No abstract

The anterior anchoring of the malleus of 30 extant species of Lagomorpha (rabbits, hares, pikas) has been studied on the basis of histological serial sections and µCT-scans. It is shown that former studies of Oryctolagus, Lepus, and Ochotona are incomplete, because the rostral part of the processus anterior of the malleus is always lacking due to damage of this extremely delicate structure. Our study shows that in perinatal stages of Leporidae the praearticulare develops a prominent processus internus that fits into a groove at the ventral side of the tegmen tympani; this "tongue and groove"-arrangement may act as a hinge. In adult stages, the rostral end of the praearticulare fuses synostotically with the medial process of the ectotympanic. Torsional strain produced by rotation around the axis of the middle ear ossicles at sound transmission must, therefore, be experienced by the extremely thin but highly elastic bony pedicle of the processus internus praearticularis. The free ending processus anterior of a late fetal Ochotona shows a short processus internus praearticularis, which does not articulate with the tegmen tympani. During postnatal development the middle ear of Ochotona becomes considerably remodelled: not only does excessive pneumatization of the tegmen tympani and tympanic cavity wall occur, but the short processus anterior is fused synostotically to a bone trabecula of the tegmen tympani meshwork. The thin and elastic bone bridges are not equivalent in Leporidae and Ochotonidae, that is, they must have evolved convergently. Fleischer's classification with Oryctolagus possessing a "freely mobile type" of middle ear ossicles cannot be supported by our observations. The same holds true for Ochotona, which does not represent a "freely mobile type" either. Thus, we suggest for the lagomorph middle ear ossicles a new category: the "bone elasticity type."

No abstract

Modern phylogenetics place the Soricidae (shrews) into the order Lipotyphla, which belongs to the relatively new superorder clade Laurasiatheria. Their most derived skull feature is the unusual position and shape of the jaw articulation: Whereas in all other mammals the glenoid region of the squamosum is more or less tightly attached to the otic capsule or petrosal, respectively, in the soricids it is attached to the nasal capsule. This new position of the jaw articulation becomes possible by the posterior extension of the nasal capsule and the rostral shift of the glenoid fossa. By the study of dated postnatal ontogenetic stages of Crocidura russula and Sorex araneus, we show that the glenoid part of the squamosal becomes fixed to the nasal capsule by the ossified alae orbitalis and temporalis. The ala orbitalis is displaced laterally by the expanded cupula nasi posterior; this posterior expansion is well documented by the lamina terminalis, which incorporates parts of the palatinum and alisphenoid. Both alae consist largely of ‘Zuwachsknochen’ (‘appositional bone’) and are then named orbitosphenoid and alisphenoid. By the forward move of the pars glenoidea and of the alisphenoid, the foramen lacerum medium (‘fenestra piriformis’) also expands rostrally. Functionally, the forward shift of the jaw joint helps to keep the incisal biting force high. Biomechanically the jaws can be considered as a tweezer, and the rostral position of the jaw joints makes the interorbital pillar and the shell-like walls of the facial skull a lever for the highly specialized incisal dentition.

a b s t r a c tEarlier studies have shown that the epitensoric position of the chorda tympani is a systematically useful apomorphic character in some mammalian orders (primates, carnivores, rodents). Newly made histological serial sections of a fetal stage now reveal that Laonastes aenigmamus (Diatomyidae), a rodent species first described in 2005, is epitensoric as well. Because Ctenodactylus gundi is the only other taxon within the Ctenohystrica having this derived character state, we conclude that this is an additional synapomorphy substantiating a sister group relationship between Diatomyidae and Ctenodactylidae.