Leptictidium tobieni. Illustration and photo-reconstruction of a Leptictidium struggling through dry foliage in ancient Europe in search of food. Leptictidium is an extinct genus of small mammals - the only known completely bipedal mammals. It varied between 60 and 90 centimetres in length and 20 centimetres in height. It was omnivorous, its diet a combination of insects, lizards and small mammals. One of the first branches to split from basal eutherians, they appeared in the Lower Eocene, a time of warm temperatures and high humidity, roughly 50 mya. Although they were widespread throughout Europe, they became extinct around 35 mya with no descendants, probably because they were adapted to live in forest ecosystems and were unable to adapt to the open plains of the Oligocene.