Comparative Functional Morphology of the Masticatory Apparatus in the Long‐snouted Crocodiles

Abstract

The masticatory muscles and their related structures of the skull were observed in the Indian gavial (Gavialis gangeticus), the false gavial (Tomistoma schlegelii), and the African slender-snouted crocodile (Mecistops cataphractus) to detail some morphological differences in comparison with the other crocodile species, and to compare and elucidate the functional strategy of themasticatory apparatus in these long-snouted species. The Musculus pterygoideus posterior was relatively smaller in the three species compared with many short-snouted crocodiles. It suggests that the masticatory power in fish-eating long-snouted species is not so high as in the short-snouted crocodiles, while the masticatory muscles were morphologically different among the three long-snouted species as follows. The M. pterygoideus posterior of the false gavial was extended in the lateral side of the lower jaw unlike the Indian gavial. The M. pseudotemporalis and the Fenestra supratemporalis were largely developed in the Indian gavial, however we suggest that the other two species possess the weak bundles in this muscle. The false gavial and the African slender-snouted crocodile have the pterygoid bone well-developed extending dorso-ventrally and it is suggested that the M. adductor mandibulae posterior attached to the pterygoid bone may be much larger than the Indian gavial. These data morphologically clarify the masticatory mechanism in the long-snouted crocodiles different from the short-snouted species, and demonstrate that the evolutional strategy to share the functional role in the masticatory muscles have been differently established between the Indian gavial and the other two species. We also obtained the morphological data in the fossil skull of the Machikane crocodile (Toyotamaphymeia machikanense) and concluded from the fossil characters that the considerable developments of the M.pterygoideus posterior and the M.pseudotemporalis in this species had not morphologically been consistent with both the Indian and false gavials.

A biomechanical analysis of the skull and adductor chamber muscles in the Late Cretaceous Plesiosaur Libonectes

“…Musculus depressor mandibulae (mDM): Origin -In Sphenodon,the depressor mandibulae originates on the posterodorsal wall of the squamosal, extending ventrally until the opisthotic (Jones et al, 2009, p. 29). In many crocodilians, the depressor mandibulae originates from the posterior region of the squamosal and quadrate (Endo et al, 2002). Libonectes has a large portion of the right squamosal and quadrate preserved, but this taxon might have extended even further dorsally as in Callawayasaurus (Welles, 1962).…”

The epipterygoid of crocodyliforms and its significance for the evolution of the orbitotemporal region of eusuchians

“…Only M. adductor mandibulae externus profundus occupies the dorsotemporal fossa in extant crocodylians (Iordansky, 1964;Endo et al, 2002;Holliday and Witmer, 2007). This pattern differs from most lepidosaurian, avian, and basal crocodyliform taxa in which a combination of muscles, including Mm.…”

Jaw mechanism and dental function in the late cretaceous basal eusuchian Iharkutosuchus

“…Gadow (1901) and later Iordansky (1964) discussed the inverse correlation of MAMEP and MPTD in relation with the length of the rostrum and the size of the supratemporal fenestra. Longirostrine crocodylians, mostly regarded as piscivorous, possess large supratemporal fenestrae and a well-developed MAMEP because this muscle has its line of action close to the jaw joint, thereby producing a rapid jaw closure (Endo et al, 2002;Mueller-Töwe, 2006).…”

“…Indeed, detailed muscular reconstruction in a fossil archosaur cannot be achieved without considering the musculature of recent forms (Bryant and Seymour, 1990) and in most cases use of the extant phylogenetic bracket is essential (Witmer, 1995). The reconstruction of the cranial adductor musculature in Iharkutosuchus was mainly accomplished by comparison of the position, as well as the size and shape of muscle scars on the particular bones with those mentioned in extant crocodilians (Iordansky, 1964;Schumacher, 1973;Busbey, 1982Busbey, , 1989Drongelen and Dullemeijer, 1982;Cleuren and De Vree, 2000;Endo et al, 2002;Holliday and Witmer, 2007). In addition, earlier reconstructions for other extinct forms were also used for comparison (e.g., Mueller-Töwe, 2006).…”