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This study presents the formulation, the numerical solution, and the validation of a theoretical framework based on the concept of variable-order mechanics and capable of modeling dynamic fracture in brittle and quasi-brittle solids. More specifically, the reformulation of the elastodynamic problem via variable and fractional order operators enables a unique and extremely powerful approach to model nucleation and propagation of cracks in solids under dynamic loading. The resulting dynamic fracture formulation is fully evolutionary hence enabling the analysis of complex crack patterns without requiring any a prior assumptions on the damage location and the growth path, as well as the use of any algorithm to track the evolving crack surface. The evolutionary nature of the variable-order formalism also prevents the need for additional partial differential equations to predict the damage field, hence suggesting a conspicuous reduction in the computational cost. Remarkably, the variable order formulation is naturally capable of capturing extremely detailed features characteristic of dynamic crack propagation such as crack surface roughening, single and multiple branching. The accuracy and robustness of the proposed variable-order formulation is validated by comparing the results of direct numerical simulations with experimental data of typical benchmark problems available in the literature.