Rex studies the relationships between diet and skull shape and structure. His fundamental interests lie in community-level ecology and he explores this through the lens of functional morphology. By employing shape analysis (geometric morphometrics) and biomechanical modelling (finite element analysis), he examines the skull across different species to identify relationships between form and function in the vertebrate masticatory apparatus. In doing so, he aims to illustrate how various adaptations and constraints associated with craniomandibular anatomy can mediate niche dynamics and ultimately influence community structure. His primary research has examined herbivorous diprotodont marsupials, with a focus on kangaroos and their relatives (Macropodiformes).