TRANSFORMATION OF THE QUADRATE (INCUS) THROUGH THE TRANSITION FROM NONMAMMALIAN CYNODONTS TO MAMMALS

The quadrate (incus) bone underwent important evolutionary transformat ions through the cynodont-mammal transition. The following character t ransformations played crucial roles in modifying the cynodont quadrate into the mammalian incus: 1) progressively greater rotation of the do rsal plate relative to the trochlea; 2) the contact facet of the dorsa l plate becomes concave; 3) development of a constricted neck between the dorsal plate and the trochlea; 4) simplification of the quadrate-c ranium joint, resulting in better mobility of the joint; and 5) introd uction of a stapedial process (crus longum). The dorsal plate rotation , the concave contact facet, the constricted neck, the mobile joint of the quadrate and the cranium are also present in some advanced non-ma mmalian cynodonts. Broad phylogenetic distributions of these features suggest that the major features of the incus of early mammals, as repr esented by Morganucodon, originated much earlier in phylogenetic histo ry among non-mammalian cynodonts. Apomorphies of the quadrate (incus) among the advanced non-mammalian cynodonts favor a sister-group relati onship of tritheledontids and mammals. The hypothesis on the postdenta ry origin of the mammalian tympanic membrane is favored by transformat ion of the quadrate through the cynodont-mammal transition. Three most important modifications of the quadrate (incus) through the cynodont- mammal transition are: formation of the concave contact facet, progres sively greater rotation of the dorsal plate, and decrease in the numbe r of cranial bones articulating with the quadrate. These modifications would simplify the quadrate-cranial joint and increase the mobility o f the quadrate (incus) relative to the cranium while a functioning tym panic membrane was maintained on the mandible, improving the sensitivi ty of the postdentary tympanum. Probainognathus is among the earliest known non-mammalian cynodonts with a concave contact facet and a rotat ed dorsal plate in the quadrate. Thus we hypothesize that it represent s a critical step in the phylogenetic transformation that led to the o rigin of the modern mammalian middle ear and tympanic membrane.