IVth International Conference on Dormice (Rodentia, Gliridae)

13 - 16 September 1999 EDİRNE - TURKEY

Trakya University Department of Biology

Severtsov Institute of Problem Ecology and Evolution, Russian Academy of Sciences, Moscow, 117071, Leninski pr., 33, Russia, e- mail: sevin@glas.apc.org

The morphology of osseous middle ear was examined in next to all recent genera of Gliridae: Graphiurus (3 species), Glirulus, Myoxus, Dryomys (2 sp.), Eliomys, Muscardinus, Myomimus (3 sp.) and Selevinia. The evolutionary transformation of auditory bulla in this family was connected with pneumatization of its tympanic and mastoid regions. In all glirids except Glirulus construction of the tympanic part has the same pattern. The tympanic bulla has round form, its cavity is separated by three septae, the wide extended lamellae cover ossicles from the outside. Morphological diversity of this part is connected mainly with extent of pneumatization of bone around the external auditory meatus and with size of bell-shaped support of the annulus. A unique tympanic part construction occurs in Glirulus: tympanic bulla is not inflated, its lateral wall is composed of trabeculated bone, external auditory meatus has tube form, the tympanic annulus attaches to the lateral bullar wall, and the meatal cylinder is absent. The mastoid part is more variable than tympanic one. Three modes of the mastoid pneumatization may be distinquished in Gliridae. One of them is characteristic for Glirulus, another is manifested by Graphiurus hueti and the third type, most typical for Gliridae, occurs in other glirids in diffderent variants. Judgements about possible courses of mastoid pneumatization are based on facts of presence/absence of chambers being connected with tympanic cavity in some particular way. Mastoid cavity of typical pattern consists of five chambers. One of them (epitympanomastoid chamber) is formed by expanding tympanic cavity from above via the epitympanic recess. Four other cells develop as a result of the growth of the tympanic cavity from below in the region of tympano-mastoid contact. The antral chamber lies behind the posterior semicircular canal. The superior mastoid chamber is situated above lateral semicircular canal and is connected with the tympanic cavity through foramen within curvature of this canal. The inferior mastoid chamber is situated ventrally of the previous one, below the lateral canal. The forth cavity develops as the tympanic cavity expanse into mastoid at the outer side of the lateral canal behind foramen for stapedial muscle. The most generalized variants of this mastoid type is that of Myoxus. It is weakly pneumatized and has but a single small chamber opening in the tympanic cavity. The remaning genera display variants derived from that of Myoxus. It is characterized by presence of all five mastoid chambers, and differ from each other by extent of mastoid inflation and various size of chambers. In some groups, antral and lateral chamsers are more developed, while in others, superior and inferior mastoid cells prevail. Glirulus differs significantly from all other glirids in respect to not only the structure of tympanic but of mastoid part, as well. In this genus, the main way of mastoid pneumatization is through foramen within curvature of lateral semicircular, and only three of five chambers are present. Antral and epitympanomastoid chambers most typical for glirids are absent in Glirulus. The third type found in G.hueti only is unique among not only Gliroidea, but in Rodentia in general. In this species, mastoid pneumatization from above via the epitympanic recess does not occur at all. Instead of four cells connected with tympanic cavity from below, there is the only one antral chamber. One more chamber is situated above the lateral canal. It is connected with the tympanic cavity via antral chamber and is not homologous to superior mastoid cell. The inferior mastoid cell and lateral expanse are absent.

So, almost all Gliridae are characterized by the same evolurionary route of middle ear development. Each of Glirulus and G. hueti demonstrates specific ways of pneumatization strongly differing from those of the other glirids.