Walking With Indonesian elephants: attribution of isolated proboscidean femurs and tibias to genus based on morphological differences

Direct comparison and biometric measurement analysis of homologue skeletal elements pertaining to distinct members of closely related vertebrate taxa (genera and species) can reveal qualitative diagnostic distinctive characteristics that reflect phylogeny and that may be related to differences in functional morphology. Apart from morphological differences, size can be an important factor in distinguishing taxa, especially when considering dwarfed insular proboscideans. Dwarfed proboscideans were once present on several islands of Indonesia during the Quaternary, but their postcranial remains have hardly been studied in detail. Most studies on fossil proboscideans focus on cranial and dental elements, which are comparatively easy to distinguish. In the case of proboscidean limb bones, morphological differences may be associated with different types of locomotion and could represent specific adaptations to insular environments. In order to investigate such adaptive morphologies in detail, a first step is to assess which differences exist between closely related genera. Establishing a list of diagnostic criteria to distinguish limb bones of different proboscidean genera and species will also aid in identifying isolated specimens of these elements. This study applied biometric analysis combined with 3D Geometric Morphometric (GM) analysis of fossil and recent skeletal parts of Indonesian proboscideans. The study aimed at characterizing the morphological differences between proboscidean taxonomic groups of two skeletal elements, the femur and the tibia. These elements were chosen because complete specimens of femur (n=10) and tibia (n=10) of known identity (based on associated dental elements) are relatively common in Indonesian fossil collections and were easily accessible. The studied sample comprises a recently excavated femur and tibia of a single adult individual of the very small-sized Stegodon sondaari from Flores. This offered the opportunity for the first time to investigate if this insular dwarf proboscidean possessed specific adaptations (apart from its small size) that are frequently encountered in insular megafauna, such as shortened distal limbs.