Locomotor reconstruction of Leptictidium from the Eocene of Messel using bone structure patterns in the postcranial skeleton of running and hopping extant mammals

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The locomotion of the extinct Leptictidium (order Leptictida), a small fossil mammal genus from the Messel Pit (Eocene, Germany) has been debated since its discovery in 1962. To date, it has been alternately suggested that Leptictidium was a bipedal hopper or runner, based on its external morphology only (App. 2). However, bone endostructures (e.g. cortical and trabecular components) also record functional specialisation related to locomotion and posture. The inner configuration and properties of the bone can be used as a reliable proxy to characterise locomotor behaviours of extinct taxa. Such inner signal remains to be explored in the appendicular elements of Leptictidium. Accordingly, the aim of this pilot study is to characterise the external and inner anatomical structures of postcranial skeletal elements in running and hopping modern mammals. The resulting comparative database will be the core of our research projects that will try to reconstruct the type of locomotion of leptictids like Leptictidium. For each studied extant taxon, the descriptive data on the external morphology of the limbs and girdles, will be associated with the inner bone structures of these elements (using μCT imaging techniques), and correlated with the known locomotor behaviour of the animal. This combination of external and internal analyses is an innovative approach for the study of the locomotion of fossil terrestrial Mammals.