?First population-level study of the ammonite genus Hildoglochiceras Spath, and the Lower Tithonian record of the Hildoglochiceras Horizon in the Kachchh Basin, India

A Hildoglochiceras -rich horizon is reported from a thin carbonate intercalation within the siliciclastic Upper Jurassic Jhuran Formation of Jara Dome, western Kachchh Mainland. The specimens have been used for first population-level study genus based on multivariate analysis. High phenotype instability in large sample confirms occurrence transient forms between morphospecies. Key morphological traits interpreting are stated, and morphospecies kobelli (Oppel) H. kobelliforme (Bonarelli) interpreted as dimorphic pair. ammonite-rich level Horizon, which related to transgressive pulse maximum flooding zone interrupting largely restrictive conditions ammonites. endemic character confirmed its environmental restriction shelf areas palaeomargins Trans-Erythraean Trough. comprehensive review biostratigraphic interpretations shows influence natural experimental forcing factors. uppermost Kimmeridgian lowermost Tithonian interval widest range assumable existing reports, but most probably it was restricted to, or at least better represented in, Lower horizons. Horizon described here correlated with lower part Albertinum/Darwini Zone Secondary Standard Scale ammonite-based bio-chronostratigraphy European West-Tethyan areas. According current state knowledge, local rather than wide regional significance favoured records before precise correlation across