Anhanguera adductor subsp. mandibulae

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. adductor mandibulae posterior (mAMP)

Origin: The origin of mAMP from the lateral surface of the quadrate is consistent across sauropsids, as seen in lepidosaurs, crocodylians and birds, although the exact position varies in all of these (e.g. Holliday & Witmer, 2007; Holliday, 2009) and further includes the postorbital and parietal in turtles ( Werneburg, 2011). Crocodylians exhibit crests and fossae on the quadrate that correlate to the origin of this muscle ( Holliday & Witmer, 2007).

Well-defined muscle scars were found on the lateral surface of the main body of the quadrate, within the inner limits of the lower temporal fossa, in the holotype of Tupandactylus navigans , Tr. mesembrinus and several pteranodontids ( Fig. 5).

Insertion: mAMP inserts at the mandibular fossa, located on the posteromedial surface of the mandibular rami, in both lizards and extant archosaurs. This strongly suggests that the same case would apply for pterosaurs. A well-preserved mandibular fossa could be found in all pterosaur specimens herein analysed.

The mandibular fossa of Th. sethi is more anteroposteriorly elongated and deeply excavated ( Fig. 10 ) than in other pterosaurs such as Aymberedactylus cearensis , Cearadactylus atrox , Ta. wellnhoferi , Tr. mesembrinus , Tupuxuara leonardii and anhanguerians (e.g. Wellnhofer, 1985; Veldmeijer et al., 2005) including An. araripensis , and the ctenochasmatid Forfexopterus jeholensis Jiang et al., 2016.

TEMPORAL ADDUCTOR MUSCLES

M. pseudotemporalis superficialis (mPSTs) Origin: Both mPSTs and mAMEP are medial to mAMEM ( Holliday & Witmer, 2007). mPSTs originates anteromedially within the upper temporal fenestra in extant lepidosaurs and birds ( Holliday & Witmer, 2007). In the probably secondarily anapsid-like skull of the Testudines (see Lee, 2013; Schoch & Sues, 2015, 2018), this muscle originates on the lateral surface of the parietal and postorbital ( Werneburg, 2011). In extant crocodylians, it originates at the dorsal part of the laterosphenoid, outside the limits of the upper temporal fenestra. Still, this condition is probably derived from the primitive condition of this muscle originating within the medial margin of the upper temporal fenestra, as inferred for primitive crocodyliforms ( Holliday & Witmer, 2007), and for dinosaurs ( Holliday, 2009). It suggests that in pterosaurs this muscle should have originated anteromedially within this fenestra as well.

Insertion: mPSTs inserts medially on the mandible, on the coronoid process, as seen in lepidosaurs, turtles (only on the surangular part of the coronoid process) and extant birds, whereas in extant crocodylians it inserts on the sesamoid transiliens cartilage, near the coronoid eminence ( Holliday & Witmer, 2007; Werneburg, 2011). For dinosaur taxa in which the coronoid eminence is immediately adjacent to the mandibular fossa, this muscle was proposed to insert on the eminence plus the anterior region of the mandibular fossa ( Holliday, 2009).

The reptilian coronoid process is not formed solely by the coronoid; instead, it also receives contributions from the anterior portion of the surangular and posterior portion of the dentary (e.g. Romer, 1956). In most pterosaurs, even though the coronoid bone itself cannot be identified, eminences formed by the surangular can be found in the same region where the coronoid eminence is found in other reptiles, such as dinosaurs (see Dalla Vecchia, 2003). This surangular

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adductor mandibulae externus superficialis posterior

Continued M mandibulae 1 Table. Muscle M adductor .

eminence is present in almost all known mandibles from Triassic forms (see Dalla Vecchia, 2003, 2014; Kellner, 2015). However, this feature was lost in later forms (see Kellner, 2015).

The Early Cretaceous tapejarid Th. sethi exhibits, in the same region where the “coronoid” process would be, a well-developed tubercle projected not dorsally, but medially. In this same taxon, the mandibular fossa is especially elongated and markedly excavated when compared to other taxa, being immediately adjacent to this surangular tubercle. It is unlikely that the whole extent of this well-developed fossa could be explained by the insertion sites of mPSTp or mAMP alone, suggesting that mPSTs probably extended from the surangular tubercle onto the anterior region of this fossa as well. In Ay. cearensis ( Fig. 9 ), An. araripensis ( Fig. 11 ) and pteranodontids ( Figs 12–15 ), the medial surface of the surangular, anterior to the mandibular fossa (where the “coronoid” process would be), exhibits well-defined muscle scars. Therefore, it seems most likely that mPSTs originated from the anteromedial region of the upper temporal fenestra (extending dorsally onto the frontoparietal crest), medial to mAMEM and anterior to mAMEP. It then passed through the subtemporal fenestra and inserted in the surangular and anterior region of the mandibular fossa.

Figures

Figure 1. Graphic representation of the method for calculating mechanical advantage of each muscle group using the skull of Ta. wellnhoferi. This image shows only its temporal musculature, taken as an example, in (A) right lateral view and (B) palatal view. Green line: inlever. Red line: outlever. Yellow lines: lines of action in each represented plane. Pink area: two-dimensional representations of muscle extension in lateral view (A) and the palatal plane (B). The latter is equivalent to the CSA.

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 11. Mandibular rami of anhanguerids in medial view: Anhanguera araripensis holotype (BSPG 1982 89) (A) left ramus and (B) right ramus; and (C) Tropeognathus mesembrinus holotype (BSPG 1987 46) left ramus. Arrowheads indicate muscle scars. Scale bars = 20 mm. Abbreviations: adfo, adductor fossa; ang, angular; glfo, glenoid fossa; part, prearticular; rapr, retroarticular process; san, surangular.

Figure 12. Right mandibular ramus of Pteranodon sp. AMNH 153 in medial view, (A) whole view of posterior region of the ramus, (B) and (C) close-ups corresponding to the rectangles shown in (A). Scale bar = 10 mm.Arrowheads indicate muscle scars. Abbreviations: glfo, glenoid fossa; san, surangular; rapr, retroarticular process.

Figure 13. Left mandibular ramus of Pteranodon sp. (A) YPM 2487, (B) LACM 51130 and (C) YPM 2401, in medial view. Arrowheads indicate muscle scars. Scale bars = 10 mm. Abbreviations: adfo, adductor fossa; an, angular; glfo, glenoid fossa; rapr, retroarticular process.

Figure 14. Left mandibular ramus of (A) Pteranodon sp. YPM 1179, (B) Pteranodon longiceps YPM 2594 and (C) YPM 2594 in detail in medial view. Arrowheads indicate muscle scars. Scale bars = 10 mm. Abbreviations: adfo, adductor fossa; glfo, glenoid fossa; rapr, retroarticular process.

Figure 15. Right mandibular ramus of Pteranodon sp. (A) YPM 1175, (B) YPM 1175 (detail of scars) and (C) YPM 2738 in medial view. Arrowheads indicate muscle scars. Scale bars = 10 mm. Abbreviations: adfo, adductor fossa; glfo, glenoid fossa; rapr, retroarticular process.