Lactodens sheni (Han and Meng 2016; Early Cretaceous) nests as a tiny mouse-like prototherian mammal, the last common ancestor of echidnas and platypuses, neither of which is mentioned in the original paper. Originally described as a spalacolestine and a symmetrodont, here it nests with prototheres (= monotremes).
‘Symmetrodontans’ are extinct mammals characterized by having a reversed-triangle molar pattern in which three main cusps define a triangular molar crown. This dental morpholgy has been regarded as being intermediate between the ‘triconodont’ tooth and the tribosphenic pattern characterizing therians.
Lactodens has a dental formula of three upper incisors, one canine, three premolars, and six molars/three lower incisors, one canine, five premolars and six lower molars, double-rooted canines, extremely low-crowned and transversely thin premolars, and acute angled molars. Deciduous canines and premolars in the slim and extremely long dentary imply a faunivorous diet. |