Triprojectacites in the Songliao Basin, Northeast China: Systematics, biostratigraphy and evolution

Triprojectate pollen plays an important role in the division and correlation of the Upper Cretaceous strata for their short stratigraphic distribution, high diversity and rapid evolution rate. Long time scale material with good age constraints that is highly valuable for studying its origin, evolution and extinction is particularly rare. In this study, we report a successive sequence of Triprojectacites evolving since its origin through the extinction during the Late Cretaceous in the Songliao Basin, northeastern China. We applied a combined observation technique using both optical microscope and scanning or transmission electron microscope on single pollen grain for the morphological observation and systematic classification. A total of seven genera, thirty-five species (including two new species, Aquilapollenites prenormalis sp. nov. and Bratzevaea pumila sp. nov.) and one conformis species are described. The occurrence and range of specific species are precisely logged on the core sectivon. Five development phases of the Triprojectacites palynoflora are recognized, ascendingly: I. Occurring phase (the earliest Campanian), II. Radiating phase (the early Campanian), III. Standing phase (the middle–late Campanian), IV. Flourishing phase (the latest Campanian–the early Maastrichtian), and V. Fading phase (the middle–late Maastrichtian). The record shows that Triprojectacites in the Songliao Basin has experienced a morphological evolution of being larger in size, more robust and complicated in ornamentation, and the development of accessory structures. The latter, especially the birth of collar ring, is indicative of the Maastrichtian in age.