New postcranial bones of the extinct mammalian family Nyctitheriidae (Paleogene, UK): Primitive euarchontans with scansorial locomotion

Abstract

New postcranial bones from the Late Eocene and earliest Oligocene of the Hampshire Basin, UK, are identified as belonging to the extinct family Nyctitheriidae. Previously, astragali and calcanea were the only elements apart from teeth and jaws to be unequivocally recognized. Now, humeri, radii, a metacarpal, femora, distal tibiae, naviculars, cuboids, an ectocuneiform, metatarsals and phalanges, together with additional astragali and calcanea, have been collected. These are shown to belong to a diversity of nyctithere taxa previously named from dental remains. Functional analysis shows that nyctitheres had mobile shoulder and hip joints, could pronate and supinate the radius, partially invert the foot at the astragalocalcaneal and upper ankle joints using powerful flexor muscles, all indicative of a scansorial lifestyle and allowing headfirst descent on vertical surfaces. Climbing appears to have been dominated by flexion of the forearms and feet. Cladistic analysis employing a range of primitive eutherian mammals shows that nyctitheres are stem euarchontans, rather than lipotyphlans, with which they had previously been classified based on dental characters. Earlier ideas of relationships with the extinct Adapisoriculidae, recently considered stem eutherians, are not upheld.