The vast Panthalassa Ocean once surrounded the supercontinent Pangaea. Subduction has since consumed most of the oceanic plates that formed the ocean floor, so classic plate reconstructions based on magnetic anomalies can be used only to constrain the ocean's history since the Cretaceous period(1,2), and the Triassic-Jurassic plate tectonic evolution of the Panthalassa Ocean remains largely unresolved(3,4). Geological clues come from extinct intra-oceanic volcanic arcs that formed above ancient subduction zones, but have since been accreted to the North American and Asian continental margins(4). Here we compile data on the composition, the timing of formation and accretion, and the present-day locations of these volcanic arcs and show that intra-oceanic subduction zones must have once been situated in a central Panthalassa location in our plate tectonic reconstructions(5-7). To constrain the palaeoposition of the extinct arcs, we correlate them with remnants of subducted slabs that have been identified in the mantle using seismic-wave tomographic models(8,9). We suggest that a series of subduction zones, together called Telkhinia, may have defined two separate palaeo-oceanic plate systems-the Pontus and Thalassa oceans. Our reconstruction provides constraints on the palaeolongitude and tectonic evolution of the Telkhinia subduction zones and Panthalassa Ocean that are crucial for global plate tectonic reconstructions and models of mantle dynamics.