A new mammaliaform from the Early Jurassic and evolution of mammalian characteristics

A fossil from the Early Jurassic (Sinemurian, similar to 195 million years ago) represents a new Lineage of mammaliaforms, the extinct groups more clo sely related to the living mammals than to nonmammaliaform cynodonts. it ha s an enlarged era nial cavity, but no postdentary trough on the mandible, i ndicating separation of the middle ear bones from the mandible. This extend s the earliest record of these crucial mammalian features by some 45 millio n years and suggests that separation of the middle ear bones from the mandi ble and the expanded brain vault could be correlated. It shows that several key mammalian evolutionary innovations in the ear region, the temporomandi bular joint, and the brain vault evolved incrementally through mammaliaform evolution and Long before the differentiation of the Living mammal groups. With an estimated body weight of only 2 grams, its coexistence with other Larger mammaliaforms with similar "triconodont-like" teeth for insectivory within the same fauna suggests a great trophic diversity within the mammali aform insectivore feeding guild, as inferred from the range of body sizes.