Postcranial skeleton of Eocene Leptictidae (Mammalia), and its implications for behavior and relationships

The postcranial skeleton of early and middle Eocene leptictids is described based on several specimens from Wyoming and Colorado. Leptictids had a strongly keeled manubrium sterni, relatively short, generalized forelimbs (intermembral index ~60), a moderately robust humerus, stout metacarpals, and ungual phalanges resembling those of fossorial mammals. The femur is slender, with a narrow, elevated patellar trochlea and a posteriorly-directed lesser trochanter. The slightly longer tibia is firmly synostosed with the fibula just distal to midshaft. The fused tibiofibula forms a stable ankle joint with the deep astragalar trochlea. The astragalar neck and the metatarsals are moderately elongate, and there is a pronounced peroneal process on Mt I, probably related to pedal eversion. These traits indicate that leptictids were terrestrial mammals that progressed by quadrupedal walking, running, and hopping (possibly bipedally at high speed), and that burrowed using their forelimbs. Cladistic analysis of postcranial traits groups leptictids with lagomorphs, zalambdalestids, and macroscelidids. Postcranial characters do not support a sister-group relationship between Leptictidae and Lipotyphla.