Note on the earliest records of genus Macrocephalites Zittel: implications for biostratigraphic correlations

Abstract

ABSTRACT The Bathonian‒Callovian (Middle Jurassic) genus Macrocephalites Zittel is distributed worldwide, and several of its species are used for inter- and intrabasinal biostratigraphic correlations. Here, the earliest occurrence of genus Macrocephalites, using an integrated approach incorporating data from ammonites, calcareous nannofossils and magnetostratigraphy from the Kachchh Basin of western India is presented. The calcareous nannofossils were extracted from the Macrocephalites triangularis Spath (m) sample, and the magnetostratigraphic data also comes from the same ammonite-bearing bed (Yellow Bed, bed A4), in the core of the Jumara Dome, Kachchh Basin. The sample yielded age-diagnostic calcareous nannofossil taxa with FOs (First Occurrence) of Pseudoconus enigma, Cyclagelosphaera margerelii, Octopodorhabdus decussatus and Watznaueria barnesiae, and the LO (Last Occurrence) of Carinolithus magharensis, thus, bracketing the age of the Yellow Bed between early to early middle Bathonian. An early middle Bathonian age was also proposed previously based on an ammonite assemblage and correlated with the Standard European early middle Bathonian Progracilis Zone. The magnetostratigraphic data yielded an early Bathonian age, but, it is of very low resolution, and possibly re-magnetised. The occurrence of M. aff. triangularis Spath from eastern Crimea from below the Bremeri Zone (middle Bathonian), corroborating the early middle Bathonian age arrived at for the earliest occurrence of M. triangularis in the Kachchh Basin, is also evaluated. The stratigraphic occurrence of a heavily encrusted M. triangularis specimen from Germany, and assigned to the lower Callovian, is also evaluated. The present data suggests that genus Macrocephalites Zittel was already well-established even in the early middle Bathonian, if not earlier, and that a relook/resampling is urgently needed with an integrated approach for southern Tethyan margin localities such as Madagascar and Sula Island.