Abstract
Among fossil primates, the Eocene adapiforms have been suggested as the closest relatives of living anthropoids (monkeys, apes, and humans). Central to this argument is the form of the second pedal digit. Extant strepsirrhines and tarsiers possess a grooming claw on this digit, while most anthropoids have a nail. While controversial, the possible presence of a nail in certain European adapiforms has been considered evidence for anthropoid affinities. Skeletons preserved well enough to test this idea have been lacking for North American adapiforms. Here, we document and quantitatively analyze, for the first time, a dentally associated skeleton of Notharctus tenebrosus from the early Eocene of Wyoming that preserves the complete bones of digit II in semi-articulation. Utilizing twelve shape variables, we compare the distal phalanges of Notharctus tenebrosus to those of extant primates that bear nails (n = 21), tegulae (n = 4), and grooming claws (n = 10), and those of non-primates that bear claws (n = 7). Quantitative analyses demonstrate that Notharctus tenebrosus possessed a grooming claw with a surprisingly well-developed apical tuft on its second pedal digit. The presence of a wide apical tuft on the pedal digit II of Notharctus tenebrosus may reflect intermediate morphology between a typical grooming claw and a nail, which is consistent with the recent hypothesis that loss of a grooming claw occurred in a clade containing adapiforms (e.g. Darwinius masillae) and anthropoids. However, a cladistic analysis including newly documented morphologies and thorough representation of characters acknowledged to have states constituting strepsirrhine, haplorhine, and anthropoid synapomorphies groups Notharctus tenebrosus and Darwinius masillae with extant strepsirrhines rather than haplorhines suggesting that the form of pedal digit II reflects substantial homoplasy during the course of early primate evolution.
Highlights
The oldest fossil euprimates are known from the Late Paleocene-Eocene of Africa and the Eocene of North America, Europe, and Asia
Using a diagnosis based on functional attributes; its morphology indicates the presence of a grooming claw
Notharctus grooming phalanx morphology might suggest that this implied transformational sequence is correct, and that Notharctus represents a transitional step towards the anthropoid condition
Summary
The oldest fossil euprimates are known from the Late Paleocene-Eocene of Africa and the Eocene of North America, Europe, and Asia. Adapiforms are a highly diverse group variously classified in a number of subfamilies: Notharctinae, Cercamoniinae, Caenopithecinae, Djebelemurinae, Asiadapinae, Adapinae, and Sivaladapinae. Notharctines are known exclusively from North America and include for example Cantius, Notharctus, and Smilodectes. Cercamoniinae has traditionally referred to many genera from Europe, North America, Asia, and Africa including Europolemur, Donrussellia, Cercamonius, Caenopithecus, Mahgarita, Djebelemur, Aframonius, and Anchomomys among others [1]. More recent analyses have suggested the separation of some into their own subfamilies: Caenopithecus, Aframonius, and Mahgarita, along with Afradapis, are considered caenopithecines by some [2,3]. Two other groups of adapiforms are recognized, the European and Asian Adapinae and the late surviving Sivaladapinae from Asia
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