Morphometric analysis of Chattian–Early Aquitanian Miogypsinidae from Iraq and their stratigraphic distribution in the Arabian Tethys

Abstract

A biometric study of Chattian–Early Aquitanian taxa from shallow marine transgressive carbonate deposits in the Oligo-Miocene sedimentary basin of Kirkuk, Iraq, provides new insight into their taxonomy, stratigraphy, and palaeobiogeography. Their evolution is based on a distinct change in general nepiont morphology, which agrees with the principle of nepionic acceleration. Each Miogypsinoides group is represented by more than one lineage. We demonstrate for the first time the parallel evolution of Miogypsina and Miogypsinoides in the late Chattian of north-eastern Iraq. We assign specimens into two lineage groups: Miogypsinoides formosensis of the Late Chattian, with smaller embryon and longer post-embryonic spirals, which evolved into Miogypsinoides bantamensis of the Early Aquitanian; and Miogypsinoides sivasensis, with shorter spirals and larger embryon, originally found in late Chattian to Early Aquitanian deposits. This group evolved into large embryon and longer post-embryonic spiral specimens of the upper Azkand Formation, assigned to Miogypsinoides dehaartii and associated with Miogypsina gunteri-basraensis and M. gunteri due to their peculiar morphological features. Miogypsina specimens are related to Miogypsina basraensis, which have much smaller embryon dimensions. This part of the sequence corresponds to Ms. formosensis and is overlain by a Miogypsina ex. interc. gunteri-basraensis succession, a composite species that evolved to Miogypsina gunteri, the most common Miogypsinids taxon in the carbonate upper Azkand Formation, thereby acting as a reference for the evolution of this taxon and its transition to M. ex. interc. tani-gunteri. Miogypsinoides evolution in the Iraqi sedimentary basin is similar to Western Tethys evolution yet differs from Indo-Pacific evolution.