by several muscles, including the gastrocnemius muscle, m. calcaneocutaneous, and m. depressor ossis styliformis. M. depressor ossis styliformis abducts the calcar towards the foot, which spreads the uropatagium. M. calcaneocutaneous works in opposition to m. depressor ossis styliformis, helping to stabilize the calcar. The gastrocnemius muscle aids in flexion of the foot, working in conjunction with m. depressor ossis styliformis to spread the uropatagium. Unlike microbats, megabats lack the m. calcaneocutaneous muscle for calcar control; megabats do share the other two muscles for calcar control, however. Function The calcar can assist the uropatagium in forming a basket or pouch to help catch and hold insects captured in flight. The calcar helps spread the uropatagium during flight, managing its camber. For species of bats that forage via trawling, such as the greater bulldog bat and the fish-eating myotis, the calcar is used to prevent the uropatagium from dragging along the surface of the water. In the hairy-legged vampire bat, the calcar is not used for flight, but rather as a sixth digit to aid in tree-climbing. Evolutionary history The oldest known ancestor to present day bats, Icaronycteris index, apparently did not have a calcar or spur as evidenced by fossil remains. The oldest known bat with a calcar is Onychonycteris, which lived in the early Eocene. Onychonycteris had a long calcar, indicative of a broad tail membrane. In describing Onychonycteris in 2008, Simmons hypothesized that its calcar was more relevant to the mechanism of flight than capturing prey. Some authors have treated the calcar as a synapomorphy, or a unique trait shared by all bats, derived from a common ancestor. However, major differences in calcar structure between the two major clades of bat (formerly Megachiroptera and Microchiroptera, now Yinpterochiroptera and Yangochiroptera) have led other authors to challenge this designation. In a paper published in 1998, Schutt and Simmons advocated for different names for this structure for the two suborders, with the Yangochiroptera (microbats) retaining "calcar" and the Yinpterochiroptera (megabats) using "uropatagial spur." The calcar-like structure is not a synapomorphy, they argued, but rather a similar structure that evolved independently in each suborder. In 2000, Adams and Thibault published a book on bat ontogeny that supported this assertion, stating that various evidence "[supports] the hypothesis of independent origins for the microchiropteran calcar and the megachiropteran ‘uropatagial spur.’ Hence, the calcar, as currently understood, does not meet the assumptions of biological homology." Instead,