Cranial Base Angulation: Examining Congruence of Interior and Exterior Measurements
Authors: Agosto Elizabeth R , Williams Samuel J , Auerbach Benjamin M 
Published: November 2023

Variation in the degree of basicranial flexion plays an important role in the analysis of hominin fossil remains. This variation in the angulation of the cranial base (CBA) is informative of clade distinctions throughout primates, and other mammals, and is informative of both developmental and phylogenetic processes. This is due to its putative covariance with traits such as encephalization and body size. Estimation of the CBA has been traditionally derived from endocranial landmarks. However, more recent analyses suggest that the CBA may be derived from ectocranial landmarks as well. The relative ease of obtaining ectocranial landmarks makes this method especially attractive when considering the time and cost of obtaining endocranial landmark data. To date there has been no comparison of these methods to determine whether or not they are reflecting the same mean angles and variance. This study assesses concordance among the four endocranial techniques and among four ectocranial techniques, and then compares endocranial and ectocranial methods for measuring the CBA.

All CBAs are extracted from a sample of 48 tamarins (Saguinus oedipus). Midsagittal radiographs are used to identify four internal landmarks, from which four planes are drawn: clivial, sella, pre‐sphenoid, and sphenoid. Each endocranial angle is calculated from the plane intersections using ImageJ. Seven three‐dimensional ectocranial landmarks are captured using a MicroScribe G2. A total of four CBAs are calculated using a vector‐based method that is then corroborated by CBA extraction using trigonometric methods. All extracted angles are converted to radians prior to statistical analysis. CBA comparisons are performed in R using a Brown‐Forsythe test. Results show no significant difference among the individual variances captured by the four endocranial methods, nor was there a difference between the vector‐based and trigonometric‐based calculations using ectocranial landmarks. However, comparisons among the individual ectocranial methods show significant differences in the variance captured between all but two of these methods. Results also indicate that the variance captured by endocranial CBAs derived from the sella, clivial, and pre‐sphenoid planes is statistically equivalent to that of the ectocranial CBAs derived from the basion, opisthion, spheno‐basion, and prosthion. Morphological dimensions and orientation of the ectocranial and endocranial landmarks are responding to different local factors; internal morphology is influenced by the brain and meninges, while the external morphology is responding to muscle attachments, spacing for soft tissue structures, and interfacing with the vertebral column. Despite these differential influences on morphology, it is possible that the position of the face relative to the neurocranium is overriding these factors. These results are not unexpected as they corroborate the leading hypothesis explaining the variation in the CBA, the spatial packing hypothesis, which has been empirically shown to demonstrate covariation between the CBA, brain size, and facial orientation (kyphosis).