Rugose corals are thought to have evolved from an ancestral anthozoan during the Middle Ordovician Epoch even though there is a lack of fossil evidence for the early evolutionary history of the Rugosa. Previously documented species of early rugose corals are all assigned to the main orders Calostylina, Streptelasmatina, Cystiphyllina and Stauriina, which had all evolved by the late Sandbian. Lambelasma? sp., a new rugose coral, was recovered from the upper Darriwilian (Middle Ordovician) part of the Shirgesht Formation of Central Iran. One of the fossils, partly embedded in rock matrix, was examined using synchrotron X-ray tomography, which is here demonstrated to be a useful tool in palaeontological taxonomic studies. The new fossils form part of a mid-latitude Gondwana fauna and are the earliest record of rugose corals to date. The specimens combine features of both the Streptelasmatina and Calostylina, but are here assigned to the Lambelasmatidae (Calostylina) on the grounds of a very deep calice, the pinnate arrangement of the septa and a lack of synapticulae and tabulae.