Insular adaptations in the astragalus-calcaneus of Sicilian and Maltese dwarf elephants

The endemic Pleistocene dwarf elephants from Sicily and Malta display numerous anatomical changes with respect to their putative mainland ancestor and contemporary Palaeoloxodon antiquus, including significant differences in the functional morphology of the astragalus-calcaneus. Here we describe the functional morphology of dwarf elephants from the Siculo-Maltese palaeo-archipelago, with a particular emphasis on Palaeoloxodon ex gr. P. falconeri from Spinagallo Cave (Hyblean plateau, Sicily). Interspecific comparisons across a wide spectrum of body mass indicate that a decrease in the mass of dwarf elephants was accompanied by a shift from mass being transmitted between the tibia-astragalus to relatively more mass being transmitted by the tibia-calcaneus. This is most evident in the calcaneus of small-bodied Palaeoloxodon ex gr. P. falconeri, which have a large and continuous tibio-fibular facet suggesting increased flexion in the ankle-joint. Synostosis between the distal tibia-fibula and the calcaneus' wide articular facet for the fibula in this species limited medio-lateral displacement of the astragalus-calcaneus relative to the tibia, suggesting possible evidence of ‘low-gear’ locomotion related to the very hilly topography of Sicily. Furthermore, differences in the functional morphology of the calcaneus between the similar-sized Palaeoloxodon sp. from Luparello Fissure (Palermo, Sicily), and Benghisa Gap (southern Malta) are possibly ecophenotypic, as a result of differing insular environments, or alternatively warrant a taxonomic revision of the material from Luparello Fissure.