On the trigeminus muscles of the lizards Xenosaurus grandis and Shinisaurus crocodilurus. American Museum novitates ; no. 2017

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New York, N.Y. : American Museum of Natural History
"The superficial layers of the throat musculature in Xenosaurus and Shinisaurus are very similar in respect to both the differentiation the arrangement of the constituent portions. In the case of Shinisaurus, the similarity with Xenosaurus places the former clearly in Camp's group Anguioidea. The detailed comparison of the trigeminus musculature, however, shows some remarkable differences between both forms. In both genera the upper temporal opening is wide, but the skull as a whole is rather flattened and broad in Xenosaurus, high and narrow in Shinisaurus. This difference is accentuated in the temporal area, the space in which most of the temporal muscles are situated. Both forms possess a levator anguli oris muscle, which is not always found in saurians and ophidians. The m. adductor mandibulae externus is extremely complex, and its often assumed (Luther, Lakjer) tripartite arrangement is blurred in many ways. Especially the boundary between the medialis and profundus portion of this muscle group is not easily determinable owing to partial fusions and parallel fiber course in certain areas of contact. Lakjer's 'Bodenaponeurose' is not very extensive in either form. The existence of an adductor posterior remains very doubtful, but deepest parts of the m. adductor mandibulae externus profundus may assume the position and the fiber arrangement of a real adductor posterior. The ramus mandibularis trigemini in its course towards the lower jaw does not overlap any deep adductor portions in such a way that would clearly separate the lateral profundus area from the medial adductor posterior component. In spite of this fact, deepest, well-separable parts of the profundus group could well represent parts of an adductor mandibulae posterior. Owing to the fact that relatively few saurians have been throughly investigated as far as the trigeminal musculature is concerned, it is still too early to try to understand the intricate patterns of differentiation, especially of the m. adductor externus in connection with the morphological aspect of the skull and with phylogeny. The deeper trigeminal muscles, otherwise of a fairly constant arrangement, differ strikingly in both forms under consideration. Both portions of the m. pseudotemporalis exhibit a certain degree of correlative development: both muscles together are of a corresponding volume in relation to the mass of the other adductors. But in Shinisaurus the m. pseudotemporalis superior is the bigger of the two muscles of this group, whereas in Xenosaurus the profundus portion is the prevalent one. Both genera, owing to a certain degree of cranial kineticism, exhibit mm. protractores and levatores pterygoidei. The protractores are fairly similar but are slightly better developed in Xenosaurus. The levator pterygoidei is weak and almost not retrahent in both forms - really a 'levator' only. The levator bulbi ventralis is bipartite in Shinisaurus, single in Xenosaurus. Ophisaurus, the only other anguid of which the trigeminal musculature has been studied, has, in contradistinction to both forms dealt with in this paper, a very narrow temporal opening. This fact alone has a deep influence on the arrangement and the differentiation of the jaw muscles. In forms with large temporal openings parts of certain temporal muscles bulge into this opening in very different patterns. Parts of the mm. adductores externi superficiales and/or mediales, and/or the m. pseudotemporalis superior, may be involved. Very often, the single or bipartite adductor externus medialis fills the opening exclusively, but other combinations may occur as well. The significance of these different penetrations into the temporal opening is so far not understood, in spite of the fact that the arrangement of the temporal arches in sauropsids constitutes one of the main systematic criteria. So both forms discussed here differ in respect to the muscular penetration into the wide temporal openings. In Shinisaurus, the m. pseudotemporalis superior shares the temporal aperture with a bipartite m. adductor externus medialis, as in the agamid Calotes or, in somewhat different distribution of the muscular shares involved, as in Sphenodon, in which the medialis is single. In Xenosaurus, however, the two portions of the m. adductor externus medialis fill the temporal fenestra exclusively, as in the agamid Uromastix, the iguanid Phrynosoma, and Chamaeleon, representing a separate group, probably connected with the Agamidae or with both the Agamidae and Iguanidae"--P. 51-53.
54 p. : ill. ; 24 cm.
Includes bibliographical references (p. 53-54).