Quantification of the position and depth of the flexor hallucis longus groove in euarchontans, with implications for the evolution of primate positional behavior

Effects of binocular cue availability on leaping performance in Cheirogaleus medius: implications for primate origins

Multiple competing hypotheses attribute the evolution of the suite of traits that distinguish primates from their closest relatives, including forward-facing eyes, which create a wide field of binocular vision, to specific behavioral and ecological factors. The grasp-leaping hypothesis suggests that the evolution of these traits in basal primates was driven by the demands of a form of leaping locomotion unique to primates. Whether the grasp-leaping hypothesis provides a viable mechanism for the evolution of primates' forward-facing eyes remains untested. To determine whether grasp-leaping locomotion may have contributed to driving the evolution of primates' forward-facing eyes, the importance of vision within the binocular field for this type of leaping was evaluated experimentally in Cheirogaleus medius, one of the cheirogaleid primate species considered reasonable living analogs of the earliest primates. Availability of binocular visual cues was experimentally restricted using a head-mounted blinder that narrowed the binocular visual field without altering the total visual field. Animals altered their launch behavior, reduced their horizontal leap speed, and were significantly more likely to select paths that offered the shortest available leaps when their binocular field was restricted. Restriction of binocular cue availability also significantly increased the probability of adverse landings even when statistically controlling for potentially confounding variables such as leap distance, horizontal leap speed, learning effects, etc. These results suggest a functional mechanism by which selection for improved grasp-leaping could also have contributed to the evolution of forward-facing eyes in the earliest crown primates.

Early euprimates already had a diverse locomotor repertoire: Evidence from ankle bone morphology

The morphological adaptations of euprimates have been linked to their origin and early evolution in an arboreal environment. However, the ancestral and early locomotor repertoire of this group remains contentious. Although some tarsal bones like the astragalus and the calcaneus have been thoroughly studied, the navicular remains poorly studied despite its potential implications for foot mobility. Here, we evaluate early euprimate locomotion by assessing the shape of the navicular-an important component of the midtarsal region of the foot-using three-dimensional geometric morphometrics in relation to quantified locomotor repertoire in a wide data set of extant primates. We also reconstruct the locomotor repertoire of representatives of the major early primate lineages with a novel phylogenetically informed discriminant analysis and characterize the changes that occurred in the navicular during the archaic primate-euprimate transition. To do so, we included in our study an extensive sample of naviculars (36 specimens) belonging to different species of adapiforms, omomyiforms, and plesiadapiforms. Our results indicate that navicular shape embeds a strong functional signal, allowing us to infer the type of locomotion of extinct primates. We demonstrate that early euprimates displayed a diverse locomotor behavior, although they did not reach the level of specialization of some living forms. Finally, we show that the navicular bone experienced substantial reorganization throughout the archaic primate-euprimate transition, supporting the major functional role of the tarsus during early primate evolution. This study demonstrates that navicular shape can be used as a reliable proxy for primate locomotor behavior. In addition, it sheds light on the diverse locomotor behavior of early primates as well as on the archaic primate-euprimate transition, which involved profound morphological changes within the tarsus, including the navicular bone.

Morphological variation of the Pan talus relative to that of Gorilla

OBJECTIVES

Differences in talar articular morphology relative to locomotion have recently been found within Pan and Gorilla. Whole-bone talar morphology within, and shared variation among, Pan and Gorilla (sub)species, however, has yet to be investigated. Here we separately analyze talar external shape within Pan (P. t. troglodytes, P. t. schweinfurthii, P. t. verus, P. paniscus) and Gorilla (G. g. gorilla, G. b. beringei, G. b. graueri) relative to degree of arboreality and body size. Pan and Gorilla are additionally analyzed together to determine if consistent shape differences exist within the genera.

MATERIALS AND METHODS

Talar external shape was quantified using a weighted spherical harmonic analysis. Shape variation both within and among Pan and Gorilla was described using principal component analyses. Root mean square distances were calculated between taxon averages, and resampling statistics conducted to test for pairwise differences.

RESULTS

P. t. verus (most arboreal Pan) talar shape significantly differs from other Pan taxa (p < 0.05 for pairwise comparisons) driven by more asymmetrical trochlear rims and a medially-set talar head. P. t. troglodytes, P. t. schweinfurthii, and P. paniscus do not significantly differ (p > 0.05 for pairwise comparisons). All gorilla taxa exhibit significantly different talar morphologies (p < 0.007 for pairwise comparisons). The more terrestrial subspecies of G. beringei and P. troglodytes exhibit a superoinferiorly taller talar head/neck complex.

DISCUSSION

P. t. verus exhibits talar morphologies that have been previously related to more frequent arboreality. The adaptations in the more terrestrial G. beringei and P. troglodytes subspecies may serve to facilitate load transmission.

Using BayesModelS to provide Bayesian- and phylogenetically-informed primate body mass predictions

An accurate prediction of the body mass of an extinct species can greatly inform the reconstruction of that species' ecology. Therefore, paleontologists frequently predict the body mass of extinct taxa from fossilized materials, particularly dental dimensions. Body mass prediction has traditionally been performed in a frequentist statistical framework, and accounting for phylogenetic relationships while calibrating prediction models has only recently become more commonplace. In this article, we apply BayesModelS-a phylogenetically informed Bayesian prediction method-to predict body mass in a sample of 49 euarchontan species (24 strepsirrhines, 20 platyrrhines, 3 tarsiids, 1 dermopteran, and 1 scandentian) and compare this approach's body mass prediction accuracy with other commonly used techniques, namely ordinary least squares, phylogenetic generalized least squares, and phylogenetic independent contrasts (PICs). When predicting the body masses of extant euarchontans from dental and postcranial variables, BayesModelS and PICs have substantially higher predictive accuracy than ordinary least squares and phylogenetic generalized least squares. The improved performances of BayesModelS and PIC are most evident for dentally derived body mass proxies or when body mass proxies have high degrees of phylogenetic covariance. Predicted values generated by BayesModelS and PIC methods also show less variance across body mass proxies when applied to the Eocene adapiform Notharctus tenebrosus. These more explicitly phylogenetically based methods should prove useful for predicting body mass in a paleontological context, and we provide executive scripts for both BayesModelS and PIC to increase ease of application.

Vertical support use and primate origins

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.

My branch is your branch: Talar morphology correlates with relative substrate size in platyrrhines at Tiputini Biodiversity Station, Ecuador

Given that most species of primates are predominantly arboreal, maintaining the ability to move among branches of varying sizes has presumably been a common selective force in primate evolution. However, empirical evaluations of the relationships between morphological variation and characteristics of substrate geometry, such as substrate diameter relative to an animal's body mass, have been limited by the lack of quantified substrate usage in the wild. Here we use recently published quantitative data to assess the relationships between relative substrate size and talar morphology in nine New World monkey species at the Tiputini Biodiversity Station, Ecuador. Within this sample, both fibular facet angle (the angle between the fibular facet and the trochlear rims) and body-mass-standardized area of the medial tibial facet decrease as average and maximum relative substrate size increases. Correlations between medial tibial facet area and relative substrate size are driven by the inclusion of callitrichids in this sample. Nevertheless, these findings strengthen the hypothesis that variation in fibular facet orientation and medial tibial facet area are functionally correlated with habitual degrees of pedal inversion. They also strengthen the notion that evolutionarily changing body mass could impact habitat geometry experienced by a lineage and thereby substantially impact major trends in primate morphological evolution. This study highlights the importance of empirical data on substrate use in living primates for inferring functional and evolutionary implications of morphological variation.

Development and Assessment of Fully Automated and Globally Transitive Geometric Morphometric Methods, With Application to a Biological Comparative Dataset With High Interspecific Variation

Automated geometric morphometric methods are promising tools for shape analysis in comparative biology, improving researchers' abilities to quantify variation extensively (by permitting more specimens to be analyzed) and intensively (by characterizing shapes with greater fidelity). Although use of these methods has increased, published automated methods have some notable limitations: pairwise correspondences are frequently inaccurate and pairwise mappings are not globally consistent (i.e., they lack transitivity across the full sample). Here, we reassess the accuracy of published automated methods-cPDist (Boyer et al. Proc Nat Acad Sci 108 () 18221-18226) and auto3Dgm (Boyer et al.: Anat Rec 298 () 249-276)-and evaluate several modifications to these methods. We show that a substantial percentage of alignments and pairwise maps between specimens of dissimilar geometries were inaccurate in the study of Boyer et al. (Proc Nat Acad Sci 108 () 18221-18226), despite a taxonomically partitioned variance structure of continuous Procrustes distances. We show these inaccuracies are remedied using a globally informed methodology within a collection of shapes, rather than relying on pairwise comparisons (c.f. Boyer et al.: Anat Rec 298 () 249-276). Unfortunately, while global information generally enhances maps between dissimilar objects, it can degrade the quality of correspondences between similar objects due to the accumulation of numerical error. We explore a number of approaches to mitigate this degradation, quantify their performance, and compare the generated pairwise maps (and the shape space characterized by these maps) to a "ground truth" obtained from landmarks manually collected by geometric morphometricians. Novel methods both improve the quality of the pairwise correspondences relative to cPDist and achieve a taxonomic distinctiveness comparable to auto3Dgm. Anat Rec, 301:636-658, 2018. © 2017 Wiley Periodicals, Inc.

Postcrania of the most primitive euprimate and implications for primate origins

The fossil record of early primates is largely comprised of dentitions. While teeth can indicate phylogenetic relationships and dietary preferences, they say little about hypotheses pertaining to the positional behavior or substrate preference of the ancestral crown primate. Here we report the discovery of a talus bone of the dentally primitive fossil euprimate Donrussellia provincialis. Our comparisons and analyses indicate that this talus is more primitive than that of other euprimates. It lacks features exclusive to strepsirrhines, like a large medial tibial facet and a sloping fibular facet. It also lacks the medially positioned flexor-fibularis groove of extant haplorhines. In these respects, the talus of D. provincialis comes surprisingly close to that of the pen-tailed treeshrew, Ptilocercus lowii, and extinct plesiadapiforms for which tali are known. However, it differs from P. lowii and is more like other early euprimates in exhibiting an expanded posterior trochlear shelf and deep talar body. In overall form, the bone approximates more leaping reliant euprimates. The phylogenetically basal signal from the new fossil is confirmed with cladistic analyses of two different character matrices, which place D. provincialis as the most basal strepsirrhine when the new tarsal data are included. Interpreting our results in the context of other recent discoveries, we conclude that the lineage leading to the ancestral euprimate had already become somewhat leaping specialized, while certain specializations for the small branch niche came after crown primates began to radiate.