Ptilocercus lowii (pen-tailed tree shrew, extant, Le Gros-Clark 1926) nests among other tree dwelling placentals (= Scandentia) close to Tupaia at the base of Glires. Tree shrews originated in the Early Jurassic, 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 Protictis, the skull of Ptilocercus had a larger orbit completely surrounded by a postorbital ring. incisors are absent, as in Palaechthon and colugos, distinct from basal pangolins and bats. The special carnivore dental relationship of upper premolar 4 shearing against lower molar 1 is lost. The diet no longer includes bones that need to be cracked.

Ptilocercus has broad flat ribs, like those found in microbats, but not in Onychonycteris, a basal bat, and Pteropus, a fruit bat.

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.

The tibia is longer than the femur. The ankles are more flexible. The astragalus and calcaneum move away from stacked one upon the other to more of a  side-by-side configuration. The tibial malleolus, which restricts ankle rotation in most mammals is not present in bats, but still present in Ptilocercus. There is no longer only one longest toe. Pedal digits 2 through 5 are equal in length and their metatarsals follow suit. The pedal unguals also deepen.

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.

Tree shrews gallop, which is basically a series of leaps in which all four feet are in the air at the same time. That's a new way to locomote.

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 animals. B ecause 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.