One of the taxonomically most diverse mammalian groups currently known from the Jurassic and Cretaceous (approximately 200 to 65 million years ago) when they were represented by 24 genera and 37 named species (all extinct). The name Eutriconodonta refers to the characteristic shape of their posterior cheek teeth, which consist of three or sometimes four cusps aligned anteroposteriorly (Fig. 1). The lower molariform teeth in many eutriconodontans are linked by a tongue-in-groove articulation formed by a projection from the posterior end of one tooth that fits into a groove in the anterior end of the following tooth.  See also: Dentition; Mammalia