Meniscoessus sp.
Multituberculate
Late Cretaceous (Maastrichtian)
Hell Creek Formation
Harding County, South Dakota, USA

Excellent 16mm jaw section with distinctive 4th lower premolar and molar with ascending process. Very fine example of an early mammal.

You might hear this Late Cretaceous mammal and its relatives referred to as “rodents” but true rodents didn’t appear until the Late Paleocene.  Meniscoessus was a genus of the Order Multituberculata, a branch of mammals entirely separate from any that live today.  They would have resembled rodents and probably had much the same lifestyle but their teeth, other than the oversized incisors, were different especially the lower fourth premolar which had become an enlarged shearing tooth perhaps for cutting into large seeds or slicing up fruit or even small prey.

Multituberculates appeared during the Jurassic, survived the K/T extinction, and died out around the end of the Eocene.