What’s in a Measurement? The Inside (and Outside) Story
In our last two posts, Kristen and Sam discussed established ways to model the effects of evolutionary forces on skeletal traits. As they both mentioned, we often ask ourselves about whether what we are measuring best represents the traits we’re interested in modeling. At first glance, this may seem to be an esoteric issue, but as I’ll argue here, we should be cautious about measurement choice when we are trying to understand evolutionary change.
The morphology or shape of the skull, for instance, is influenced by a myriad of factors, ranging from the evolutionary history of a species to the interactions between bone and the soft tissues of the head during growth and development. Anthropologists have devoted numerous studies to identify and parse out the effects of these different factors, as significant changes to the shape, relative size, and postural orientation of the cranium are observed throughout human and primate evolution. In choosing measurements to assess these topics, we make assumptions about what those dimensions capture. While researchers are deliberate in their measurement choice, it is possible that in restricting observations to one set of traits, we miss potentially informative observations inherent to the traits we have chosen to omit. By considering alternate suites of traits, we may discover previously undetected influences on morphology. One such example of this is observed in the way the angulation of the cranial base is captured and measured.
The cranial base is centrally located in the cranium between the brain case and the bones of the face, and the angle of flexion at this location appears to reflect the orientation of the face to the rest of the cranium. The degree of flexion differs among primates and so is of particular interest concerning interspecies evolutionary relationships. Humans display the most extreme amount of flexion, where the face is shifted inferior to the brain. Researchers including Schultz (1942), Moss (1958), Beigert (1963) and Gould (1977) proposed mechanisms to explain this interspecific variation in the past, but more recent studies suggest that the primary factors contributing to basicranial flexion are relative brain size and facial size (Ross and Ravosa 1993; Ross and Henneberg 1995; Bastir et al. 2010).
Landmarks (readily identifiable features on bone) used to measure the angle of flexion of the basicranium are typically taken from the internal surface of the cranial base, captured using X-rays or computed tomography (CT) scans. External landmarks of the cranial base are not used to measure the degree of flexion, rather they are thought to better reflect pharyngeal (throat) dimensions as they do not capture the same angle of flexion as internal landmarks (Laitman et al. 1978, 1979; Laitman and Heimbuch 1982; Lieberman and McCarthy 1999; Pagano and Laitman 2015). This may be due to Moss’s (1960) observation that growth of the internal aspects of the cranium respond to soft tissue; their morphology is reflective of the growth and development of that soft tissue (e.g., the functional matrix hypothesis). An extension of this idea is that the observed evolutionary changes in cranial form may in fact be secondary to changes in related soft tissues—the brain and the pharynx, for instance. However, because the flexion of the cranial base has such a marked effect on the overall shape of the cranium, is it not possible that internal and external measures of the cranial base angle could be reflective of the same underlying signal?
As a first step in addressing this question, my colleagues and I (Agosto et al. 2016) compared different measures of the angulation of the cranial base to see whether they capture similar information. We used tamarins (Saguiunus oedipus) as a model organism. Using the standard practice of taking mid-sagittal X-rays, we captured the internal cranial landmarks and measured the angles between them. An external measure of the cranial base angle was captured using three-dimensional landmarks from the external surface of the cranium and calculating the angle among them. Despite the different influences on morphology, both internal and external cranial landmarks capture similar variance that may be related to the relative position of the face to the brain case. However, due to the effect of different influences on the bony anatomy, they do not measure the same angles of flexion in the cranial base.
To understand the interactions between brain growth, pharyngeal development, and variation in the basicranial skull requires developmental models and experimental approaches. Our study indicates the presence of a shared, underlying signal among internal and external measures of the degree of flexion in the cranial base. While the observed differences in the angles supports the idea that different soft tissue interfaces have an influence, the equal variance among them points to the fact that the underlying covariance of these traits—both within a region of the body as well as the whole organism—has an important effect that shouldn’t be overlooked. Consideration of such effects in our studies of evolutionary change may lead to the discovery of unexpected and interesting relationships that ultimately affect organismal morphology. The measurements we choose for our studies have the potential to expand our understanding of the processes and relationships that affect evolution, and so we benefit from testing the assumptions behind our measurement choice.
References
Gould SJ. 1977. Ontogeny and Phylogeny. Cambridge: Belknap Press.