Their ancestors appeared around the same time as the first small mammals, and they were probably able to survive because they were evolving— and growing—at the same time as their mammalian predators.
نیای آن‌ها حدوداً در زمان اولین پستانداران ظاهر شدند و آن‌ها توانستند زنده بمانند چون همزمان با شکارچیان پستاندارشان تکامل پیدا می‌کردند و بزرگ می‌شدند.