Ptilocercus lowii (pen-tailed tree shrew, extant, Le Gros-Clark 1926) was derived from a sister to Protictis from among the civets and phylogenetically preceded Palaechthon and the colugo, Cynocephalus among other tree dwelling placentals (= Scandentia).
Scandentia is a clade of tree shrews basal to Volitantia (bats + pangolins + colugos), Primates, and Glires (rodents + rabbits + multituberculates and kin). Tupaia is a tree shrew basal to Glires. Microcebus is a tree shrew/tiny lemur basal to Primates. Tree shrews originated in the Early Jurassic, but their fossils go back only 3 million years.
Scandentia is derived from basal Carnivora/basal Placentalia and also gives rise to Maelestes and the rest of the placentals. Most of these are ground-dwelling and includes odontocetes, edentates and ungulates.
Ptilocercus is ancestral to colugos and bats, but lacks their extradermal membranes.
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.
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