The fossil of a complete cranium of a North American haramiyid - a group of early mammals - from the Cretaceous period is reported in a paper this week in Nature. This cranium, dated to around 139-124 million years ago, is the youngest known specimen of a haramiyid and suggests that the species persisted in the northern continents after the breakup of Pangaea.

Haramiyids represent an enigmatic group of mammals known from the Triassic and early Jurassic periods. For a long time they were known just from fossilized teeth, until specimens from China were found that preserved entire animals complete with soft tissues. Some of these creatures were gliders, similar to modern flying squirrels. There has been much debate about the relationships of haramiyids with other mammals: whether they were close to the successful rodent-like multituberculates, or far more primitive.

Adam Huttenlocker and colleagues describe the fossilized cranium of a haramiyid found in the Andrew’s Site quarry in Utah, USA, from the lower Cretaceous. It provides the basis of a new genus and species of a haramiyid: Cifelliodon wahkarmoosuch. The authors show that this specimen fills substantial morphological and spatiotemporal gaps in the common understanding of this group. Analysis of the skull reveals that another even more enigmatic group of mammals called hahnodonts are actually haramiyids, not multituberculates, as commonly understood.