Speculative Evolution: Herbivorous Eutriconodont
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In some other timeline, a lineage of eutriconodonts (specifically, within Triconodontidae) survive the KT event in Asia. Though succeeding as carnivores in the absence of carnivorans, one genus, Cynoailurus, begins specializing as bamboo specialists. Free green protein, y'all.
Now the triconodont type tooth has evolved into the numerous conditions seen in other mammals, from the turbercules of multituberculates to the talonid of therians to whatever toothed monotremes were doing. The triconodont type dentition was adapted for shearing meat, after all, so it had to change.
Cynoailurus took the path of least resistance and simply dulled and strengthened its cusps. No fancy alterations, just blunt points. Its adapted to pierce through bamboo after all, so no there was no need of fancy adaptations beyond perforation and crushing. The animal doesn't undergo extensive oral processing, so it kept just a slightly modified triconodont dentition.
To help grab bamboo it simply developed an opposite thumb. Several species ranging from the size of a cat to a panda bear exist, all solitary animals taking advantage of multiple species of bamboo. They give birth to altricial young (just like real pandas!) and reach sexual maturity late, as late as ten years in some species.
Most are endangered, but managed to snuggle their way into the hearts of the sophonts of their timeline (intelligent symmetrodonts) because that's how life finds a way.