From the Ancient Tethys to Modern Mountain Springs: A New Operculum‐Projection‐Bearing Family of Truncatelloidea

ABSTRACT The superfamily Truncatelloidea represents one of the most diverse radiations among microgastropods, yet its systematics remains challenging due to extensive morphological homoplasy and small body size. Yunnan Province in south‐western China, located at the intersection of three global biodiversity hotspots, is known for its high endemism of freshwater truncatelloideans. However, most taxonomic surveys have focused on ancient lake systems, leaving mountain spring streams largely unexplored. Here, we describe a new truncatelloidean species from a mountain spring stream in Kunming City, Yunnan, based on an integrative taxonomic approach combining shell morphology, radular characters, operculum morphology and multi‐locus phylogenetic analyses (COI, 16S, 18S, 28S and H3). The new species possesses distinct dorsal projections on the operculum, a morphological trait previously considered a synapomorphy of the families Squamapicidae and Stenothyridae. Phylogenetic analyses, including mitogenome data, strongly support the placement of the new species in a separate clade (PP = 1/BS = 99) that is only distantly related to Squamapicidae, warranting recognition at the family level. Accordingly, we erect Chiyanidae fam. nov. (type genus: Chiyana gen. nov.) and describe Chiyana yilong sp. nov. Time‐calibrated phylogenetic analysis estimates that the common ancestor of Chiyanidae, Squamapicidae and Stenothyridae—the three families characterized by opercular projections—originated at approximately 138.6 Ma; Chiyanidae and Squamapicidae subsequently diverged at around 97.3 Ma. These divergence times coincide with the Late Jurassic to Early Cretaceous closure of the Tethyan Bangong Lake–Nujiang Ocean and subsequent uplift of the Yunnan–Guizhou Plateau. Our findings highlight that mountain spring streams in Yunnan harbour previously unrecognized, relict lineages of Truncatelloidea and underscore the importance of integrating microhabitats beyond ancient lakes for a more complete understanding of the region's hidden biodiversity and evolutionary history.