• astragalus;
  • calcaneum;
  • Eocene;
  • England;
  • hindfoot reversal

Astragali and calcanea from the English late Eocene, attributed to the extinct ‘insectivoran’ family Nyctitheriidae, are described for the first time. They contrast with those of the strict sense insectivorans, the Lipotyphla, in which order nyctitheres have usually been placed, and compare more closely with those of Scandentia (tree shrews) and the extinct Plesiadapiformes. Functional analysis demonstrates that inversion of the foot was possible between the astragalus and calcaneum of nyctitheres, allowing them to be interpreted as having had a tree-dwelling, probably scansorial, mode of life. These tarsal bones are compared with those of other placental mammals. Cladistic analysis of tarsal characters places nyctitheres as sister group to the Plesiadapiformes within the superorder Archonta, excluding Chiroptera (bats). An independent analysis of dental characters places them as sister group to the rest of the Archonta, but still excluding bats. Combining the dental and tarsal characters places nyctitheres as sister group to Plesiadapiformes and all modern groups of archontans except bats. A new osteological synapomorphy is proposed for the Archonta, which is thus considered to comprise Chiroptera, Deccanolestes, Nyctitheriidae, Plesiadapiformes, Dermoptera (including Mixodectidae), Scandentia and Primates. Insectivorans s.l. have long been at the centre of arguments on placental origins, although lipotyphlans are usually regarded as a monophyletic group, rather than paraphyletic stem placentals. Reidentification of an extinct lipotyphlan family as having archontan relationships raises the possibility of advances in other areas of insectivoran phylogeny when more postcranial elements become known. The early Oligocene extinction of nyctitheres may be causally related to the rise of insectivorous microchiropteran bats, which, because of their flying ability, would have been able to forage more widely.