The jaw articulation of the Docodonta and the classification of Mesozoic mammals

Abstract

Re-examination of the specimens of Docodon from the American Upper Jurassic (Morrison) has shown the presence of a trough running along the medial side of the posterior part of the dentary. A similar, although relatively larger trough, is found in a mammal related to Morganucodon , from the Rhaeto-lias of South Wales. Comparison with the most advanced of the mammal-like reptiles shows that this groove must have been for the malleus (articular), which in Docodon and in Morganucodon still functioned as part of the jaw articulation, although in both the main part of this hinge would have been formed by the well-developed mammalian condyle on the back end of the dentary. A new subclass, the Eotheria, is created for such mammals with a dual jaw articulation. Morganucodon is related to the present-day monotremes, and is probably close to the ancestral stock of the latter. Morganucodon may also show triconodont affinities.