The Unilateral Torque Hypothesis for Proto‑Wing Function in Terrestrial Theropods

The Wing as a Torque-Vectoring Organ: A New Biomechanical Paradigm How did the proto-wing survive millions of years of selection before it could ever clear the ground? The Unilateral Torque Hypothesis shifts the focus from the limits of bilateral lift to the mechanical advantages of asymmetric force. By treating the feathered forelimb as a high-leverage atmospheric anchor, this technical note demonstrates how terrestrial theropods could generate whole-body angular momentum that no unwinged rival could match. Key highlights include: • The Fulcrum Effect: A detailed look at how unilateral wing strokes coupled with ground reaction forces (GRFs) at the stance foot to create rapid, sub-millisecond heading changes. • Mechanical Leveraging: An analysis of how the long lever arms of proto-wings transformed modest aerodynamic resistance into significant rotational torque (τ = F r). • The Level 4 Transition: Integrating this mechanism into a progressive model of kinetic energy harvesting—moving from vibrotactile sensing to active torque-vectoring. • Quantifiable Falsification: Clear, testable predictions for the fossil record, including deltopectoral crest/hindlimb correlations and specific trackway asymmetries. This paper provides the "missing link" in functional morphology, offering a continuous evolutionary pathway where the wing’s primary value was not in escaping the earth, but in dominating it.