). As usual in dicynodonts, the basipostsphenoid forms the posterior end of the bone, and the parasphenoid rostrum and basipresphenoid (PRB) forms its anterior part (Cox, 1959).
The PRB is mediolaterally broad in Myosaurus (length-to-width ratio 1.1 in Myosaurus vs. 1.5 in Pristerodon). In dorsal view, it is excavated by an anteroposteriorly elongated and thin sulcus (pss). This sulcus is bordered by two low lateral crests (psc, Fig. 6L). Posteriorly, this sulcus ends in a small, anteroposteriorly oval foramen in the parasphenoid (psf). This pit opens on the ventral side of the bone within a circular cavity (Fig. 6N). As this pit is located anterior to the carotid foramina in BP/1/2690 (Fig. 6L), it forms a notch on the anterior margin of the carotid foramen in BP/1/2701a and may represent a relictual pituitary fossa. In contrast to what is found in several dicynodont taxa (e.g. Niassodon), the PRB and the basipostsphenoid are fused in Myosaurus. The position of the dorsal exit of the carotid foramina gives a rough indication of the suture between the PRB and the basipostsphenoid, and these foramina open dorsally into a single large circular orifice. Posterior to the carotid foramina, in dorsal view, lies the sella turcica (stu), which is a shallow excavation bordered posteriorly by a shallow inconspicuous ridge corresponding to the dorsum sellae (Fig. 6L). The sella turcica extends a short distance anteriorly onto the carotid foramina, but it is not confined anteriorly. There is no tuberculum sellae in Myosaurus. The sella turcica in Myosaurus is limited laterally by the dorsally projecting clinoid process (clp) of the basipostsphenoid. The clinoid process sharpens dorsally, its dorsal edge is turned medially and has a concave recess along with its dorsal extension. There is a gap of 0.3 mm that separates the clinoid process from the pila antotica, and the two processes are separated posteriorly by the dorsum sellae (ds, Fig. 6L). In dorsal view, the dorsum sellae is interrupted posteriorly by the transverse suture with the basioccipital. The basisphenoidal tubera (bpt) project posteroventrally and have a slightly concave dorsal surface that articulates with the basioccipital. As such, the basisphenoidal tubera do not border the fenestra ovalis, because the anteromedial wall of the fenestra is entirely formed by the basioccipital. In lateral view, the parasphenoid rostrum is straight but has a dorsoventrally expanded anterior tip (Fig. 6M). The clinoid process is offset from the parasphenoid rostrum, forming a notch where they meet. It forms an acute angle with the parasphenoid rostrum in Myosaurus.
In ventral view, the PRB tapers more anteriorly than posteriorly. It is excavated by a median trough that envelops the parasphenoid foramen (Fig. 6N). The ventral aspect of the base of the PRB is not flat but is irregular due to the interdigitating suture with the pterygoid median plate (Fig. 6N). This suture obscures most of the ventral anatomy of the PRB. The basipostsphenoid in Myosaurus does not form the ventral depression of the basicranium, which is instead composed of basioccipital. The basisphenoidal tubera border the ventral depression only laterally. The carotid foramina are bordered by thin ridges and they diverge ventrally and follow, to some degree, the basisphenoidal tubera (Fig. 6N).
Basioccipital: The basioccipital (bo) in Myosaurus forms more than half the length of the braincase floor at the back of the skull (Figs 2, 7). It is subtrapezoidal in dorsal and ventral views, with the posterior wall being the shortest. Both anteriorly and posteriorly, the basioccipital forms an inverted U-shape. The basioccipital contacts the parabasisphenoid anteriorly, the opisthotic laterally, the prootic anterodorsally and the exoccipital posterodorsally.
The basioccipital condyle (boc) is the most prominent structure in posterior view and is excavated by two concavities (bocc) that host the exoccipital in posterior view (Fig. 7A). Dorsal to these concavities, the condyle is traversed by an M-shaped ridge (bocdr) that articulates with the anterior tuberosity of the exoccipital (Fig. 7B). In dorsal view, the dorsal surface of the basioccipital is slightly concave (Fig. 7B). This concavity is divided by a blunt median ridge (bomr). The lateral margin of the basioccipital has sigmoidal ridges in dorsal view. The vestibule of the bony labyrinth (vea) deeply notches the lateral aspects of the basioccipital and opens ventrally as the fenestra ovalis (fo).
In ventral view, the basioccipital of Myosaurus is excavated by a Y-shaped depression (bod, Fig. 7C). The two branches of the Y-shaped depression delineate the semispherical condyle posteriorly. Four nutritive foramina (bonf) aligned on the sagittal axis perforate the basioccipital along the median depression. The basioccipital contacts the parabasisphenoid anteroventrally along an undulating suture with the basisphenoidal tubera extending backward to overlap the basioccipital tubera. The fenestra ovalis is ellipsoidal (3.3 × 1.5 mm) with its long diameter extending along the coronal axis of the skull.
In posterior view, there is a symmetrical pit (spi) on each side of the supraoccipital, dorsal to the foramen magnum. These have been interpreted as foramina for blood vessels bringing nutrients to the brain (Hammer & Cosgriff, 1981; Angielczyk & Kammerer, 2017) in other dicynodonts, but in Myosaurus the pits do not form a hole in the bone anteriorly. Cluver (1974b: fig. 3C) previously described the nutrient channel on the suture between the supraoccipital and the squamosal in Myosaurus, also present in BP/1/2701a. However, in specimen BP/1/2690 there are other pits located more medially within the supraoccipital (Fig 7J). Interestingly, there is a channel in Compsodon that starts in a homologous position as in BP/1/2690 but continues laterally to reach the position described by Cluver (Angielczyk & Kammerer, 2017: fig. 2). Whether these structures are part of the same system has yet to be confirmed. The supraoccipital is bounded laterally by shallow vertical depressions (svd) in posterior view that descend from the dorsal edge of the supraoccipital onto the exoccipital (Fig. 7J). The lateroventral corner of the supraoccipital forms the posttemporal fenestra together with the exoccipitals, but the lateral wall of this fenestra is bounded by the opisthotic. As in other dicynodonts, the foramen magnum is dorsoventrally oval in posterior view. In Myosaurus, the foramen magnum is 3.9 mm at its widest point. Ventrally, the supraoccipital is excavated by a conspicuous sulcus for the posterior semicircular canal (pscc, Fig. 7K).
The supraoccipital has a lateral vertical recess (slr) formed by the lateral and anterior edges of the supraoccipital, best seen in dorsal view (Fig. 7L). The recess descends from the dorsalmost aspect of the supraoccipital to nearly half the height of the foramen magnum.
Prootic: The prootic of Myosaurus (pr, Fig. 7) is a plate-like, vertically oriented bone in lateral and medial views, that forms the lateral wall of the braincase. As is typical in dicynodonts, the prootic borders the anterior wall of the foramen magnum. It is formed by the pila antotica anteriorly and the alar process posteriorly. The prootic of Myosaurus specimen BP/1/2690 is not completely fused to the surrounding bones (except the opisthotic) as it displays a gap of a few millimeters from the supraoccipital dorsally and the basioccipital and parabasisphenoid anteroventrally on CT images. We cannot exclude the possibility that the specimens here described are subadult (see discussion below).
In anterior view, the pila antotica (pa) of Myosaurus is tall, slightly curved medially, mediolaterally compressed and tapers dorsally. Its anteroventral half is bordered by medial and lateral crests (prac) that join dorsally half way up the pila antotica, flanking a shallow vertical trough (Fig. 7M).
In lateral view, the main body of the prootic sends an alar process (ap) posteriorly, which sutures with the opisthotic on its posteroventral part. The suture with the opisthotic is serrated and is oblique in lateral view (see the orientation of the right margin of the prootic where the suture is in Figure 7N). The alar process of the prootic has a sigmoid posterodorsal margin, and its dorsal and ventral margins are horizontal. Its anterior margin is straight and obliquely oriented in lateral view (Fig. 7N). A notch for the passage of the trigeminal nerve (CNV) is present at theanteroventralpartofthealarprocessandposteriortothe pila antotica. In lateral view, the pila antotica of Myosaurus appears as an isosceles triangle; the anterior border is relatively straight and the posterior border slightly convex (Fig. 7N). The pila antotica typically overlaps the clinoid process of the basisphenoid, but in BP/1/2690 they are separated by a 0.3 mm gap (gp, Fig. 7R). The medial face of the prootic is slightly depressed in lateral view (best seen in dorsal view, Fig. 7P), and ventral to this depression, the prootic is perforated by the circular facial foramen (fa). Also, the prootic forms the lateral wall of the conical fenestra ovalis which opens on the ventral side of the bone.
In medial view, the posterior and anterior margins of the prootic are oblique, whereas the ventral margin is horizontal. The dorsal margin of the prootic is V-shaped as it is traversed by the trigeminal nerve (Fig. 7O). The prootic forms the anterior wall of the horizontal, large and deep floccular fossa (flo) in medial view. The dorsal and ventral limits of the floccular fossa bear conjoined sigmoidal crests (prmc), with the ventral one being the most prominent. The dorsal crest serves to articulate with the supraoccipital. The suture with the supraoccipital appears sigmoidal in medial view (see the shading in Figure 7O). A small circular cavity for the anterior semicircular canal (ascc) perforates the prootic dorsal to the floccular fossa in medial view. This canal connects to another larger cavity located ventral to the floccular fossa, which hosts the vestibule of the inner ear (ve, Fig. 7O). As the sutural area between the prootic and opisthotic is visible in medial view, it appears trapezoidal (see the grey shading in Figure 7O). The base of the pila antotica is robust in medial view, and it overlaps the basioccipital along a nearly rectangular contact (see the green shading in Figure 7O). Dorsal to this suture and anterior to the vestibule is the facial foramen.
In posterior view, the prootic is excavated by a semicircular excavation (ppex), which exits through the posttemporal fenestra (Fig. 7Q). There is a bulge (prpb) at the concave part of the excavation which contributes to the anterior components of the posttemporal fenestra wall (Fig. 7Q). The prootic of BP/1/2690 is separated from the supraoccipital by a 0.4 mm gap posteriorly. Along its posteroventral corner, the prootic contacts the opisthotic in posterior view. This contact appears triangular. At the medial part of the suture, the prootic and opisthotic are perforated by the lateral semicircular canal (lscc, Fig. 7Q).
In dorsal view, the prootic forms a vertical buttress (pabt) and sends the alar process laterally. This buttress hosted the anterior semicircular canal. The area between the buttress and the alar process is depressed in dorsal view (Fig. 7P).
Opisthotic: The opisthotic in Myosaurus (op, Fig. 7) displays a subtrapezoidal shape in posterior view. It bears a well-developed ventromedial process. In anterior view, the lateral margin of the opisthotic forms the posteroventral wall of the posttemporal fenestra. The suture with the prootic is located in the middle of the anterior face of the opisthotic. This surface appears jagged in Figure 7S as the two bones were partially fused as in most dicynodonts, and was thus difficult to segment. The anteromedial aspect of the opisthotic is characterized by conjoined medial (omc) and lateral crests (olc). The lateral crest is sigmoidal, whereas the medial one is linear and subvertically oriented (Fig. 7S). The lateral crest forms the lateral margin of the opisthotic that sutures with the supraoccipital. The medial one borders the articulation with the prootic. The two crests meet dorsally and ventrally to form a large cavity that hosts the vestibular organ (ve). The ventromedial process of the opisthotic (ovmp) is a thick and robust tuber-like knob in Myosaurus. The medial tip of the process expands anteroposteriorly to receive the basioccipital along its medial flat edge and the exoccipital condyle along its dorsal edge.
In posterior view, the opisthotic is shaped like a horizontal hourglass as its dorsal and ventral aspects are concave (Fig. 7T, U). The ventral margin is the longest in posterior view. The stout oblique body (ob) is flanked by posteromedial (opmb) and lateral vertical buttresses (lvb). The posteromedial buttress of the opisthotic descends onto the dorsal component of the exoccipital ventrally, but remains separated from it by a 0.4 mm gap in specimen BP/1/2690. The lateral vertical buttress sutures with the squamosal and descends ventrally to articulate with the quadrate. The two opisthotic buttresses terminate dorsally forming rounded bulges visible in dorsal view (obu). These bulges comprise the posterior part of the posttemporal fenestra posterior to the prootic bulge. The posteromedial aspect of the opisthotic is excavated by an oblique sulcus (osu) that separates the main body of the opisthotic and the ventromedial process. This sulcus extends dorsally to join the exoccipital posterior sulcus. The ventromedial process is more vertical in specimen BP/1/2701a. In ventral view, the process borders the fenestra ovalis (fo) posteriorly and the jugular foramen (jf) anteriorly (Fig. 7V).
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 13-20, DOI: 10.1093/zoolinnean/zlac033, http://zenodo.org/record/7922143