Whose lower left jaw is this? That's the question ancient mammal expert Jaelyn Eberle asked herself when a colleague found this little lower-left jaw in Late Cretaceous sediments in Wyoming's Hanna Basin. So she set out on a taxonomic and systematics journey -- to identify and create a family tree for this "beautiful" 3.5-centimetre fossil.

Teeth tell a tale so specific that mammal species, both modern and ancient, can be identified from teeth alone.

At first glance Eberle knew that these teeth were mammalian. Fish and reptile teeth are most often conical or pointed. In contrast, the molars and premolars of these teeth are more complex, having numerous cusps, or bumps. In fact, they have so many that Eberle judged that the animal was a member of the ancient mammalian order known as the multituberculates, which means "animals with many cusps on their teeth". They are an unusual group of rodent-sized mammals with strange teeth. They went extinct in the Eocene (about 35 million years ago) leaving no modern descendants.