The dissipative and self-healing properties of mussel byssal threads are critical for their function as anchoring fibers in wave-battered habitats and central to their emergence as an exciting model system for bio-inspired polymers. Much is now understood about the structure-function relationships defining this remarkable proteinaceous bio-fiber; however, the molecular mechanisms underlying the distinctive tough, viscoelastic and self-healing behavior are still unclear. Here, we investigate elastic and dissipative contributions from the primary load-bearing proteins in the distal region of byssal threads (the preCols) using X-ray diffraction (XRD) combined with in situ tensile testing. Specifically, we identified cross beta-sheet structure in the preCol flanking domains that functions as an elastic framework, providing hidden length. Dissipative behavior was associated with a strain-rate dependent phase transition of a sacrificial network stabilized by strong, reversible cross-links. Based on these findings, we posit a new model for byssal thread deformation and self-healing. (C) 2016 Elsevier Inc. All rights reserved.