M. pterygoideus ventralis

Pêgas, Rodrigo V, Costa, Fabiana R & Kellner, Alexander W A, 2021, Reconstruction of the adductor chamber and predicted bite force in pterodactyloids (Pterosauria), Zoological Journal of the Linnean Society 193 (2), pp. 602-602 : 602-

M. pterygoideus ventralis (mPTv)

Origin: In all extant diapsids this muscle originates from the pterygoid, plus the palatine in birds and turtles, and the quadrate in squamates. In crocodylians, mPTv originates from both medial and lateral surfaces of their vertically oriented pterygoid. Such a condition is different from the one seen in the pterygoid of pterosaurs, in which the bone is horizontally oriented, composing the bony palatal floor. In birds, mPTv originates from the ventral surface of both the pterygoid and the palatine. This allows us to assume that such a muscle would be present at least on the ventral surface of the pterosaur pterygoid. Given that the pterosaur pterygoid represents a flattened version of the vertical pterygoid of crocodylians and most dinosaurs, then its ventral surface would indeed correspond to the lateral and medial surfaces of the pterygoid of the latter forms.

Muscle scars can be seen on the ventral surface of the pterygoids in An. araripensis (BSP 1982 89 and MN 4735-V), An. blittersdorffi , C. ybaka , Th. sethi and Tr. mesembrinus . These muscle scars extend onto the ventral surface of the palatines at least in An. araripensis and C. ybaka , in which these scars are more conspicuous relative to the other mentioned specimens ( Figs 7 , 8 ). In C. ybaka , the pterygoid also exhibits an oval fossa on its main body, near the lateral process of the pterygoid. On the other hand, in An. araripensis the main body of this bone forms an elevated keel that could also be related to muscle attachment. Additionally, well-defined muscle scars can be recognized on the ventral surface of the ectopterygoids ( Fig. 7 ) in An. araripensis (BSP 1982 89) and Th. sethi , strongly suggesting that the origin of mPTv expanded onto this bone.

Insertion: MPTv wraps around the posterior region of the mandible and then attaches to the ventral and lateral surfaces of the mandibular rami, especially the angular in lepidosaurs, crocodylians and extant birds (see Holliday & Witmer, 2007). It extends even into some skull bones such as the jugal and the maxilla in lepidosaurs, turtles and possibly some dinosaurs (see Holliday & Witmer, 2007; Holliday, 2009; Werneburg, 2011). The attachment also involves the surangular in crocodylians and the medial mandibular process of the articular in birds, as well as the articular in lepidosaurs and the prearticular and surangular in turtles (see Holliday & Witmer, 2007).

The angular in Th. sethi is a thin, small bone, located ventral to the surangular, which is mostly occupied by a fossa related to m. adductor mandibulae externus superficialis (mAMES), as will be further discussed. Therefore, mPTv in Th. sethi most likely did not reach the surangular, unlike in crocodylians. Thin, although well-defined muscle scars upon the ventrolateral region of the articular and angular can be seen in Ay. cearensis ( Fig. 9 ), Th. sethi ( Fig. 17 ) and An. araripensis (BSP 1982 89; Fig. 17 ). Of particular note is An. araripensis in which the angular displays a strong ridge that separates two fossae, of which we interpret the dorsal one as related to mAMES and the ventral to mPTv.

The presence of mPTv in pterosaurs, as well as its origin from the ventral surfaces of the pterygoids and insertion laterally on the angular and lateroventrally on the articular, is a level I inference. The extended area of origin comprising specifically the ventral surface of the ectopterygoids in Th. sethi is a level III inference. However, because the osteological correlates are evident, we interpret them here as compelling evidence supporting this inference. The muscle would pass medial to both m. adductor mandibular dorsalis and m. adductor mandibulae posterior (mAMP; see below) on its way to the mandible (being the most medial palatal muscle), and wrap around the ventral margin of the lower jaw to finally insert on its lateral side.

Figures

Figure 7. Palatal bones in occlusal view of Anhanguera araripensis holotype (BSPG 1982 89). A, left pterygoid, palatine and ectopterygoid. B, right pterygoid, palatine and ectopterygoid. Scale bars = 10 mm. Arrowheads indicate muscle scars.Abbreviations: ch, choannae; ec, ectopterygoid; pl, palatine; pplf, postapalatinal fenestra; pty, pterygoid; stf, subtemporal fenestra.

Figure 8. Palatal bones in occlusal view of Caupedactylus ybaka holotype (MN 4726-V).Scale bar = 10 mm.Arrowhead indicates pterygoid fossa. Abbreviations: ec, ectopterygoid; m, maxilla; pplf, postapalatinal fenestra; pty, pterygoid; stf, subtemporal fenestra.

Figure 9. A, right mandibular ramus of Aymberedactylus cearensis holotype (MN 7596-V) in ventral view. B, left mandibular ramus of Aymberedactylus cearensis holotype in ventral view and (C) medial view. Arrowheads indicate muscle scars. Scale bars = 20 mm. Abbreviations: adfo, adductor fossa; ang, angular; art, articular; d, dentary; rapr, retroarticular process; san, surangular.

Figure 17. Left mandibular rami in lateral view. A, Thalassodromeus sethi holotype (DGM 1476-R). B, Anhanguera araripensis holotype (BSPG 1982 89). C, Ludodactylus sibbicki holotype (SMNK PAL 3828). Arrowhead indicates muscle scars. Scale bars = 20 mm. Abbreviations: sanfo, surangular fossa.