Vertical support use and primate origins

Abstract

Adaptive scenarios of crown primate origins remain contentious due to uncertain order of acquisition and functional significance of the clade’s diagnostic traits. A feature of the talus bone in the ankle, known as the posterior trochlear shelf (PTS), is well-regarded as a derived crown primate trait, but its adaptive significance has been obscured by poorly understood function. Here we propose a novel biomechanical function for the PTS and model the talus as a cam mechanism. By surveying a large sample of primates and their closest relatives, we demonstrate that the PTS is most strongly developed in extant taxa that habitually grasp vertical supports with strongly dorsiflexed feet. Tali of the earliest fossils likely to represent crown primates exhibit more strongly developed PTS cam mechanisms than extant primates. As a cam, the PTS may increase grasping efficiency in dorsiflexed foot postures by increasing the path length of the flexor fibularis tendon, and thus improve the muscle’s ability to maintain flexed digits without increasing energetic demands. Comparisons are made to other passive digital flexion mechanisms suggested to exist in other vertebrates. These results provide robust anatomical evidence that the habitual vertical support use exerted a strong selective pressure during crown primate origins.

New records of a lost species and a geographic range expansion for sengis in the Horn of Africa

“…The method has been widely applied to both categorical and continuous data. A best practice used when modeling quantitative traits against species-level phylogenies is to code tree-tips with mean values derived from multiple intraspecific measurements ( Boyer et al, 2013 ; Yapuncich et al, 2019 ). A dedicated geographical module (which accommodates geodesic distances and associations around any directional horizon) uses a Brownian motion model to estimate ancestral positions in a three-dimensional Cartesian coordinates system.…”

The Central Role of Small Vertical Substrates for the Origin of Grasping in Early Primates

“…Furthermore, the terminal-branch environment is characterized by substrates of varying diameter and orientation with an overall high degree of spatial complexity, which significantly influences the locomotor behavior of an animal [14][15][16]. Recent studies have questioned the small-branch niche environment and proposed that prehensile foot proportions and nails on the lateral digits may well have been acquired after the radiation of crown primates, i.e., in parallel in different euprimate lineages, and that leaping specializations and large vertical substrate use evolved prior to nails on the lateral digits [17][18][19][20]. Understanding the relationships between hand and foot postures during climbing on vertical substrates is important, as many fundamental primate morphological features have been associated with climbing [17,18,21].…”

Locomotion of an adult female and juvenile male aye‐aye (Daubentonia madagascariensis) in Torotorofotsy, Madagascar

“…Some have argued that primate ancestors were likely not vertical clingers and leapers, but rather generalized quadrupeds capable of leaping, climbing, clinging, and hindlimb suspension (Cartmill, 1972;1974a, 1974bFleagle & Lieberman, 2015;Gebo, 2011;Godfrey, 1988;Sussman, 1991). Other studies on features of the talus (Boyer et al, 2017;Yapuncich et al, 2019) and manus skeletons of ancestral mammals (Boyer et al, 2013), along with hand and foot pressure analysis of postural behaviors in extant mammals, have shown that early primates may have evolved to use vertical supports more than horizontal, fine branches. However, this association is complicated by the strong correlation between clinging behaviors and leaping locomotion in extant primates (Yapuncich et al, 2019).…”

“…Other studies on features of the talus (Boyer et al, 2017;Yapuncich et al, 2019) and manus skeletons of ancestral mammals (Boyer et al, 2013), along with hand and foot pressure analysis of postural behaviors in extant mammals, have shown that early primates may have evolved to use vertical supports more than horizontal, fine branches. However, this association is complicated by the strong correlation between clinging behaviors and leaping locomotion in extant primates (Yapuncich et al, 2019). Aye-ayes' intermembral index clearly identifies them as quadrupedal (Fleagle, 2013;Glander, 1994), yet they are anatomically distinctly different from other strepsirrhines (Oxnard, 1981).…”