Vertical support use and primate origins

Gabriel S Yapuncich,Doug M Boyer,Henry J Feng,Rachel H Dunn,Erik R Seiffert

doi:10.1038/s41598-019-48651-x

Gabriel S Yapuncich, Doug M Boyer + Show 3 more

Open Access

https://doi.org/10.1038/s41598-019-48651-x

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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.

Highlights

Adaptive scenarios of crown primate origins remain contentious due to uncertain order of acquisition and functional significance of the clade’s diagnostic traits
The talus is an important element for reconstructing positional behavior throughout primate evolution because the bone’s morphology correlates well with locomotor and postural behaviors of living euarchontans and it is frequently preserved in fossil assemblages[1,2,3,4]
Our finding that the highest posterior trochlear shelf (PTS) indices are observed in euarchontan species that habitually grasp vertical supports provides evidence for the hypothesis that the PTS can function as a cam mechanism when hypertrophied

Summary

Introduction

Adaptive scenarios of crown primate origins remain contentious due to uncertain order of acquisition and functional significance of the clade’s diagnostic traits. If the flexor fibularis can stretch the additional distance necessitated by a hypertrophied PTS, the cam mechanism could shift the muscle to a more efficient position along its length-tension curve. Four extant primate families (Indriidae, Lemuridae, Galagidae, and Tarsiidae), as well as a non-natural group of the remaining lemuriform taxa (Cheirogaleidae, Lepilemur, and Daubentonia) have mean PTS indices significantly greater than 1 (Table S8).

Results

Conclusion