Comments on the evolution of the jaw adductor musculature of snakes



The aim of this study is to provide a general view of the adductor musculature of the alethinophidian snakes. The aponeurotic system present in anilioid snakes is here described as being also present in colubroid and booid snakes. Although modified in various groups, this aponeurotic system retains the same topographical pattern in the anilioids, booids and colubroids, and is thus hypothesized to be homologous. An analysis of the aponeurotic system and related muscular bundles within the alethinophidian snakes is given. A new terminology is proposed for the jaw adductor muscles where the muscles levator anguli oris and adductor mandibulae externus superficialis (proper) of snakes (sensu Lakjer, 1926; Haas, 1962) retain these names even if this fails to reflect the presumed homologies with the bundles of the same name in lizards (see Rieppel, 1988b); the fibres originating from the temporal tendon in the Anilioidea, and presumed to form a bundle of composite nature (Rieppel, 1980b), are named the M. adductor mandibulae externus temporalis (lost by the Macrostomata); the M. adductor mandibulae externus medialis is a composite muscle in the Anilioidea (Rieppel, 1980b) which give rise to two different muscles in the ‘booids’, the M. adductor mandibulae externus medialis, pars anterior and the M. adductor mandibulae externus profundus, the former being secondarily lost by the Caenophidia which retains only fibres homologues of the 3b and 3c heads of the profundus layer of lizards; the so-called M. adductor mandibular externus profundus of snakes (sensu Lackjer, 1926; Haas, 1962) is also a composite muscle in the Anilioidea (Rieppel, 1980b), in the alethinophidians it is essentially made of fibres homologous with the posterior pinnate part of the medialis layer of lizards, and is here named the M. adductor mandibulae externus medialis, pars posterior. As a result from this analysis it follows that: (1) the Macrostomata are characterized by the downward extension of the fibres forming the M. adductor mandibulae externus medialis, pars anterior and the loss of the M. adductor mandibulae externus temporalis: (2) the Xenopeltidae are set apart from the remaining macrostomatan snakes by the retention of the M. levator anguli oris and of a well developed lateral sheet of the quadrate aponeurosis; (3) the ‘booids’ form a monophyletic group comprising only the Boidae and Bolyeriidae (with the exclusion of the Xenopeltidae and Tropidophiidae) which is characterized by a differentiated M. adductor mandibulae externus medialis, pars anterior inserting on the lateral surface of the compound bone via its own aponeurosis; (4) the Tropidophiidae are set apart from all other snakes by the peculiar course of their lateral head vein; however, they belong to the Caenophidia as they show a facial carotid artery which passes dorsally to the mandibular and maxillary branches of the trigeminus; (5) a possible additional character in favour of an Acrochordoidea + Colubroidea monophyletic unit may be given by the pattern of innervation of the jaw adductor muscles in these two taxa; (6) a new interpretation of the compressor glandulae muscular complex of Atractaspis resulted in a morphologically similar pattern to that of the viperids; the phylogenetic implications of such similarity are discussed in detail.