A functional analysis of a well-preserved snout of the early-diverging mammaliaform Morganucodon watsoni, with matching upper and lower dentitions, and of the holotype of Megazostrodon rudnerae, showed that both taxa had a primarily orthal occlusal path. In Morganucodon, the direction was individually variable and either strictly orthal or slightly distally or mesially inclined. An analysis with the Occlusal Fingerprint Analyser (OFA) software confirmed an earlier hypothesis that the main cusp A of the upper molars occluded between cusps b and a of the lower antagonists. According to the OFA analysis, there was more extensive contact between cusp a and the preceding anterior upper molar than previously assumed, showing some similarities to the two-on-one pattern described for Megazostrodon. According to our analyses, the molars of Morganucodon and Megazostrodon had an adaptation to piercing, as well as shear-cutting. ‘Shearing flanks,’ which were the focus of previous studies, seem to be a result of attrition, rather than functional areas in themselves. The posterior upper molars in Morganucodon were rotated along their longitudinal axis and lingually inclined within the tooth row, resulting in a triangle between M1 and M2 into which the large m2 can occlude. Together, this suggests a predetermined tooth placement and that contrary to previous hypothesis Morganucodon did not rely on extensive wear in order to form a precise occlusion.
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