In this study, the axial skeleton of 14 species of tupaiids (tree shrews) was analysed functionally and compared to that of other archontan mammals. Several differences that relate to differential substrate use were found in the ribs and vertebrae. These differences included cranio-caudal width of the ribs; number of thoracic, lumbar, and caudal vertebrae; cranio-caudal width of the atlas; orientation of the spinous process of the axis; length and cranio-caudal width of the spinous processes of the thoracic vertebrae; length of the spinous processes of the lumbar vertebrae; length and orientation of the transverse processes of the lumbar vertebrae; and the number of sacral vertebrae that articulate with the ilia. The ribs and vertebrae of the arboreal Ptilocercus lowii, the only ptilocercine, exhibit adaptations for a stable thorax that probably facilitate bridging locomotion. The vertebral columns of tupaiines, on the other hand, are more mobile and allow more flexion and extension of the spine; this increased flexion and extension increases stride length, which in turn increases speed in bounding or galloping mammals such as terrestrial tupaiines. It is proposed here that the attributes of the thorax of Ptilocercus are primitive for the Tupaiidae, that the ancestral tupaiid was arboreal, that the tupaiine condition is derived, and that the ancestral tupaiine was terrestrial. It is also proposed that: Ptilocercus may be primitive for the Archonta in its axial skeletal features; a stable thorax was first evolved in an arboreal ancestral archontan; the adaptations for stability of the thorax were retained in the Volitantia (dermopterans and chiropterans) for certain locomotor types, including gliding or flying; a mobile thorax evolved in conjunction with the shift to graspleaping in the ancestral euprimate. These scenarios may be further tested by quantitative analyses of vertebral osteology, as well as myological analyses of the epaxial musculature.
