Adapid

Adapids (members of the taxon Adapidae) are a diverse group of extinct primates that primarily radiated during the Eocene epoch between about 55 and 34 million years ago. However, one specialized endemic Asian group (sivaladapines) survived into the Miocene. Fossils of adapids are known from North America, Europe, Asia, and Africa. Adapids are one of two groups of Eocene primates with a geographic distribution spanning holarctic continents, the other being the omomyids (Omomyidae). Early representatives of the Adapidae (e.g., Cantius and Donrussellia) and Omomyidae (e.g, Teilhardina and Melanerimia) are some of the earliest known crown primates.

Features that characterize many adapids include small orbits (eye sockets), elongate rostra, cheek teeth adapted for folivorous or frugivorous diets, and relatively large body mass (i.e., greater than 1 kg). However, the endemic radiation of adapids in the early and middle Eocene of Europe included a number of taxa (e.g., Anchomomys) that were very small (about 250 g or less) and partly insectivorous. Small orbits in genera such as Notharctus, Smilodectes, Adapis, Leptadapis, and Mahgarita indicate that these taxa were probably diurnal. At least one adapid genus from the late Eocene of Europe (Pronycticebus) had large orbits and was probably nocturnal.

Like living primates, adapids had grasping hands and feet with digits tipped by nails instead of claws. Other features of the skeleton suggest that most adapids lived in trees. North American endemic adapids (notharctines) like Notharctus had extremely long digits and skeletal proportions that superficially resemble those of living lemurs. In contrast, some authors have suggested that one group of late Eocene European adapids (adapines) had adaptations for suspension and slow climbing.

Adapid systematics and evolutionary relationships are controversial, but there is fairly good evidence from the postcranial skeleton (everything but the skull, or cranium) that adapids were stem strepsirrhines (members of the group including the living lemurs, lorises, and bushbabies). In particular, the anatomy of the adapid wrist and ankle (e.g., position of the groove for the flexor fibularis tendon on the talus, the presence of a sloping talo-fibular facet) show derived similarities with those of living strepsirrhines. However, ancient adapids lacked many of the anatomical specializations characteristic of living strepsirrhines, such as a tooth comb, a grooming claw on the second pedal digit, and a reduction in the size of the promontory branch of the internal carotid artery.