"There are several ways and extents in which the tarsometatarsal fusion is accomplished throughout bird evolution. Namely, in Neornithes (modern birds) the fusion is most thorough at the distal (metatarsal) end though present along the entire length of the bone. In the Enantiornithes, a group of Mesozoic birds, the fusion was complete at the proximal (tarsal) end but the distal metatarsi still were somewhat separate."
"The third long bone in the [modern bird] leg is the tarsometatarsus. It is unique to birds and consists of the fused remnants of the metatarsal bones (long bones of the foot) and some small anklebones or tarsals. It is thought [incorrectly] to have evolved from comparable bones in dinosaurs that also walked on their toes. In many birds, the tarsometatarsus is the only bone that extends beyond the skirt of feathers. Like other parts of the avian foot, it has no muscles of its own but includes grooves and other structures to guide long tendons that stretch between the toes and the thigh muscles."