THE ANATOMICAL RECORD 300:160–170 (2017) Zygomatic Root Position in Recent and Fossil Hominids 1,2 1,2 GERHARD W. WEBER * AND VIKTORIA A. KRENN 1Department of Anthropology, University of Vienna, Austria 2University of Vienna, Core Facility for Micro-Computed Tomography, Austria ABSTRACT The relative position of the zygomatic root to the dentition plays a crucial role in determining the overall strength of the face in response to bite forces. The powerful superficial head of the masseter arises there and the zygomaticoalveolar crest (ZAC) is discussed as a buttressing fea- ture of the face. For instance, a more forwardly or backwardly positioned zygomatic root or a lower or higher vertical distance to the dentition could be indicative for evolutionary adaptations to particular loading regimes which are associated with diet. We therefore examined the mor- phology of the maxilla using state-of-the-art 3D Geometric Morphometric methods. The data set was reduced to a minimum of relevant measure- ments and includes five landmarks (pr, ol, zm, lingual and buccal mid- point of second molar alveoli) and three curves with semilandmarks along the lingual and buccal alveolar rim and the ZAC. Results show a stun- ning overlap in shape variation. We find no clear pattern of shape that would allow separating different hominid groups with confidence, except two extreme forms—Paranthropines and Neanderthals. We also find no clear trend over time. Australopithecines, Habilines, Erectines, and Mid- dle Pleistocene Homo can be very similar to modern humans. Even great apes are within or not far from the central shape distribution of Homo, but they separate clearly from gracile and robust Australopithecines. We discuss the shape factors underlying our data. The geometry studied allows simple measurements and analyses and is thus potentially inter- esting for classification purposes of extreme forms. Anat Rec, 300:160– 170, 2017. VC 2016 Wiley Periodicals, Inc. Key words: zygomatic process; maxilla; zygomatic arch; zygo- maticoalveolar crest; human evolution; buttress; biomechanics; mastication INTRODUCTION Cranial diversity among primates is largest in homi- Associate Editor: Prof. Jeffrey T. Laitman noids, particularly if Homo sapiens is included (Fleagle Grant sponsors: Oesterreichische Nationalbank, Anniversary et al. 2010). The modern human cranium is character- Fund; Grant number: 16121. ized by a large globular braincase and a retracted face *Corresponding author: Univ. Prof. Dr. Gerhard W. Weber, with a small jaw (Lieberman 2011), while extant apes Department of Anthropology & Core Facility for Micro- Computed Tomography, University of Vienna, Althanstr. 14, such as Pan troglodytes, Gorilla gorilla or Pongo pyg- A-1090 Wien, Austria, Tel: 143-1-4277-54701, Fax: 143-1-4277- maeus, as well as most Australopithecines (except Au. 9547, Email: [email protected] sediba), show a marked facial prognathism. With early Received 14 March 2016; Revised 1 June 2016; Accepted 14 members of the genus Homo the face became more June 2016. orthognathic, narrower and taller, but Mid-Pleistocene DOI 10.1002/ar.23490 Homo still exhibits considerable prognathism. The max- Published online in Wiley Online Library (wileyonlinelibrary. illary bone shows mainly depository bone remodeling in com). VC 2016 WILEY PERIODICALS, INC. WEBER AND KRENN 161 Pliocene and early Pleistocene hominins, while modern inferior point of the sutura zygomaticomaxillaris [zygo- humans exhibit extensive bone resorption (Lacruz et al. maxillare (zm)] there is often a well-palpable crest run- 2015). All together, we can recognize a reduction in size ning down the lateral wall towards the alveolar process and robusticity of the face, the postcanine dentition, and of the maxilla. In other cases only a slightly elevated, the chewing forces in the genus Homo (Demes & Creel rounded column that fades out long before reaching the 1988; McHenry & Coffing 2000; Lieberman 2011). Likely alveoli can be seen. The crest is called the reasons for the masticatory reduction are dietary shifts “zygomaticoalveolar crest” (ZAC; Rak 1983; synonymous along the way from Australopithecines to early Homo, with “crista zygomaticoalveolaris”) or “inferior zygomati- Middle Pleistocene Homo, and modern humans (Rak comaxillary margin” (IZM; Pope 1991). Both authors 1983; Brace et al. 1987; Wrangham et al. 1999; Lieber- focused on the curvature of this crest and on its height man 2008; Ungar et al. 2006; Lucas 2004; Wrangham & in relation to the dental arch in Australopithecines, ear- Carmody 2010; Ungar 2012). Easier digestible food and ly Homo, and modern humans. The zygomaxillary region the invention of extra-oral food processing techniques in Neanderthals is tall and swept back (Rak 1986) which (e.g. tool use, cooking) are reasonable factors discussed in contributes to their typical appearance of the midface. hominin evolution leading to a reduction of loading Trinkaus (1987) described a posterior migration of the regimes related to mastication (Lucas 2004; Lieberman zygomatic root in Neanderthals compared to Middle et al. 2004; Zink et al. 2014). Pleistocene hominids and Proto-Neanderthals as one fac- The zygomatic (malar) process of the hominid maxilla tor determining this distinct configuration. The zygomat- is the anterior root of the zygoma and connects via a ic root in Neanderthals arises typically above second to rough articulation surface with the zygomatic bone. The third molar (M2-M3; Trinkaus 1987) rather than first to latter joins posteriorly with the zygomatic process of the second molar (M1-M2; Lieberman 2011). temporal bone. One of the most powerful muscles of the Today, by 3D measurements, structures can be related skull, the superficial head of the masseter, arises by a to each other considering their actual spatial geometry thick, tendinous aponeurosis from the zygomatic process quantitatively. From a functional-morphological view on of the maxilla and from the anterior two-thirds of the hominid evolution, two questions with regard to the rela- inferior border of the zygomatic arch. Its fibers pass infe- tive position of the anterior root of the zygoma can be rior and posterior to insert into the angle of the mandi- raised: 1) where in anterior-posterior direction is its ble and the inferior half of the lateral surface of the location relative to the postcanine dentition, and 2) how ramus of the mandible. Other muscles originating in the is the zygomaticoalveolar crest shaped, which includes region of the zygomatic root are mimic muscles, such as the height of the zygomatic root above the alveolar the musculus levator anguli oris, the musculus zygoma- plane? The location of the zygomatic root is often found ticus, and the musculus caninus (Fig. 1). between M1 or M2 (e.g., in recent humans, see also Lie- The location of the zygomatic root, particularly its rel- berman 2011), but can be also seen above the premolars ative position to the postcanine dentition where major (e.g., in Paranthropines), or between M2 and M3 (e.g., in strains during chewing occur, certainly plays a crucial Neanderthals). Using advanced morphometric methods, role in determining the overall strength of the face in some authors dealt implicitly with the topic, e.g. by response to bite forces (Ledogar et al. 2016b, this vol- studying the face of Middle Pleistocene hominins, and ume). The whole zygomatic arch is a functional require- found temporal clusters between them (Freidline et al. ment and surely not an anachronistic relic of phylogeny 2012). Unfortunately, in this study the question of the (Witzel et al. 2004). It has to sustain bite forces, it must relative position of the anterior zygoma was not explicit- withstand bending in a plane parallel to the occlusal ly addressed. Noback and Harvati (2015) recently pub- plane, and it also redirects tensile forces produced by lished works on cranial shape to investigate its relation the masseter and the temporal fascia. Facial dimen- to subsistence. Their 3D measurements included a sub- sions, e.g. a more forward or backward position of the set capturing the dental arch and zygomatic root, but zygomatic root as well as a wider or narrower mid-face, their sample focused on modern humans only. Beside a influence the pattern of stresses and strains which the few studies touching the topic, there is a lack in litera- face experiences during chewing (Hylander 1977). Fur- ture using state-of-the-art technologies to answer the thermore, the position of the zygomatic root influences questions raised above. This prompted us to address the the mechanical advantage of the superficial head of the issue if a clear evolutionary trend (probably even useful masseter which elevates the mandible. Due to its diago- for classification purposes) exists, or if we find rather an nal direction, it also moves the mandible forward inconsistent pattern of variation across hominid taxa. (protrusion). Our study is a first attempt to examine the morphology It seems reasonable to assume that the position of the of the maxilla with regard to the relative position of the zygomatic root in relation to the dentition might reflect zygomatic process to the dentition by Geometric Morpho- evolutionary adaptations in response to biomechanical metric methods. The data set was reduced to a minimum demands. For instance, Rak (1983) advocated that the of relevant measurements to include as many as possible placement of the zygomatic root relative to the rostrum recent and fossil specimens. Our goal is to provide results might influence the strain patterns in the Australopithe- on the existing variation in extant modern humans and cine face, particularly a laterally expanded zygomatic apes, and in some extinct hominin taxa. root together with a straight and steeply-inclined zygo- maticoalveolar crest (see below) would act as a bony but- MATERIALS AND METHODS tress (cf. anterior pillars in some Australopithecines and Paranthropines). Tracing the inferior border of the zygo- The sample to cover morphological trends across dif- ma from posterior to anterior, in many hominids a more ferent hominid taxa included extant apes [Pan troglo- or less sharp ridge is easily detected.