Morphology and molecules revealed a remarkable new genus of Lamiaceae, a surprising discovery in south‐eastern Europe

Abstract Petrolamium crnojevicii gen. & sp. nov., a new distinct and remarkable monotypic genus of Lamiaceae, was discovered in the Dinaric karst of southern Montenegro. This taxon exhibits a unique combination of morphological traits, including eglandular, mostly glabrous reniform leaves with 5–7 lobes and prominent hydatodes at their tips. Its small pedicellate flowers feature included anthers and styles within the corolla tube, a straight, narrow, and emarginate to bifurcate posterior corolla lip, and an anterior lip with entire, broadly elliptic‐oblong lateral lobes. The trigonous mericarps have a truncated apex and bear fatty tissue. Notably, Petrolamium has a unique chromosome number of 2 n = 32. A molecular phylogenetic analysis, based on 79 plastid protein‐coding genes and 5 chloroplast markers placed Petrolamium in the subfamily Lamioideae, where it forms a distinct lineage, sister to the other members of the tribe Lamieae. Petrolamium is also unique from an ecological point of view, as it exclusively inhabits floristically depauperate calcareous rock crevices in a very restricted distribution range. This makes P. crnojevicii a critically endangered species according to IUCN criteria and emphasizes the need for immediate conservation efforts. This study highlights the importance of ongoing floristic exploration, demonstrating that even in regions long considered botanically well‐known, important discoveries can still emerge, including within extensively investigated plant families like the Lamiaceae.