Locomotor Adaptations as Reflected on the Humerus of Paleogene Primates

Abstract

Examination of Paleogene distal humeri and a survey of homologous articulations in living primates allows some anatomical correlation with elbow mechanics and the occurrence of these during specific locomotor behaviors in living species. Claw climbing is postulated to be the ancestral primate locomotor mode from which the ancestral euprimate (strepsirhines and haplorhines) evolved a grasp leaping locomotor pattern. This pattern, which depends on a powerful grasp either during climbing or when landing after a jump, is still the most pervasive form of primate locomotion. Morphological evidence of Paleogene humeri suggest that vertical clinging and leaping behavior derived from grasp leaping at least six times independently.

New Paleocene skeletons and the relationship of plesiadapiforms to crown-clade primates

“…Acquisition of grasping hands and feet would have allowed easy access to terminal branches where such resources are typically found. The first euprimates have convergent orbits that are larger relative to skull length and more frontated than those of plesiadapiforms (5,42) as well as features indicative of specialized leaping (18). These differences from plesiadapiforms could indicate a shift to visually directed predation (5) and/or a change in locomotor mode to include grasp-leaping on small-diameter supports (18).…”

“…The first euprimates have convergent orbits that are larger relative to skull length and more frontated than those of plesiadapiforms (5,42) as well as features indicative of specialized leaping (18). These differences from plesiadapiforms could indicate a shift to visually directed predation (5) and/or a change in locomotor mode to include grasp-leaping on small-diameter supports (18). The former explanation is weakened by the inferred omnivorous or herbivorous habits of many early euprimates, even at small body size (43), and the concomitant lack of evidence for a clear transition to more effective predation and greater insectivory at the euprimate node.…”

“…The association between vertical claw-clinging and exudate-feeding is well established (12,27). Such a habitus has been postulated as the primitive plesiadapiform condition (18) and applies specifically to paromomyids.…”

“…All known plesiadapiforms have features suggesting that they were committed arborealists, adapted in part for locomotion on large-diameter tree trunks like extant claw-climbing callitrichine primates (18). These features include (i) a humerus with tuberosities that do not extend above the head, a spherical capitulum, and an extended entepicondyle, suggesting mobile shoulder and elbow joints, and the capacity for powerful flexion of the fingers during grasping; (ii) an innominate with a shallow, yet cranially buttressed, elliptical acetabulum, suggesting a mobile hip joint capable of a great range of abduction and lateral rotation during orthograde postures on vertical supports; (iii) a femur with a distally positioned and medially extended lesser trochanter, a posteroproximally extended articular surface of the head, and a shallow patellar groove, suggesting habitual flexion and abduction of the thighs, and infrequent full, forceful extension of the knee; (iv) an astragalus with an elliptical head, short neck, confluent sustentacular and navicular facets, and a shallowly grooved trochlea, suggesting lower ankle joint mobility for inversion of the foot and infrequent exposure to large sagittal loads (as experienced in cursorial locomotion); (v) a calcaneum with a short shaft distal to the ectal facet and a cuboid facet oriented perpendicular to the long axis of the tuberosity, suggesting mobility at the transverse tarsal joint and infrequent cursorial or leaping locomotion; and (vi) terminal phalanges II-V that are mediolaterally compressed and dorsoventrally high, suggesting use in claw-clinging and climbing on large-diameter, vertical supports in which high tensile, sagittal loads were experienced.…”

“…Well preserved crania have been documented for only three families: Plesiadapidae, Microsyopidae, and Paromomyidae (1, 9-11). Postcrania are known from a taxonomically limited sample of North American and European plesiadapids (1), from a sample of North American paromomyids and micromomyids (3, 4, 12) the identification and associations of which are still controversial (13,14), from a recently published North American carpolestid skeleton (15, 16), and from a few other isolated and questionably identified elements (7,17,18). Following the suggestion that paromomyid and micromomyid plesiadapiforms were mitten-gliders closely related to modern dermopterans (3, 4, 12), interpretations of locomotor modes from this limited postcranial sample have played a central role in evolutionary arguments about the group.…”