Members of the three families of squirrel-like rodents share the same relatively unspecialised arrangement of jaw muscles: The 273 species of arboreal and terrestrial squirrels belong to the family Sciuridae; Two species of beavers belong to the family Castoridae; The single species of mountain beaver belongs to the family Aplodontidae. This burrowing animal, which lives in the mountains of Western North America. is thought to be the most primitive of modern day rodents.

Skulls and dentition

The scuirognath skull is unspecialised. The lateral masseter muscle extends in front of the eye onto the snout and moves the lower jaw forward during gnawing. The deep masseter muscle is short and only serves to close the jaw. The cheek teeth are simple - lacking cusps or enamel ridges. One of the two premolars is retained in each row, a primitive feature found only in sciurognaths. The dental formula of the Grey Squirrel is I1/1; C 0/0; P3/2;M3/3.

These animals are represented in our Collections by the European red squirrel, the grey squirrel and the Eurasian beaver.