All squirrels are characterized by their relatively primitive jaw structure. The skull has an broad, tilted zygomatic plate that serves as the attachment point for the lateral branch of the masseter muscle . The superficial branch of masseter muscle originates on a prominent bump of bone of the side of rostrum called the masseteric tubercle .
Tree squirrels have long, bushy tails, sharp claws and large ears.
Some have well-developed ear tufts. Ground squirrels are generally more robust than tree squirrels and often have short, sturdy forelimbs that are used for digging. Their tails, while fully furred, generally are not as bushy as those of tree squirrels. Like many other rodents, all squirrels have five functional toes on the hindfeet and four on the forefeet, with a well-developed claw on each digit. Squirrels occur throughout the world except for Australia, southern South America, and some desert regions.