The last non-mammaliaform cynodonts included the tritylodontids, an advanced family of herbivorous cynodonts. They were close to mammals and shared several mammal-like traits, but they remained outside Mammaliaformes. Their extinction around 125 million years ago marked the end of the non-mammaliaform cynodont line, while mammals—the surviving branch—continued on.

By the Early Cretaceous, around 125 million years ago, the world of the last tritylodontids was no longer the old synapsid world. It was a dinosaur-dominated landscape of warm seasonal forests, floodplains, lakeshores, and volcanic basins. Tritylodontids would have shared these environments with early birds, lizards, amphibians, turtles, pterosaurs, and dinosaurs such as small theropods and herbivorous ornithischians. In places like Early Cretaceous East Asia, the ecosystem also included mammaliaforms and true mammals that were becoming surprisingly diverse, so tritylodontids were not simply “waiting for mammals.” They were living beside them. The Cretaceous ran from about 145 to 66 million years ago, and early mammals and mammal-relatives had already diversified into many ecological roles well before the dinosaur extinction.

Tritylodontids were advanced herbivorous cynodont therapsids, close to mammals but usually placed outside Mammaliaformes. They had small-to-medium bodies, strong jaws, large cheek teeth with multiple cusps, and a well-developed secondary palate, all pointing to a specialized plant-eating lifestyle. Some later forms, such as Fossiomanus sinensis, even show digging adaptations, suggesting they could occupy burrows or fossorial niches. They were among the last non-mammaliaform therapsids, surviving long after true mammals had appeared. That makes them a wonderful “living fossil” style branch in the mammal-side story: not mammals, not primitive throwbacks, but late-surviving cousins carrying an older version of the mammal-line experiment into the age of dinosaurs.