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 externus superficialis (mAMES)

Origin: mAMES is the lateralmost of the temporal muscles, which originates from the temporal bar – more precisely from the lateral surface of the squamosal and ventromedial surface of squamosal and postorbital in lepidosaurs, from the squamosal and quadrate otic process in modern birds, and from the anterolateral surface of the quadrate and quadratojugal in crocodylians ( Holliday & Witmer, 2007). The configuration in crocodylians is most likely shifted from the squamosal as a derived feature, given its origin on the squamosal in both lepidosaurs and modern birds, while the absence of this muscle on the surface of the postorbital in modern birds is likely due to the loss of the temporal bar in these animals. The conservative morphology of the temporal bar in pterosaurs, comprising both the postorbital and the squamosal, most likely indicates that these two bones served as origin areas for mAMES. Unfortunately, the origin of this muscle is usually fleshy and rarely leaves osteological correlates.

Insertion: mAMES runs lateral to the skull and inserts on the lateral surface of the mandible in modern birds, on the dorsolateral surface of the surangular in lepidosaurs, on the dorsal surface of the surangular in crocodylians ( Holliday & Witmer, 2007) and most likely on the dorsolateral surface of the angular in non-avian dinosaurs ( Holliday, 2009). Insertion sites for this muscle generally do not leave clear osteological correlates, only rarely leaving smooth shallow fossae ( Holliday & Witmer, 2007). Considering the phylogenetic bracketing, this muscle would most likely insert dorsolaterally on the surangular of pterosaurs.

Extensive scarring can be seen in the dorsal surface of the surangular of An. araripensis and pteranodontids ( Fig. 16 ), which seems to correlate at least partially to mAMES. The lateral surface of the bone is gently excavated by a depression, which extends further ventrally to part of the angular. This depression bears some faint muscle scars. A similar depression can be seen in the holotypes of Th. sethi and Ludodactylus sibbicki ( Fig. 17 ), although is the latter specimen the surface is damaged and no scars can be seen except on the dorsal region of the surangular. This surangular/ angular depression is situated dorsal to the probable insertion site of mPTv, which was previously discussed. As mentioned, in An. araripensis , a strong ridge seems to separate the insertion areas of these two muscles.

ESTIMATED BITE FORCES AND BFQ

Reconstructed CSAs ( Fig. 2 ), inlevers, outlevers, mechanical advantages and estimated bite forces are given in Tables 3–4. The values of stylopodial diameters are given in Table 5, and estimated body masses and BFQs are given in Table 6.

Considering the mean values of bite force (tip and 70% jaw-length), the lowest bite forces were found in the two smallest specimens: 8.67– 12.40 N (tip and 70% jaw-length, respectively) for Ta. wellnhoferi MN 6595-V (2.23 kg in body mass) and 5.0– 7.9 N for Nyctosaurus gracilis FMNH P 25026 (3.2 kg). The same values for D. weii IVPP V 4065 were 61 – 86 N, despite being only slightly larger than the former two (4.18 kg). The mean mid-jaw length bite force calculated for D. weii , due to its unique dentition, was 185 N. Pteranodon longiceps YPM 1177 (15.12 kg) and An. blittersdorffi MN 4805-V (10.45 kg) bear similar mean tip and 70% jaw-length bite forces (~23–~33 N), although the estimated bite forces of the similar-sized C. ybaka MN 4726-V (13.61 kg) were over twice as high (~52–74 N), close to the estimated bite forces of Tr. mesembrinus BSPG 1987 46 (58–82 N), which is larger than the former three (~ 23 kg). The two largest analysed specimens, Th. sethi DGM 1476 -R and Tupuxuara leonardii IMNH 1052 (~ 30 kg), exhibit disparate estimated bite forces: 157–224 N and 29–41 N, respectively.

The largest BFQs were estimated for D. weii (26.88) and Th. sethi (20.61), followed by Ta. wellnhoferi (13.55), C. ybaka (9.81) and Tr. mesembrinus (8.68). Estimated values were lower for the other taxa: Tupuxuara leonardii (3.94), N. gracilis (3.18), Pteranodon longiceps (4.04) and An. blittersdorffi (5.14).

Figures

Figure 2. Jaw muscle reconstruction of the analysed taxa, with skulls in lateral and palatal views. These reconstructions are based on the present work and were used as the basis for calculating CSA, α, β, inlever and outlever. Pink represents the temporal musculature; green, M.adductormandibulae posterior; and blue,the pterygoideusmusculature.A,B, Caupedactylus ybaka (based on Kellner, 2013). C, D, Tapejara wellnhoferi (based on Kellner, 1989; 2004). E, F, Dsungaripterus weii (based on Young, 1984). G, H, Thalassodromeus sethi (based on Kellner & Campos, 2002; Pêgas et al., 2018). I, J, Tupuxuara leonardii (based on Kellner, 2004; Pinheiro & Schultz, 2012). K, L, Tropeognathus mesembrinus (based on Wellnhofer, 1987). M, N, Anhanguera blittersdorffi (based on Kellner & Tomida, 2000). O, P, Pteranodon longiceps (based on Bennett, 2001). Q, R, Nyctosaurus gracilis (based on Williston, 1902; Bennett, 2003). All scale bars = 200 mm.

Figure 16. Mandibular rami in dorsal view. A, Aymberedactylus cearensis holotype (MN 7596-V), right ramus. B, Thalassodromeus sethi holotype (DGM 1476-R), left ramus. C, Anhanguera araripensis holotype (BSPG 1982 89), left ramus. D, Pteranodon sp. YPM 2410, left ramus. E, Pteranodon sp. LACM 51130, left ramus. Arrowheads indicate muscle scars. Scale bars = 10 mm. Abbreviations: adfo, adductor fossa; defo, fossa depressoria; lc, lateral cotyle; mc, medial cotyle; rapr, retroarticular process.

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.