Lumbar vertebral morphology of flying, gliding, and suspensory mammals: implications for the locomotor behavior of the subfossil lemurs Palaeopropithecus and Babakotia.

Lumbar vertebral morphology has been used as an indicator of locomotor behavior in living and fossil mammals. Rigidity within the lumbar region is thought to be important for increasing overall axial rigidity during various forms of locomotion, including bridging between supports, inverted quadrupedalism, gliding, and flying. However, distinguishing between those behaviors using bony features has been challenging. This study used osteological characters of the lumbar vertebrae to attempt to develop fine-grade functional distinctions among different mammalian species in order to make more complete inferences about how the axial skeleton affects locomotor behavior in extant mammals. These same lumbar characters were measured in two extinct species for which locomotor behaviors are well known, the sloth lemurs (Palaeopropithecus and Babakotia radofilai), in order to further evaluate their locomotor behaviors. Results from a principal components analysis of seven measurements, determined to be functionally significant from previous studies, demonstrate that inverted quadrupeds in the sample are characterized by dorsoventrally short and cranio-caudally expanded spinous processes, dorsally oriented transverse processes, and mediolaterally short and dorsoventrally high vertebral bodies compared with mammals that are relatively pronograde, vertical clingers, or gliders. Antipronograde mammals, dermopterans, and chiropterans also exhibit these traits, but not to the same extent as the inverted quadrupeds. In accordance with previous studies, our data show that the sloth lemur B. radofilai groups closely with antipronograde mammals like lorises, while Palaeopropithecus groups with extant sloths. These findings suggest that Palaeopropithecus was engaged in inverted quadrupedalism at a high frequency, while B. radofilai may have engaged in a more diverse array of locomotor and positional behaviors. The osteological features used here reflect differences in lumbar mobility and suggest that axial rigidity is advantageous for suspensory locomotion and possibly flight in bats.