Uropatagium

The interfemoral membrane, or uropatagium, is the membrane that stretches between the legs of bats, pterosaurs and other animals like flying squirrels and colugos.

In bats, the uropatagium extends between the legs and tail, though in tail-less/short tailed species the uropatagium either connects both legs directly (such as in phyllostomids), or is reduced to a pair of membranes along each inner side of the leg (such as in flying foxes). The uropatagium is supported by the calcar, a rod of cartilage that helps expand this membrane. The uropatagium is primarily used in flight control, allowing the bat to steer to a high degree of efficiency in the air. Insectivorous species also use the uropatagium to capture insects, forming trapping pouches that hold their prey.

In pterosaurs, clear evidence of a similar membrane exists in most fossils that preserve soft tissues. However, the exact extent of this membrane in non-pterodactyloid pterosaurs isn't clear: some authorities consider that it encompassed the tail much like in most bats, while a Sordes specimen seems to indicate that it involved the hindlimbs to the exclusion of the tail. In the latter case the membrane is referred to as a cruropatagium. Unlike the other wing membranes, cr/uropatagia were fairly simple, generally lacking the specialised tissues like actinofibrils, so they're less well preserved. Meanwhile, in pterodactyloids, the membranes have clearly been reduced, and are instead an independent pair running along the inner side of each leg.

Pterosaur cr/uropatagia were supported by an elongated, opposing fifth toe, which took a similar role to that of the bat calcar. In non-pterodactyloid pterosaurs, the fifth toe is long and curved, supporting a well developed cr/uropatagium, while in pterodactyloids the fifth toe is very reduced if not outrightly lost, and the cr/uropatagia are instead reduced to a pair of membranes running along each leg. The large, unified cr/uropatagium of non-pterodactyloids seemingly offered a more stable flight, while the smaller, divided cr/uropatagia of pterodactyloids allowed a more acrobatic flight.

Several flying mammals like colugos and flying squirrels have uropatagia, usually similar to the arrangement in most bats, connecting the hindlegs to the tail.

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