Ptilocercus lowii (pen-tailed tree shrew, extant, Le Gros-Clark 1926) nests among other tree dwelling placentals (= Scandentia) with Tupaia at the base of Glires and derived from pouchless marsupials, like Glironia. Tree shrews must have originated in the Early Jurassic, prior to the earliest multituberculates, but their fossils go back only 3 million years.

Tree shrews act like squirrels, but instead of eating nuts, they eat insects. Pouncing on prey while perched on a tree branch requires greater skills and a larger brain. To find prey tree shrews rely on their eyes and their hands.

Distinct from Glironia, the skull of Ptilocercus had a larger orbit completely surrounded by a postorbital ring. Incisors are reduced from 4 to 2. The canine is a vestige.

The radius is longer than the humerus. The ulna is reduced distally, to no more than one third the width of the radius (as in bats). The short fingers of Ptilocercus demonstrate the ability to spread so widely that digits 1 and 5 oppose one another by 180º, a character unheard of in mammals other than bats, by convergence.

A tree shrew cannot retract its eyballs back into their sockets. Nor is a nictating membrane (clear third eyelid) present. Color vision makes a return, as does better binocular vision. A pair of feathery extra tongues are found beneath the main tongue. These are also found in lemurs by convergence.

The pen-tailed tree shrew lives in trees full time. The first digit is somewhat thumb-like, able to grasp objects. Pen tails retain a scaly tail up to the last inch which is tipped with long feathery hairs.

Tree shrews are active during daylight, which sets them apart from more primitive relatives, like Glironia. Because of this their internal temperatures are set higher, closer to the 98.6 degrees that is normal for humans. To maintain their temperature and their active lifestyle a tree shrew must eat its weight in food each day.

Tree shrews produce a few sounds, from a snarling hiss to a piercing squeal. They also chatter when alarmed. Most have cartilage that shapes the outer ear. Like a dog, tree shrews can draw back their upper lip to expose their teeth, which is their only facial expression.