Xilmaxilligidae

Xilmaxilligidae is a family of small, extinct mammals that lived during the Late Cretaceous period, approximately 99 to 66 million years ago. They are known from fossils found in North America, particularly in the Hell Creek Formation of Montana and the Dakotas. Xilmaxilligidae is characterized by its small size, with the largest known species, Xilmaxilligia maxima, reaching a length of about 10 centimeters (4 inches).

The family is named after the genus Xilmaxilligia, which was first described in 2009. The name is derived from the Navajo word "xilmaxillig," meaning "small mouse," reflecting the small size of these mammals. The family is further divided into two genera: Xilmaxilligia and Xilmaxilligia minor.

Xilmaxilligidae is considered to be a member of the order Multituberculata, a group of mammals that are characterized by their complex, molar-like teeth. The family is notable for its small size and the fact that it is one of the earliest known members of the order to have evolved in North America. Despite their small size, Xilmaxilligidae played an important role in the evolution of mammals, helping to shape the diversity of the group in the Late Cretaceous.