Mouse-sized mammals known as multituberculates evolved 165 MYA. The earliest species were meat eaters, but plants were constantly upping their game with innovations. As plants became tastier, many multituberculates moved to fresh greens. By 84–66 MYA, multituberculates on a plant-based diet had grown from the size and girth of a mouse to that of an oversized groundhog: some 5 kilograms.
Surviving for 130 million years, multituberculates had an exceptionally good run: the longest lasting of all mammals. Owing to their prodigious diversity, they are often considered one of the most successful mammal groups.
Alas, their good fortune did not last. Multituberculate diversity dropped during the Eocene extinction event 56 MYA. Competition from predatory eutherians – the extant mammal clade – was the insurmountable problem. Multituberculates went extinct 35 MYA.