The evolution of precise tooth occlusion in mammals was a key innovation enabling effective processing of a wide variety of foods to fuel their high metabolic rate. Such exact occlusion likely requires tight developmental control over tooth size and shape as well as the coordination between upper and lower teeth. The developmental origins of this innovation are still unclear. We examine the developmental basis of occlusion using the evolutionary-developmental rule called the inhibitory cascade. The inhibitory cascade ensures that teeth do not change in size randomly and so are integrated along the row. If both the upper and lower tooth rows were under the control of the inhibitory cascade, then relative changes in tooth size could be synchronised between the two. This coordination would result in the maintenance of occlusion during development, facilitating evolutionary change to more precise occlusion. We analysed relative tooth size in two major clades of mammals that diverged ~160 Ma, primates (including fossil hominoids and subfossil lemurs; n=65 species) and marsupials (n=70 species). Our results show that the inhibitory cascade patterning is synchronised between the upper and lower jaws as evidenced by similarities in the slope and reversal point of the inhibitory cascade. We establish that the integration of mammalian teeth is largely controlled by the inhibitory cascade, enabling more complex interdigitation required for precision chewing in mammals. This mechanism must have been present from very early in the history of mammals and is a fundamental reason for the success of mammals throughout the Cenozoic.