Meeting Abstract
The derived postcranial morphology of bats reflects their specialized ecology – flying mammals with an inverted roosting position. While adaptations of the bat forelimb have been researched extensively, fewer studies have explored the evolution of the bat hindlimb. In particular, the calcar (a cartilaginous or bony spur projecting from the calcaneum into the hindlimb membrane) has received little attention, even though it varies considerably among bat species and may be a novel skeletal feature in mammals. We address two major knowledge gaps regarding the evolution of the calcar-calcaneum joint and shed light on their functional significance. First, is the calcar a bone or a cartilaginous element? Second, does ecological specialization explain the morphological diversity of the calcar and calcaneum in bats? To answer these questions, we integrated analyses of comparative histology and 3D morphology across 21 bat species from 16 families. By comparing the tissue anatomy of calcars in a subset of these species, we found evidence of ossification in at least one bat species, Noctilio leporinus. This raises questions about the homology of the calcar among mammalian tarsals. Our comparisons of the 3D morphology of the calcanea revealed that their shape is influenced by both phylogeny and ecological specialization. For example, the calcaneum of N. leporinus has a uniquely broad calcaneal tuberosity that corresponds with the laminar shape of its calcar. N. leporinus is a fishing bat, and its stiff, bony calcar may be adapted to support the hind limb membrane as it skims the water surface.
