Elsevier

Quaternary International

Volume 481, 10 July 2018, Pages 91-100
Quaternary International

A 3D form comparative analysis of the Neandertal glenoid fossa in the context of the genus Homo

https://doi.org/10.1016/j.quaint.2017.07.031Get rights and content

Abstract

Morphological differences between the Neandertal and modern human glenoid fossa are mostly attributed to functional factors. However, the intimate relationship between the pectoral girdle and the shape of the thorax on which it rests calls for a structural analysis of the morphology of the glenoid fossa. Using both 3D Geometric Morphometric techniques and classical osteometry we carry out an analysis of the morphology of Neandertal scapular glenoid fossa, considering the effects that independent parameters such as allometry, sexual dimorphism and laterality could exert on the glenoid fossa shape. We also discuss how the morphological configuration of the thorax together with other functional factors could explain some of the differences between Neandertals and H. sapiens, and we assess the morphological evolution of this structure in the context of the genus Homo. To this end, 3DGM methods were applied in a comparative framework including the new SD-2101 + specimen from the Sidrón site (Spain) dated to 49,000 years ago, other Neandertal, Homo sapiens, KNM-WT 15000 (Homo ergaster/erectus) and Pan troglodytes specimens. A total of 75 sliding semilandmarks were used to collect the morphology of the glenoid fossa. Morphological comparison and variability were assessed through principal component analysis. Our results confirm the metric and morphological features of the Neandertal scapular glenoid fossa and point to a multifactorial effect in the glenoid morphology. We also detect a morphological trait of the Neandertal glenoid fossa that has not been reported previously: the projection of the upper part of the ventral border shared with modern humans and influenced by structural factors related to the configuration of the rib cage and shoulder. These results are consistent with the evolutionary framework of the hominin glenoid fossa, with some morphological changes being caused by functional factors, and others by structural factors such as the configuration of the thorax.

Introduction

The pectoral girdle is an important source of information about the body plan of fossil specimens, as well as being involvement in different behavioural aspects. This is particularly true in the case of human fossils, which is why this structural complex holds such interest in Paleoanthropology. The pectoral girdle has an intimate relationship with the shape of the thorax on which it rests; the individual parts of the shoulder girdle, on the other hand, could be affected by different factors associated with the participation of the shoulder in the use of tools or in hunting strategies (for example, the introduction of high-speed weapon technology) (Young et al., 2015).

In this context, several differences have been detected in the shape of the scapular girdle, and more specifically in scapular glenoid fossae (SGF) of the human species. The analysis of evolutionary and intrapopulation variation of the SGF of Neandertals and modern human populations might clarify the possible influence of structural or functional factors in the configuration of the phenotype in the different species of the genus Homo.

Neandertal SGF tend to be antero-posteriorly narrow and shallow and supero-inferiorly elongated compared to those of Homo sapiens, which are more dorsoventrally expanded and concave (Vallois, 1928-46, Stewart, 1962, Smith, 1976, Trinkaus, 1983, Churchill and Trinkaus, 1990, Trinkaus, 2006). These differences have traditionally been associated with functionality, i.e. the habitual use of specific subsistence techniques: Neandertals could have developed close range hunting whereas anatomically modern humans (AMHs) could have used throwing-based projectiles (Trinkaus, 1977, Churchill and Trinkaus, 1990, Churchill and Rhodes, 2009, Rhodes and Churchill, 2009). However, Di Vicenzo et al. (2012) suggested that the main morphological difference between Neandertals and H. sapiens could be a particular case within the general evolutionary-developmental growth pattern, because SGF shape exhibits a temporal tendency from the plesiomorphic configuration (narrow and craniocaudally elongated SGF) shown in Australopithecus and archaic Homo to the opposite condition exhibited in modern humans (see also Trinkaus, 2006). In this context, the functional hypothesis as result of the differences in subsistence strategies between Neandertals and modern humans is unlikely to be the main factor affecting the transformations observed in the morphological evolution of the human scapular glenoid fossae (Di Vincenzo et al., 2012).

However, factors related to functionality, such as the variation in load and contraction forces of specific muscles and ligaments, would act in specific ways and probably cause minor changes in the morphology of the SGF, such as the expansion of the coracoid portion of the SGF or the degree of development of the glenoid notch. Macias and Churchill (2015) proposed that functional adaptations to habitual behaviour could explain the morphological configuration of the SGF and the differences detected between Neandertals and H. sapiens. Nonetheless, they noted the importance of evaluating other factors such as allometry, laterality or sexual dimorphism before functional morphology could be addressed.

The morphological variation of the SGF in Neandertals associated with changes in adjacent structures, such as the rib cage, has traditionally received little attention in research. The Neandertal thorax is characterized by volumetric expansion, which is probably caused by a medio-lateral expansion of the central and caudal parts (Franciscus and Churchill, 2002, Gómez-Olivencia et al., 2009, García-Martínez et al., 2014, Bastir et al., 2015, García-Martínez et al., 2017) compared to modern humans. Even though this morphology has been hypothesized to be adapted to factors such as cold climate or high energetic expenditure, the morphological adaptations of the rib cage could also alter the spatial configuration of the pectoral girdle due to the hypothetic anatomical integration between both structures (Churchill, 1994a, Churchill, 1994b, Rosas et al., 2016). The medio-laterally expanded Neandertal thorax could induce a lateralized spatial position of the scapulae, and a more anteriorly oriented SGF (Churchill, 1994a, Churchill, 1994b, Vandermeersch and Trinkaus, 1995). Other factors, such sexual dimorphism, cause morphological variation in the rib cage and could also cause modifications in the shoulder configuration. Specifically, in the thoracic configuration of H. sapiens, previous researchers have observed that the male thorax is larger and medio-laterally wider than the female thorax, probably because of differences in body composition and energy consumption (Bellemare et al., 2003a, Bellemare et al., 2003b, Bellemare et al., 2003c, Bellemare et al., 2003d, Shi et al., 2014, Weaver et al., 2014, García-Martínez et al., 2016a). Therefore, the spatial configuration of the scapula and the morphology of the SGF could be affected by the aforementioned anatomical integration of the shoulder and the rib cage. As a result, the thoracic configuration should be considered as a structural factor affecting the SGF morphology, and should be evaluated when studying Neandertal-H. sapiens differentiation.

The unearthing of a complete and well-preserved Neandertal scapular glenoid fossa from the El Sidrón site (∼49 kya) in Northern Spain (Rosas et al., 2006, Rosas et al., 2012) has encouraged us to carry out an analysis of the morphology of Neandertal scapular glenoid fossa using both 3D Geometric Morphometric (3DGM) techniques and classical osteometry. In this paper we study the morphological comparison of the Neandertal-H. sapiens SGF by analysing the effects that allometry, sexual dimorphism and laterality could exert on the glenoid fossa shape, and the overall evolution of the pectoral girdle morphology within the genus Homo.

In particular, we will test where the new specimen SD-2101+SD-2030b (from now on it will be called SD-2101+, pictured in Fig. 1) lies within the Neandertal variation range, and also how 3DGM contributes to defining the variability range for the SGF. We will also analyse the shape differences between Neandertals and H. sapiens by quantifying the influence of allometry, sexual dimorphism and laterality. In addition, we will discuss how the morphological configuration of the thorax could explain some of the differences observed in the SGF of Neandertals and H. sapiens in combination with the possible functional factors.

Section snippets

The El Sidrón site

The El Sidrón cave is located within a karstic system in Asturias, Northern Spain (Fortea et al., 2003, Rosas et al., 2006, Rasilla et al., 2011). More than 2550 Neandertal remains have been retrieved from a single deposit there. Direct dating of Neandertal teeth and bone has confirmed the age of the site to be approximately 49kya (Wood et al., 2013), and it represents the most complete Neandertal sample of the Iberian Peninsula (Rosas et al., 2006, Rosas et al., 2012, Rosas et al., 2013, Rosas

Description of El Sidrón SD-2101+SD-2030b scapula

SD-2101+SD-2030b (SD-2101+) is a complete SGF and part of the axillary border of an adolescent right scapula (Table 1 and Fig. 1) with a maximum length of 113.8 mm (from the coracoid process to the end of the preserved axillary border). The maximum SGF height is 35.8 mm and the maximum SGF breadth is 22.7 mm (see Table 3). This SGF is laterally oriented with an axilloglenoid angle of 138° for SD-2101+ (Table 3) and continues inferiorly with part of an axillary border where a well-developed

Discussion

This study shows that the SD-2101 + glenoid fossa of the El Sidrón site is within the known parameters of morphological variability for H. neanderthalensis (Table 1, Table 3), which previous studies (Vallois, 1928-46, Stewart, 1962, Trinkaus, 1983, Churchill and Trinkaus, 1990) have defined as high and narrow (low Glenoid Index), with a shallow and lateral-oriented SGF. Allometry, laterality and sexual dimorphism have been analyzed as potential independent parameters that can act on the

Summary and conclusions

In light of the newly presented fossil remain SD-2101 + from the El Sidrón site, this study confirms the metric and morphological features of the Neandertal scapular glenoid fossa, which fall into the variability range known for Neandertals (Vallois, 1928-46, McCown and Keith, 1939, Trinkaus, 1983, Churchill and Trinkaus, 1990, Trinkaus, 2006). Our analyses of allometry indicate that there is no influence of size on SGF shape both at the inter- and intra-specific levels (sexual dimorphism and

Acknowledgments

We are grateful to everyone who participated in the El Sidrón fieldwork seasons and to other members of the Group of Paleoanthropology MNCN-CSIC. We are also grateful to the Digital Morphology Museum, KUPRI, Kyoto. Finally, we are very thankful to Davorka Radovčić for allowing us to study the formidable collection of Krapina. This work was funded by the Ministerio de Economía y Competitividad of Spain (CGL2012-36682 and CGL2012-37279).

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