Functional and Phylogenetic Implications of Ankle Morphology in Goeldi’s Monkey (Callimico goeldii)

Abstract

Callimico behaves like callithrichid but its ankle looks like a larger non-callithricid plat. Compares Callimico's ankle to 14 other platyrrhine species. The size and morphology of the tibial malleolus and medial astragalar facet are distinct in Callimico. Medial Tibioastragalar Joint: Large malleolus, typical of non-callithricid plats. Bulbous tibial malleolar articular surface like P. pithecia. Largest medial astragalar facet. Behavioral translation: An enlarged tibioastragalar articular surface area provides a larger attachment for the deltoid and medial ligament and a stable displacement-resistant brace during plantar and dorsiflexion. Adding a bulbous malleolar surface further stabilizes the ankle during dorsiflexion which occurs during VCL. The morphological complex of Callimico's ankle is absent in other callithricids who also VCL. So this morphology must be Callimico derived, otherwise why would the rest retain the behavior but loose the morphology, the author reasons. Possible form/function body size threshold in which larger species require a more stable medial tibioastragalar joint for habitural ankle dorsiflexion while smaller species may maintain this activity w/out the supportive features in the ankle. Superior Tibioastragalar Joint: long astragalar trochleae (more than half the lenght), like non-callithricids. Facilitate increased plantar and dorsiflexion. This benefits VCL, leaping, climbing, and hindlimb susp. Because of this it may be a phylogenetic instead of functional trait. Posterior narrowing of the trochlea (astragalar trochlear wedging) or whether parallel-sided, correlated w locomotion as well (145). Wide distal trochlea increases weight transmitting surface during dorsiflexion. A narrowed proximal trochlea reduces joint sruface contact are during plantarflexion and inclreases mobility, advantageous for hindlimb hanging, which Callimico does. Neither does the tibial stop nor the trochlear wedging give clear signal of locomotion anymore.