Clinical Anatomy 28:593–601 (2015)

VIEWPOINT

Evaluating Sexual Dimorphism in the Human Mastoid Process: A Viewpoint on the Methodology ANJA PETAROS,1* SABRINA B. SHOLTS,2 MARIO SLAUS,3 ALAN BOSNAR,1 4,5,6 € € SEBASTIAN K.T.S. WARML ANDER 1

AND

Department of Forensic Medicine and Criminalistics, Rijeka University, School of Medicine, Rijeka, Croatia 2 Department of Anthropology, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia 3 Anthropological Center, Croatian Academy of Sciences and Arts, Zagreb, Croatia 4 Division of Biophysics, Arrhenius Laboratories, Stockholm University, Stockholm, Sweden 5 € ping University, Linko € ping, Sweden Division of Commercial and Business Law, IEI, Linko 6 The Cotsen Institute of Archaeology, UCLA/Getty Conservation Programme, University of California in Los Angeles, California

The mastoid process is one of the most sexually dimorphic features in the human skull, and is therefore often used to identify the sex of skeletons. Numerous techniques for assessing variation in the size and shape of the mastoid process have been proposed and implemented in osteological research, but its complex form still presents difficulties for consistent and effective analysis. In this article, we compare the different techniques and variables that have been used to define, measure, and visually score sexual dimorphism in the mastoid process. We argue that the current protocols fail to capture the full morphological range of this bony projection, and suggest ways of improving and standardizing them, regarding both traditional and 3D-based approaches. Clin. Anat. 28:593–601, 2015. VC 2015 Wiley Periodicals, Inc. Key words: forensic anthropology; mastoid process; osteology; sexual dimorphism; standardization

INTRODUCTION Identification of human remains is a fundamental task in both legal medicine and forensic anthropology. In the absence of soft tissue, skeletal material is usually analyzed to construct the individual’s biological profile, which includes information about genetic ancestry, sex, stature, age at death, disease history, and anything else that can contribute to identification (Scheuer, 2002). One of the first steps in analyzing skeletal remains is sex determination, mainly because subsequent steps - such as estimating age and stature-depend directly on the sex of the individual (Iscan and Steyn, 2013). The pelvic bones are the most diagnostic of sex, though the best sexing results are obtained when the complete skeleton is analyzed (Dawson et al., 2011). However, in many situations, a missing or poorly preserved pelvis requires the exam-

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iner to rely on the second-best element for sex determination: the skull (Bass, 2005; Byers, 2008; Shearer et al., 2012; White et al., 2012). The skull can be sexed by either visual inspection or metric measurements (Byers, 2008). In both approaches the mastoid process is important, since—together with the pars petrosa of the temporal bone—it is one of the most robust and sexually dimorphic features of the *Correspondence to: Anja Petaros, MD. Department of Forensic Medicine and Criminalistics, Rijeka University, School of Medicine, Brac ´e Branchetta 20, 51 000 Rijeka, Croatia. E-mail: [email protected] Received 5 December 2014; Revised 5 March 2015; Accepted 11 March 2015 Published online 10 April 2015 in Wiley Online (wileyonlinelibrary.com). DOI: 10.1002/ca.22545

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skull (Wahl and Graw, 2001; Walrath et al., 2004; Noren et al., 2005; Kemkes and Gobel, 2006; Williams and Rogers, 2006; Walker, 2008; Saini et al., 2012; Garvin et al., 2014). Although standard protocols for measuring and visually assessing the mastoid process have been published and accepted by the osteological community (Buikstra and Ubelaker, 1994), many studies still employ alternative approaches. In this article, we discuss the different methods and landmarks used to define, measure, and visually score the mastoid process. We also discuss variation in the sexual dimorphism of the mastoid process between different human populations, and argue that the current protocols for mastoid process analysis do not capture the full morphological range of this bony projection. Analytical standardization is desirable both for scientific research and in legal settings when forensic cases are investigated, and here we suggest how improved and standardized mastoid process analysis protocols could be devised.

THE MASTOID PROCESS The mastoid process is a prominent, conical bony projection located posteroinferiorly to the external auditory meatus of the temporal bone. It is part of the broader mastoid portion, which includes the posterior part of the temporal bone. Many muscles such as the sternoclediomastoid, splenius capitis, and longissimus capitis are attached to the mastoid part (Standring, 2008), mostly to the mastoid process itself, exerting forces that affect the development of its size and shape, specifically by pulling the temporal bone downward (Lieberman, 2011). Variable growth patterns in the mastoid cells, which create the hollow interior spaces of the mastoid process, also contribute to morphological differences between sexes and among individuals (Cinamon, 2009). The mastoid process is absent in neonatal skulls: it forms postnatally, growing directly from the flat mastoid portion of the temporal bone (Scheuer and Black, 2004). At birth, the petromastoid forms a separate region from the squamotympanic parts. It ossifies independently via enchondral ossification and fuses with the squamotympanic parts during the first year of life. Sometimes a fissure or slight separation remains between the two parts, which then disappears after the early postnatal period (Sperber, 2001). The mastoid process is formed from the petromastoid region between the first and fifth years of the child’s life, and the process is closely associated with the growth of the cranial external plate and the extension of the air space of the tympanic antrum to the mastoid process and subsequent formation of the mastoid air cells (Scheuer and Black, 2004; Sperber, 2001). As a continuation of the mastoid part, the mastoid process is not a separate center of ossification, which makes its boundaries hard to delineate clearly. The absence of close osteometric landmarks makes it difficult to develop stringent protocols for analyzing the mastoid process. In traditional osteology, only the external appearance of the mastoid process is

assessed, via linear measurements, categorical scores, or subjective descriptors. The details and drawbacks of these different approaches are discussed below.

METRIC EVALUATION As early as 1973, Howells noted in his book Cranial Variation in Man that his measurements of the mastoid process differed in part from the protocols offered by contemporaneous researchers such as Keen in 1950, Giles and Elliot in 1963, and Vallois in 1969 (Howells, 1973). He also noted a list of difficulties in measuring mastoid process height (which he also refers to as “length”) and width, ranging from the effects of size and other parameters on the measurements to the inconsistency of available landmarks in this region, especially those in the inner part of the mastoid process. As discussed by Nagaoka et al. (2008), the limitations described by Howells still persist. Mastoid length is the most commonly used measurement of the mastoid process, although it has not always been as successful as subjective visual assessment for sex determination (Spradley and Jantz, 2011). This variation might be related in part to intraand inter-observer errors in placing defining landmarks (Bernard, 2008). In Standards for Data Collection from Human Skeletal Remains (Buikstra and Ubelaker, 1994), mastoid length is defined as the linear distance from porion (the superior point on the external auditory meatus) to mastoidale (the lowest point on the mastoid process), measured perpendicularly to the Frankfurt Horizontal (FH) plane (Fig. 1). However, it might not be possible to measure the linear distance between these landmarks directly with Vernier calipers if mastoidale is too medially or laterally removed from vertical alignment with porion (Hoshi, 1962). In these situations, mastoid length might have to be estimated, just like in cases where there is damage to the lower border of the orbit—a fragile part of the facial skeleton that defines the orientation of the FH plane. Many disagreements about mastoid process measurements originate from inconsistencies in terminology and definitions among studies (Fig. 1). Some scholars refer to the measurement from porion to mastoidale as mastoid length (Giles and Elliot, 1963; Keen, 1950; Buikstra and Ubelaker, 1994; MooreJansen et al., 1994), while others refer to it as mastoid height (Howells, 1973; Nagaoka et al., 2008). Mastoid length has also been measured from the upper rim of the zygomatic arch rather than from porion (Saini et al., 2012), or even from the posterior end of the incisura mastoidea (IM) to porion (Nagaoka et al., 2008; Sujarittham et al., 2011) (Fig. 2). Moreover, some studies have measured the porionmastoidale distance using Vernier calipers with the jaws parallel to the FH, as recommended by the Standards procedure (Buikstra and Ubelaker, 1994), while others have measured projective linear distances between the landmarks both directly and diagonally (de Paiva and Segre, 2003; Saini et al., 2012; Jaja et al., 2013; Fig. 1). Nagaoka et al. (2008)

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Although single linear measurements of the adult mastoid process can sometimes produce accurate classifications by sex, the results differ considerably

Fig. 1. Different measurements of mastoid length and height, illustrating inconsistencies in measurement methods and nomenclature (ast: asterion; ms: mastoidale; PEIM: posterior end of incisura mastoidea; po: porion). (a) Standard mastoid length (po – ms; projection; Buikstra and Ubelaker, 1994), also referred to as mastoid height by Nagaoka et al. (2008). (b) Mastoid length (upper rim of the zygomatic arch – ms; Saini et al., 2012). (c) Mastoid length (PEIM – po; Nagaoka et al., 2008). (d) Mastoid length (ast – po; Demoulin, 1972). (e) Mastoid height (po – ms; direct; Saini et al., 2012). (f) Mastoid height (ms – po/PEIM; Nagaoka et al., 2008). (g) Mastoid height (ms – PEIM/nearest point of the external auditory meatus; Nakahashi and Nagai, 1986). (h) The visual method proposes assessment of the mastoid process from the lower border of the external auditory meatus. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

measured mastoid dimensions in three different ways, setting the caliper jaws both parallel and non-parallel to the FH plane. Less common measurements such as mastoid breadth, width, and thickness also are inconsistently defined in the literature, and are occasionally used synonymously (Howells, 1973; Patil and Mody, 2005; Nagaoka et al., 2008; Kittoe, 2012; Saini et al., 2012). These disagreements can perpetuate confusion among researchers and preclude direct comparisons between studies.

Fig. 2. Examples of morphological variation in the mastoid process: (a) pointed (male), (b) obtuse (male), (c) wide (male), (d) narrow with prominent paramastoid process (female), (e) long (male), (f) short mastoid process (female), (g) projecting anteriorly (male), (h) projecting vertically (male), (i) thin (female), and (j) thick (male). All individuals are from the same Portuguese population Photos by AP.

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between populations. The asterion-mastoidale distance has achieved greatest accuracy in sex classification in samples from North India (Saini et al., 2012), Thailand (Manoonpol and Plakornkul, 2012) and Nigeria (Jaja et al., 2013). This linear distance classified 77% of the females and 75% of the males correctly in the North Indian population (n 5 138). In Thais (n 5 150), it achieved 77% average accuracy, while in Nigerians (n 5 102) it correctly classified 86% females and 45% males. Mastoid width performed best for Japanese crania (n 5 87), achieving 92% and 86% correct classifications for females and males respectively (Nagaoka et al., 2008). The mastoidale-porion distance provided the best classification rates for a population from southern India (n 5 80), yielding 100/ 80% correctly classified females/males using a stepwise method (Sharma et al., 2013). For Europeans and Brazilians, mastoid process measurements have not proved very accurate. In a sample of Brazilians (n 5 81), mastoidale-porion distance measurements yielded very different numbers of correct classifications for females, 18%, and males, 93% (Galdames et al., 2008). In contrast, mastoid triangle measurements correctly classified females and males in a European sample (n 5 197) with nearly equal accuracy, i.e., 67/64% (Kemkes and Gobel, 2006). In most studies, the classification accuracy has been better for females than for males, possibly indicating more variation in the male mastoid process. Newer ways of measuring the mastoid process and its surroundings have been proposed in recent studies, ranging from calculating the mastoid triangle area (i.e., between asterion, mastoidale, and porion; de Paiva and Segre, 2003) to measuring the mastoid radius (Bernard, 2008). The triangle approach achieved 95% accuracy in sex classification when it was first proposed, but later work on other samples yielded lower accuracy and more variation (de Paiva and Segre, 2003; Kemkes and Gobel, 2006; Deepali et al., 2013; Jaja et al., 2013). Failure to replicate the initial successful results could be related to the position of asterion, one of the landmarks employed, which is highly variable among populations and individuals (Day and Tschabitscher, 1998; Sripairojkul and Adultrakoon, 2000; Mwachaka et al., 2010). Methodological differences related to instrumentation are another potential source of error (Kemkes and Gobel, 2006; Saini et al., 2012; Deepali et al., 2013; Jain et al., 2013; Jaja et al., 2013), as the original work was based on xerography (de Paiva and Segre, 2003) and did not address the possibility of variation related to positioning of the skull on the copying device (e.g., different distances between the copy machine and the mastoid; Stephan, 2014).

pinger, 2006; Byers, 2008; Pickering and Bachman, 2009; Dawson et al., 2011; Garvin, 2012; White et al., 2012; Christensen et al., 2014). Despite its inherent subjectivity, studies have demonstrated that visual inspection of the mastoid process can produce very good results in sexing unidentified remains, often achieving both good accuracy (around 80%) and high precision (above 90%) in different age groups and populations (Williams and Rogers, 2006; Walker, 2008; Ramsthaler et al., 2010; Garvin et al., 2014). To make visual scoring of the mastoid process less subjective and more standardized, an ordinal scale ranging from one (the least pronounced and most feminine expression) to five (most pronounced and most masculine) is presented in the Standards volume (Buikstra and Ubelaker, 1994). In the scoring instructions, observers are advised to inspect the mastoid process below the inferior margin of the meatus acusticus externus and the digastric groove (Buikstra and Ubelaker, 1994; Walker, 2008; Fig. 1). Thus, the upper edge of the meatus should not be used as the superior border of the mastoid process, as is commonly done in metric analysis (Buikstra and Ubelaker, 1994). Moreover, observers are encouraged to view the mastoid process in relation to its surrounding features (such as the meatus and the zygomatic process) and to consider its volume more than its length (Walker, 2008). This reiterates the observations of the earliest anthropological authors, who recognized the overall shape of the mastoid process to be the feature most useful for sex determination (Hoshi, 1962). As in conventional metric approaches, the scoring method of mastoid process analysis focuses on specific aspects of overall form. When it is assessed visually in the five-score system, the mastoid process is scored according to the closest match to two-dimensional drawings of different trait expressions, presented in lateral view. These diagrams emphasize differences in length more than other variables, leading observers to treat the mastoid process as a twodimensional structure. Consequently, many subtle shape and size differences that can be observed in nonlateral views are neglected during standardized visual scoring. Owing to the complexity of mastoid process morphology (as demonstrated in Fig. 2), observers should be aware of different aspects of variation apart from length, such as inclination, breadth, thickness, and general outline (pointed or obtuse). These features are rarely examined or discussed in osteology texts, though they contribute to human sexual dimorphism. In a study focusing on aspects other than length, sex was found to correlate satisfactorily with visually assessed mastoid process volume but not with apex prominence (Hoshi, 1962). These results have so far not been validated by other researchers.

VISUAL SCORING In contrast to the metric approach, visual scoring of the mastoid process for sex identification involves a number of qualitative descriptors. The male mastoid process is typically described as long, prominent, projecting, robust, large, broad, and/or wide, whereas the female one is typically described as gracile, small and/or thin (Stewart, 1979; Wilkinson, 2004; Kle-

SUGGESTIONS FOR FURTHER RESEARCH AND OPTIMIZATION OF MASTOID PROCESS ASSESSMENT Although current methods for visual and metrical assessment of the mastoid process have proven

Evaluating Mastoid Process Morphology useful for sex determination and other applications, both approaches have limitations. The metric approach yields continuous parameters (e.g., linear distances), which are essential for most statistical analyses, and facilitates comparisons among studies (Garvin et al., 2014). However, a large amount of morphological variation cannot be captured by simple two-dimensional measurements. There is a related problem with the visual approach, which relies on two-dimensional reference diagrams and ordinal scores than do not reflect the inherent continuity of trait expression. Although the visual scoring method is rapid, inexpensive, and generally reliable, the subjectivity of scoring assignments raises concerns about repeatability and interobserver errors. Given these shortcomings, together with recent advances in 3D technology, it is tempting to suggest that a 3D-based approach would improve mastoid process analysis. Today, a range of techniques for recording 3D data and constructing 3D anatomical models are available for anthropologists and anatomists interested in virtual examination of skeletal remains. Suitable recording techniques include CT and microCT scanners, 3D laser scanners, and 3D photogrammetry (Sholts et al., 2011; Weber and Bookstein, 2011; Katz and Friess, 2014). Regardless of the technology used for 3D data acquisition, 3D models can be conveniently examined using specialized software originally designed for industrial reverse-engineering and 3D-based CAD drawing, but which can also be used for skeletal analysis (Virtual-Anthropology, 2009). Digital approaches to skeletal morphometrics have helped to quantify the morphological attributes of many cranial traits used in sex determination. For example, the browridge/glabella (Shearer et al., 2012; Garvin et al., 2014), the mental eminence (Garvin et al., 2014), and even teeth (De Angelis et al., 2014) have been quantified through their volumes and shapes. Because the mastoid process is one of the most reliable sex indicators (Walker, 2008; Garvin et al., 2014), and because its dimorphism is expressed via differences in volume and shape rather than size alone (Walker, 2008), it appears well suited for digital 3D analysis. Previous studies have demonstrated the usefulness of employing contours (Sholts et al., 2010) and surface shapes (Kimmerle et al., 2008; Bigoni et al., 2010; Garvin and Ruff, 2012) in cranial morphometrics, and such digital analyses would be likely to succeed in objectively quantifying differences in mastoid length, width, shape, development, and projection, thereby combining the respective advantages of both the traditional metric and visual methods for sex estimation. However, 3D-based skeletal analysis is timeconsuming and relatively expensive, requires a certain technical proficiency, and has its own limitations and methodological difficulties such as noise in 3D models, problems with bone segmentation in CT scans, difficulties in capturing complex and internal bony structures, the large file size of the final model—which requires computers with high computational power and good graphics cards—long acquisition, processing, and postprocessing times, loss or dislocation of landmarks due to inappropriate simplification processes,

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differences in mesh quality, and the presence of mesh artifacts (Sholts et al., 2010; Jantz et al., 2013; Katz and Friess, 2014). Consequently, anthropologists and anatomists still search for cost-effective and simple yet reliable methods of skeletal analysis for practical use (Kemkes and Gobel, 2006). Both traditional osteometry and anthroposcopy represent straightforward and time-sparing ways of assessing skeletal remains. With the above limitations in mind, it is therefore worthwhile for researchers to optimize existing methods by improving their reliability, repeatability, and accuracy in various aspects, such as those discussed below. First, a standardized nomenclature for mastoid analysis is needed, as proper measurements, applications, and comparisons of data are currently handicapped by differences in terminology and landmarking among studies. To define the dimensions of the mastoid process, we suggest adhering as far as possible to the most commonly used definitions of various distances (Table 1). Thus, in view of the wide acceptance and use of the Standards protocols (Buikstra and Ubelaker, 1994), we recommend researchers to delineate the upper limit of the mastoid process with porion and the lower with mastoidale. We also recommend using the posterior end of IM (PEIM) as the posterior endpoint of the mastoid process, the posterior border of the external auditory meatus (PMAE) as the anterior border, and the IM)as the internal border. Standardization of measurement protocols and definitions can be impeded by terminological synonyms or by using different terms for the same metric distance. A case in point is the confusing use of the terms “mastoid height” and “mastoid length” in the osteology literature (see Fig. 1 and Table 1 for clarification). To remedy this, we suggest that distance measurements should be denoted by the endpoints employed during the measurement (e.g., distance asterion-porion; distance asterion-mastoidale), thereby eschewing such terms as superior length, height, etc. However, in view of the widespread use of Standards in osteological data collection, we suggest continuing to refer to the mastoidale–porion distance as “mastoid length”, even though the term height (“distance from base to top”) is arguably better than length (“longest dimension of something fixed in place”, cf. Table 1). To avoid confusion, we recommend completely avoiding the term “height” in mastoid process measurements, since no distance other than mastoidale–porion could properly describe it. Nevertheless, new protocols could benefit from investigating more straightforward (direct linear) measurement(s) of mastoid length/shape, as the mastoidale– porion measurement often involves error-prone projections. Second, we suggest that the entire mastoid process, or at least a large part of it, should be examined during analysis. For visual assessment, this means inspection from different directions and not just the commonly used lateral view. To visualize the mastoid process in its entirety the observer should also examine it from its antero-superior aspect (where the thickening of the process is more visible) and from its inferior aspect (where, beside the thickness, the

The extended 2dimensional outer boundary of a 3-dimensional object

Surface

de Paiva and Segre (2003)

This study

This study

Bernard (2008)

Keen (1950), Giles and Elliot (1963), Vallois (1969), Howells (1973) Nakahashi and Nagai (1986), Nagaoka et al. (2008), Saini et al. (2012) Howells (1973), Nakahashi and Nagai (1986)

Buikstra and Ubelaker (1994) Demoulin (1972), Nagaoka (2008), Saini et al. (2012)

Referencea

Howells, Nakahaski and Nagai all describe as mastoid width the distance from the skull base measured perpendicular to the incisura mastoidea, i.e., the distance from the incisura mastoidea to the most prominent point on the mastoid process external surface. Even though “width” and “breadth” are synonyms, we favor using “breadth.” However, the most appropriate term for this measurement may be mastoid “thickness,” given the length-breadth-thickness convention, where length is the longest dimension, breadth the second longest, and thickness the shortest dimension. In that case, mastoid breadth could be defined as the distance between the posterior border of the external auditory meatus and the posterior end of incisura mastoidea. We propose to avoid this measurement, since the mastoid is not a sphere or circle and therefore the measurement may not be consistently reproduced and may lead to misinterpretations. The mastoid projection could be defined as the distance between the mastoidale and the lower bony border of the external auditory meatus (as defined by the visual scoring system). The mastoid volume could be calculated easily in 3D models with certain 3d modeling software. The best 3D model-based methods are to be defined and verified by future studies. The surface of the so-called mastoid triangle (area between mastoidale, porion, and asterion).

Although the measurement could easily be defined as the distance from porion to mastoidale, we propose to avoid its use and instead use the term length.

The vertical projection of the porion – mastoidale distance (as defined by Buikstra and Ubelaker, 1994)

Proposal for future useb

When measuring mastoid length, the skull must be aligned along the FH plane. Length and breadth should be measured from the lateral side of the skull (looking at the mastoid process as a two-dimensional object). Mastoid thickness should be measured by laying the skull with its base facing up. a see Figure 1 for details. b see Figure 3 for details.

The amount of 3dimensional space occupied by an object

Volume

Projection

Radius

Thickness

Breadth/width

The length of a line segment between the center and circumference of a circle or sphere Any solid convex shape that juts out from an object

The linear extent in space the longest dimension of an object that is fixed in place The vertical dimension of extension – distance from the base to the top of an object The extent of an object from side to side The dimension through an object as opposed to its length and width

Length

Height

Definition

Distance

TABLE 1. Suggested Standardization for Geometric Measurements of the Mastoid Process

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Fig. 3. Computed tomography (CT) models, recorded by AP with an MDCT unit (Sensation 16, Siemens AG Medical Solutions, Erlangen, Germany) in continuous layers without overlapping (12 3 0.75 mm collimation, 120 kV and 320 mA) and processed with 3D Doctor imaging software (Able Software Corp., 1998–2011), showing the measurements of different mastoid process distances proposed in Table 1 (ML: mastoid length; MB: mastoid

breadth; MP: mastoid projection; MTS: mastoid triangle surface; MV: mastoid volume; MT: mastoid thickness; po: porion; ms: mastoidale; PMAE: posterior end of external auditory meatus; PEIM: posterior end of incisura mastoidea; LBMAE: lower border of external auditory meatus; ast: asterion; im: incisura mastoidea). Differences in shape and outline can be noted. All individuals are from the same Croatian population.

development of the surrounding muscle attachment structures can be seen). From Figure 3, it is clear that some shape attributes (e.g., differences in the mastoid point and outline, mastoid breadth and thickness, rugosity of the paramastoid area) can be observed only from non-lateral projections. These other projections enable the massiveness and “voluminosity” of the mastoid process to be assessed more clearly and more objectively, as suggested by the Standards procedures. We furthermore suggest examining the dimorphism of the region surrounding the mastoid process, as it contains many muscle attachment points. It is well known that bone expression in muscle insertion areas partly accounts for human sexual dimorphism and morphological variation (Wilczak, 1998; Suazo Galdames et al., 2008), and that muscle attachment spreads through a larger area in males

than females (Franklin et al., 2006). The surrounding features may deserve to be assessed separately, but they also affect analysis of the mastoid process, especially during visual inspection. For example, the paramastoid process, which is located medially to the mastoid process, is influenced by muscle activity and is sometimes very prominent. When significantly developed it stretches more laterally and posteriorly than the mastoid process, so the mastoid process visually appears comparatively smaller and more feminine. Clearly, creating improved protocols for assessing the mastoid process is not a job for a single researcher, especially since different analytical approaches (visual, metric, and 3D-based) are involved. Moreover, new protocols should be independently tested by other researchers, and regional

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variations in mastoid process dimorphism (Kemkes and Gobel, 2006; Kittoe, 2012; Garvin et al., 2014) should be taken into account. It could prove useful to develop separate and complementary protocols for comparing regional populations (cf. (Warmlander and Sholts, 2011). Thus, the measurements listed in Table 1 are not presented as new standards for measuring the mastoid process, but are offered to provide insight and guidance. We hope that the ideas and opinions presented here will contribute to a deeper discussion of the problems of defining and measuring morphology in the more complex parts of the human skull.

ACKNOWLEDGMENTS The authors thank Prof Ana Luisa Santos and the Department of Life Sciences, Faculty of Science and Technology, University of Coimbra, Coimbra, Portugal for allowing us to study the skulls from the Human Identified Collection of the Department, which led to the writing of this viewpoint. Figure 2 shows the mastoid processes of some of the skulls housed at the Human Identified Collection of the Department of Life Sciences, Faculty of Science and Technology, University of Coimbra. ˇic The authors thank also Prof Boris Brkljac ´ and Dr  Mislav Cavka from the Department of Diagnostic and Interventional Radiology, University Hospital Dubrava, Zagreb, Croatia, who allowed us to CT-scan skulls housed at the Osteological Collection of the Croatian Academy of Sciences and Arts, Zagreb, Croatia. These 3D models are presented in Figure 3.

CONFLICT OF INTEREST The authors declare no conflicts of interest related to this manuscript.

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