New insights on the palaeobiology and biostratigraphy of the acritarch Trachyhystrichosphaera aimika: A potential late Mesoproterozoic to Tonian index fossil

The late Mesoproterozoic to Tonian (∼1100 Ma to ∼720 Ma) witnessed a critical evolutionary transition in Earth history. Several fossil taxa, including acritarchs and vase-shaped microfossils, have been suggested as potential diagnostic fossils for this time interval. The acanthomorphic acritarch Trachyhystrichosphaera aimika has become a focus of recent biostratigraphic investigations, showing great potential to assist the definition of the Tonian Global Boundary Stratotype Section and Point (GSSP). Although T. aimika has been extensively reported in the Proterozoic sequences, little is known about its phylogenetic interpretation. Its palaeogeographic and stratigraphic distributions need to be critically scrutinized with recently emended diagnosis of the taxon and new published age constraints. In this study, we report new palaeobiological data of T. aimika specimens from the Tonian Liulaobei Formation in the Huainan region, North China, using transmitted light microscopy, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Our analyses reveal a multilayered ultrastructure from the vesicle wall of T. aimika that has experienced advanced diagenesis to low-grade metamorphism with the peak palaeotemperature of ∼212 °C. In addition, a compilation of palaeogeographic occurrences of T. aimika shows the taxon is preserved in a wide range of palaeoenvironments and has a broad palaeogeographic distribution with a relatively limited stratigraphic range from ∼1150 Ma to ∼720 Ma, suggesting that T. aimika has great potential to become an index fossil for the late Mesoproterozoic to Tonian interval.