Posterior maxillary (PM) plane and anterior cranial architecture in primates

Abstract

This study tests several hypotheses of integration between the cranial base and face in primates. After reviewing the definition and anatomical basis for the posterior maxillary (PM) plane, which demarcates the back of the midface at its junction with the sphenoid, we demonstrate how the PM plane can be identified accurately on radiographs, and confirm that it maintains a 90°angle relative to the Neutral Horizontal Axis of the orbits in all primates. In addition, we use the PM plane to test Dabelow's (1929) hypothesis that the orbits and anterior cranial base are more highly integrated in anthropoids than in strepsirrhines, and we test the hypothesis that the midline anterior cranial base (planum sphenoideum) and anterior cranial floor (planum sphenoideum plus cribriform plate) in primates are highly correlated with each other relative to the PM plane. The mean angle between the anterior cranial base and the PM plane does not differ significantly from 90°in anthropoids, but differs significantly in strepsirrhines. The anterior cranial base and anterior cranial floor, however, correlate well with each other relative to the PM plane in both suborders of primates, independent of orbital orientation and configuration. The PM plane, anterior cranial base, and anterior cranial floor, therefore, form an integrated structural complex, a "facial block," whose orientation relative to the posterior cranial base influences craniofacial shape among anthropoids in which orbital orientation influences the orientation of the anterior cranial base. One such effect is that increases in cranial base flexion shorten the antero-posterior length of the nasopharynx. Anat Key words: posterior maxillary (PM) plane; neutral horizontal axis; integration; "facial block"The posterior maxillary (PM) plane 1 is defined as the line connecting two radiographically determined termini projected onto the midsagittal plane: (1) the average most posteroinferior point on the maxillary tuberosities, and (2) the average point on the anterior-most extent of the greater wings of the sphenoid (Fig. 1). Although the PM plane was developed for, and has principally been used in, radiographic studies of cranial growth in humans Enlow and Moyers, 1971;Enlow and McNamara, 1973;Enlow and Azuma, 1975;Enlow, 1990), it may have a wider utility for comparative studies of primate and hominin craniofacial growth and architecture 1 Technically, the PM plane is not a plane (which is defined by three points), but is really a line defined by the average of two points inferiorly and two points superiorly. We use the term "plane" because it is established in the literature. Bromage, 1992;Lieberman, 1998). According to Enlow (1990:171), the PM plane is "one of the most basic and important planes in the whole head" for several architectural and developmental reasons. Spatially, the PM plane is an attempt to characterize the posterior margin of the ethmomaxillary complex (the ethmoid, maxilla, and palatine) where it articulates with the boundary between the anterior...

Nasal Septal and Premaxillary Developmental Integration: Implications for Facial Reduction in Homo

“…the chondrocranium plays a key integrative role in craniofacial development and evolution in genus Homo (Moss and Young, 1960;Enlow, 1990;Ross and Ravosa, 1993;Lieberman et al, 2000;McCarthy and Lieberman, 2001;Spoor, 2002, 2004;Bastir et al, , 2010Lieberman et al, 2008), the influence of the nasal septum, as one component of the chondrocranium, has not been as widely considered. There is, nevertheless, fossil evidence to suggest that an integrated nasal septal/premaxillary complex may account for variation in facial size between archaic and recent modern humans as Neandertal and recent human subadults exhibit taxonomic variation in the timing of premaxillary suture fusion.…”

“…A variety of ultimate causal mechanisms have been proposed to account for this evolutionary dynamic (see Lieberman, 2008); however, our ability to test hypotheses regarding morphological change in Homo relies on our understanding of the proximate causal mechanisms that underlie phenotypic variation in fossil forms. To this end, the development and integration of the chondrocranium has been a primary focus in detailing developmental (i.e., proximate) changes upon which evolutionary (i.e., ultimate) mechanisms operate (Moss and Young, 1960;Enlow, 1990;Ross and Ravosa, 1993;Lieberman et al, 2000;McCarthy and Lieberman, 2001;Spoor, 2002, 2004;Lieberman et al, 2008).…”

Variation in avian brain shape: relationship with size and orbital shape

“…5) had more anteroposteriorly shorter orbits than those with a more dorsally flexed cranial base. In the case of mammals, cranial base angulation is closely related to brain size and facial length (Moss & Young, 1960;Biegert, 1963;Ross & Ravosa, 1993;Ross & Henneberg, 1995;Lieberman, 1998;Lieberman et al 2000a,b;McCarthy & Lieberman, 2001;Ross et al 2004;. Architecturally, the cranial base provides growth space for the brain and face.…”

Speculations about the selective basis for modern human craniofacial form

“…This flexion has several implications for skull shape, in part because the top of the face is the base floor of the anterior cranial base and also because the back of the face is always roughly perpendicular to the anterior cranial base. [21][22][23] The combined effect of these constraints is that a more flexed cranial base reorients the entire face as a block more ventrally beneath the anterior cranial fossa and decreases the length of the nasopharynx behind the Note the relatively longer face and orbits of the archaic human (as shown by white bars), which generates a more square-shaped orbit; the modern human face is also larger and more projecting, and the neurocranium is more spherical. B. graph of centroid size versus the first principal component of cranial shape in Pleistocene modern H. sapiens (stars) and archaic Homo (circles), H. erectus (squares), and H. habilis (pentagon) following a Procrustes superimposition of the landmarks.…”