Date of Degree
PhD (Doctor of Philosophy)
Robert G. Franciscus
Variation in the shape and position of the internal nasal floor relative to the lower border of the piriform aperture in the genus Homo has been described as having three primary shape configurations: level, sloped, or depressed. The high frequency of depressed nasal floors among Neandertals relative to other fossil and extant groups (>80%) had originally led to the idea that nasal floor depression was related to an overall enlarged nasal capsule - an adaptive feature that would have been under selection among Neandertals living in cold, glacial climates. For a variety of reasons, subsequent research has found little empirical or theoretical support for this adaptive idea. Recent research on extant humans has also demonstrated that nasal floor shape variation, unlike many other midfacial traits, does not arise until well after birth, with nasal floor depression (when it occurs) appearing at the earliest around 3.0 years of age. Furthermore, nasal floor depression was also shown to correspond with a vertically expanded premaxillary region. Thus, it was hypothesized that nasal floor depression might be related to variation in key developmental and morphological aspects of the anterior maxillary dentition. This study metrically quantifies nasal floor topography for the first time in order to more objectively examine patterns of shape variation and to test explicit hypotheses regarding potential causative factors for nasal floor variation. The variables examined include anterior tooth dimensions, dental developmental rate, aspects of midfacial shape, overall facial size, and patterns of premaxillary/post-maxillary integration. It was found that among these, only dental developmental rate was clearly correlated with internal nasal floor shape. This result indicates that aspects of anterior dental development may indeed be a causative factor in the development of nasal floor shape variation. The existing visual discrete coding system for nasal floor topography was also evaluated in light of the new, quantitative data produced by this study as well as a critical comparison of the consistency of nasal floor topography definitions used previously in the literature. While it is suggested that quantitative data are preferable to qualitative data for this trait when possible, limitations in research methods for collecting quantitative data on osteological and fossil collections remain difficult to overcome. Thus a new, two-category presence/absence based system for describing nasal floor shape is proposed.
While the midface (the nose and the region around it) is argued to be an aspect of anatomy that is highly constrained by the need to function in respiration, there are obvious differences in midfacial form between both fossil human species (e.g., Neandertals) and populations of living humans. The cause of this variation is still not entirely understood. One nasal trait in particular, a distinctly depressed shape to the bottom of the inside of the nose (the internal nasal floor), has previously been argued to be a Neanderthal adaptation to cold climates to help warm inhaled air. Subsequent research, however, has cast this into doubt. The bones which make up the majority of the nasal capsule, the paired maxillae, also house the upper dentition and must respond to the competing biological constraints that respiration and mastication place on them. Recent research has suggested that the development of the front teeth during childhood may be the ultimate cause of shape differences in the internal nasal floor. This project tested hypotheses regarding potential causes for nasal floor shape variation (e.g., the size of the front teeth, timing of tooth eruption, aspects of midfacial shape, overall facial size), focusing especially on aspects of the anterior dentition. It was found that only the timing of tooth eruption was clearly correlated with internal nasal floor shape. This result indicates that aspects of anterior dental development may indeed be a causative factor in the development of nasal floor shape variation.
publicabstract, Dentition, Extant human, Internal nasal floor, Midface, Neandertal, premaxilla
xx, 207 pages
Includes bibliographical references (pages 193-201).
Copyright 2015 Christina L. Nicholas