Covariance and Modularity in Rhesus Macaque (Macaca mulatta) Thoracolumbar Vertebrae

Abstract

ABSTRACT COVARIANCE AND MODULARITY IN RHESUS MACAQUE (MACACA MULATTA) THORACOLUMBAR VERTEBRAE by Ashley Smith The University of Wisconsin-Milwaukee, 2024 Under the Supervision of Professor Emily R. Middleton Elucidating the evolutionary processes governing complex biological architecture remains a central pursuit in scientific inquiry. The vertebral column is a novel region for anatomical and evolutionary analysis since it is highly conserved and critical to locomotion and posture in mammals. The thoracolumbar region, a transitional zone characterized by distinct biomechanical demands and functional requirements, provides a valuable framework for examining patterns of morphological variation, covariance, and modularity. The purpose of this thesis was to assess covariation of transitional vertebrae at the thoracolumbar boundary and to provide a unique perspective on variation at the diaphragmatic and thoracic-lumbar junction. This research employed 3D geometric morphometric analyses to investigate covariation and modularity in thoracolumbar vertebrae (T10-L3) of Rhesus macaques, a taxon that shares significant similarity in vertebral morphology with humans. By examining spinous processes, transverse processes, and facet morphology, this research sought to elucidate the mechanisms shaping vertebral variation and evolution. Results from statistical analyses revealed that the spinous and transverse processes exhibit substantial variation both within individual vertebrae and between vertebral types, supporting that these regions are highly variable in thoracolumbar boundary vertebrae. Contrary to expectations, a higher thoracic count (T13) did not demonstrate a more gradual transition in zygapophysis orientation, likely due, in-part, to the low frequency of nonmodal thoracic formulae in the sample. Facet morphology was found to covary less strongly with other regions in caudal thoracic compared to lumbar vertebrae, with integration increasing from T10-L3. These findings contribute to the growing literature on covariation and modularity in nonhuman primates, shedding light on the intricate interactions between developmental, functional, and evolutionary factors that shape vertebral morphology and diversification. Future research should expand on these findings by investigating the relationship between osteoarthritis prevalence and facet orientation, as well as the heritability of nonmodal positioning of the diaphragmatic vertebra.