Evolution of the hominoid scapula and its implications for earliest hominid locomotion

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Spear and Williams (2018) are incorrect in stating that we (Selby & Lovejoy, 2017) argued that "a mediolaterally narrow scapula is unrelated to suspensory locomotion and therefore. . .early crown hominoids could not have been suspensory." We clearly stated that scapular breadth is related to suspension, but is not a primary determinant of suspension. We posited instead that scapular narrowing was a fundamental change in the bauplan of primates that abandoned strict above branch quadrupedality in favor of more diverse arboreal locomotor postures, especially above and below branch palmigrade clambering.And, of course, early crown hominoids could have been suspensory; the point of import is, however, that they were most likely not. As we have argued elsewhere, the morphology of the wrist, elbow, foot, hip, and thigh, as well as the limb proportions of Ardipithecus, all point to a included long bone lengths (approximating Spear and Williams' GMusing femoral head, proximal tibia breadth, anteroposterior and proximodistal humeral head breadths, distal humeral articular surface breadth, lengths of the femur, tibia, humerus, and radius, SWGM) and one that did not (i.e., only joint dimensions of the same bones) to compare to body mass. When each was log transformed for linearity, the joint surface GM better approximated body mass (r 5 0.986 vs. 0.978, both p < 0.05). Not surprisingly, there is a similar pattern for mediolateral breadth and spine angle when either size correction method is used (compare Figure 1a, b).Spear and Williams also point to a significant correlation between size-corrected mediolateral breadth and spine angle in their atelid sample, suggesting that both characters serve as adaptations for suspension. We found a similar correlation between these two variables in both ceboids (r 5 20.844, p < 0.05) and cercopithecoids (r 5 20.652, p < 0.05), but not in hominoids (r 5 0.134, p > 0.05). Such a lack of association between blade shape and spine orientation in hominoids (see also Green, Spiewak, Seitelman, & Gunz, 2016) suggests that mediolateral narrowing and spine angulation evolved independently, as we argued previously.We tested this lack of association further by employing a second, but equally reasonable vicar for scapular breadth: the transect from the scapula's superior angle to its glenoid (Figure 1c, 1d). Note that FIG URE 1 Mediolateral blade breadth and spine angulation in anthropoids. Spine angle (SPINANG) is compared to (a) scapular breadth from spine at vertebral border to glenoid (SPGLN) normalized by a geometric mean (GM) of fore-and hind-limb joint dimensions (see Selby and Lovejoy 2017) and by (b) a GM approximating that used by Spear and Williams (SWGM); (c) superior border (SUPBOR) length normalized by GM, and (d) by vertebral border (VERTBOR). Regression lines and R 2 values are provided in a and b for each superfamily. Scapular breadth does not discriminate hominoids based on either metric or method of normalization 2 | SELBY AND LOVEJOY 198 SELBY AND LOVEJOY normalization of this ...

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Spear and Williams (2018) are incorrect in stating that we (Selby & Lovejoy, 2017) argued that "a mediolaterally narrow scapula is unrelated to suspensory locomotion and therefore. .

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confidence: 88%

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confidence: 99%

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confidence: 99%

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confidence: 99%

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Scapular breadth does not discriminate suspension from clambering in hominoids: A response to Spear and Williams

Selby

Lovejoy

2018

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confidence: 99%

Scapular breadth is associated with forelimb‐dominated suspensory behavior in Atelidae: Comments on Selby and Lovejoy (2017)

Spear

Williams

2018

Scapular breadth is associated with forelimb-dominated suspensory behavior in Atelidae: Comments on Selby and Lovejoy (2017) Recently, Selby and Lovejoy (2017) argued that most apparent synapomorphies in the hominoid scapula are not independent adaptive traits but rather secondary developmental characters driven by a few underlying adaptive changes. According to their argument, chief among these adaptive changes is the mediolateral narrowing of the hominoid scapula. This interpretation is consistent with recent work suggesting that scapular blade shape is the most phylogenetically conserved aspect of the hominoid scapula (Young, Capellini, Roach, & Alemseged, 2015). Selby and Lovejoy (2017) then assert, contrary to previously published work (e.g., Hunt, 1991;Green, 2013), that a mediolaterally narrow scapula is unrelated to suspensory locomotion and therefore that early crown hominoids could not have been suspensory and were instead clambering arboreal quadrupeds. This conclusion is based on their assertion that among extant border height, while both are narrower than either Lagothrix or Cebus (P < .0001). When the ratio of scapular border size relative to geometric mean is calculated, this relationship disappears. Instead, Ateles has the narrowest scapula, followed by Lagothrix, then Alouatta, and finally Cebus (Figure 1B; P < .002 for all pairwise comparisons). Based on published descriptions of atelid positional repertoires (Cant, Youlatos, and Rose, 2001;Defler, 1999;Fontaine 1990;Gebo, 1992), the taxa with the narrowest scapulae are also the most suspensory ( Figure 1B). This pattern holds whether only suspensory locomotion (P < .0001, R 2 5 .81), only suspensory positional behavior (P < .0001, R 2 5 .80), or the full positional repertoire (P < .0001, R 2 5 .82) of each genus is considered.The discrepancy between our results and those reported by Selby and Lovejoy (2017) is driven by the relatively tall vertebral border of Alouatta ( Figure 1C). Further, the relationship we observed between scapular narrowness and suspension is not explained by allometry or other factors potentially associated with suspension, such as the angle of the vertebral spine or long arms. Selby and Lovejoy (2017) found a slightly positively allometric trend in the vertebral border of anthropoids, but the smallest primate analyzed here, Cebus, has the second largest relative vertebral border height while the largest primate, Ateles, Am J Phys Anthropol. 2018;1-3. wileyonlinelibrary.com/journal/ajpa V C 2018 Wiley Periodicals, Inc. | 1

An anatomical and mechanical analysis of the douc monkey (genus Pygathrix), and its role in understanding the evolution of brachiation

Byron

Granatosky

Covert

2017

The adoption of arm-swinging represents a major shift within at least three anthropoid clades and little data exist about its transition. Across species, few mechanical differences are observed during arm-swinging. Apparently, there are limited functional solutions to the challenges associated with moving bimanually below branches, especially in more proximal forelimb regions. Morphological data support this idea that the Pygathrix distal forelimb differs from apes more than its proximal end. These results can inform other studies of ape evolution, the pronograde to orthograde transition, and the convergent ways in which suspensory locomotion evolved in primates.