Modularity and Integration of the Sphenoid Body in Human Cranial Variation: The Confluence of Functional Matrices

In the Functional Matrix Hypothesis (FMH), Moss suggests that the formation and development of soft tissues of the head influences variation in the bony features in the skull. How multiple soft tissues with different developmental origins interact to influence skull variation remains unresolved. In this study, we investigate the covariation of the anterior intracranial space with the anatomical shape and location of the orbits and palate, two regions under local influence by soft‐tissue‐hard‐tissue interfaces. We obtained 12 virtual landmarks from a sample of 30 microCT scans of human crania. Using geometric morphometrics to capture the shape and location, we examined the sphenoid, which interfaces with both the dura mater and the mucosa lining of the sphenoidal paranasal sinus, along with dimensions reflecting the orbits and the palate. Regions were compared through Partial Least Squares analyses and Klingenberg's measures of morphological integration. Given their developmental separation, we predicted the shape and location of the orbits to be independent of variation in the position of the sphenoid, while the sphenoid body and sinus may shift as part of the anterior cranial fossa due to its developmental relationship with brain growth. We also expected the position and shape of the palate to be independent of the spatial position of the sphenoid, though not independent of the orbits given developmental processes mediated through the midface. Results show that most of cranial variance occurs in the anteroposterior length and superoinferior depth for the cranium as a whole. This is reflected by the entire sphenoid body shortening and rotating superoposteriorly along with increases in the cranial base angle (CBA). This morphological variance provides covarying changes in the orbits and palate, which lengthen with increased anteroposterior space for the face. When CBA does not vary, most of the variation is in anterosuperior expansion of the lesser sphenoid wing, and an inferior expansion of the sphenoid body which we hypothesize to be driven by sinus variation. Shifts in the position of the sphenoid results in changes in the anteroposterior length of the anterior cranial fossa, but no anteroposterior changes in the rest of the face. These results indicate that the sphenoid body is significantly modular (p=0.045, RV=0.37) from the facial skeleton, but also has significant covariation (p=0.0034) with the same features that we hypothesize to be driven by interaction between the dura mater and the endocranial fossae making up the basicranium and posterior face. These results preliminarily indicate that variation of the dimensions of the sphenoid body are largely driven through interactions with brain development. This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .