Morphological Specificity of the Auditory Capsule of Sciurid (Sciuridae, Rodentia)

Abstract

Specific features of the sciurid auditory capsule have been analyzed based on 63 species from 20 genera. Its morphological specificity is characterized by a peculiar combination of primitive and advanced features stable within the group. They are (1) a rounded and inflated bulla with internal septa and a primitive attachment of the tympanic ring to the inner tympanic wall; (2) a transverse position of the bulla relative to the axis of the promontorium, resulting in the presence of deep petrosal fossae in front of and above the promontorium which are not covered by the tympanic bone; (3) a fully formed osseus facial canal and a developed bone tube of the stapedial artery; (4) the presence of a meato-cochlear bridge connecting the promontorium to the posterior wall of the acoustic duct which, in a fully developed form, does not occur in any other recent rodents; (5) pneumatization of the mastoid due to the expanding of the epitympanic chamber both anteriorly and posteriorly, with the formation of a premeatal pocket and a large epitympano-mastoid chamber divided into parts. The sciurid auditory capsule can generally be considered as morphologically advanced. Its diversity within the family concerns functionally significant features and is manifested in the structure of the external acoustic meatus, the size of the tympanic membrane, the length of the processes of auditory ossicles, and, most importantly, the degree of pneumatization of the auditory capsule. This is associated with quantitative variations of the characters and does not affect its structural plan, as a rule.