Where the giants lived: the worlds largest ammonite Parapuzosia and its palaeoenvironment
Final Report Abstract
Within this project we documented, collected and determined 330 ammonoids, 230 inoceramid bivalves, ca. 200 other molluscs and ca. 10 echinoids from the Tepeyac section across the Santonian/Campanian boundary. This high number of macrofossils allows for precise biostratigraphic subdivision. It was combined with biostratigraphy on planktonic foraminifers and nannofossils, and with chemostratigraphy, i.e. by δ13C stratigraphy and Sr- isotopic dating, and with cyclostratigraphy. The Gulf Coast ammonoid zonation is now formally defined at Tepeyac. We proposed that faunal interchange across the Atlantic was more intense than previously thought in the early Campanian. Repeated faunal connections to Madagascar and central Europe were also identified and are explained by a gradual increase in oceanic circulation towards the South Atlantic, in particular by submergence of the Walvis Ridge located between Western Africa and Brazil, and by increasing connection between the North and South Atlantic. The ontogeny, evolution and palaeobiogeographical distribution of the world’s largest ammonite Parapuzosia (P.) seppenradensis was revised with the historical specimens from Germany and the United States of America, but also including hitherto unknown specimens from England and Mexico, mostly with stratigraphic information. We documented 66 specimens of Parapuzosia from Tepeyac. The specimens have diameters between 10 and 150 cm and show five ontogenetic stages and size dimorphism, as well as gradual evolution from the Santonian Parapuzosia leptophylla into Parapuzosia seppenradensis in the latest Santonian. An additional significant increase in shell sizes and subsequent decrease in diameters was identified in the lower Campanian. Based on the identification of the five ontogenetic stages in Parapuzosia seppenradensis we concluded that the high number of species names for ammonites from this genus was entirely artificial, and that giant Parapuzosia is monospecific and occurs on both sides of the North Atlantic. >45 specimens were documented from Kent (Santonian) and Sussex (lower Campanian). The detailed stratigraphy of the English Chalk allowed for precise correlation of the sites with Mexico and Germany. Our data also point to a semelparous reproduction strategy in this giant cephalopod, similar to the modern giant squid Architeuthis which also has a wide palaeogeographic distribution combined with a high intraspecific variability. Tepeyac is the only section in which ammonoids and inoceramids are abundant and allow for detailed zonation and can be correlated with the stable carbon isotope record. The detailed multistratigraphic record across the Santonian-Campanian boundary favours Tepeyac as a reference section for the base of the Campanian. It was thus chosen as future Associated Boundary Stratotype Section and Point for the Americas by the International Subcommission on Cretaceous Stratigraphy in Milano, July 2019 and will contribute to the definition of the base of the Campanian stage.
