Morphology of Triprojectate fossil pollen: Form and distribution in space and time

Abstract

During the Late Cretaceous (65–85 million years ago, Ma) Triprojectacites, a group of now-extinct fossil pollen, were among the most dominant elements of Northern Hemisphere circumboreal palynological assemblages. Triprojectates represent an unknown group of parent plants of imprecise albeit angiosperm affinities. Although long considered restricted to the Late Cretaceous, recent reports suggest the group persisted until as late as the Oligocene (30–35 Ma) in Arctic areas. Reaching their evolutionary and geographic acmes during the Campanian to Maastrichtian (70-65 Ma), triprojectates were present on all continents except Antarctica. The plants that produced triprojectates apparently spread from the then-contiguous Siberia-western North America landmass (an area referred to as theAquilapollenites province) during the later Maastrichtian. Several monophyletic groups of triprojectates are recognized, with only one having a possible connection with the modern flora. Despite intensive study, precise morphological delimitation of fossil pollen in the Triprojectacites group has not been achieved in the more than 30 years since their first description. Application of light, scanning, and transmission electron microscopy facilitates more critical examination of these fossil pollen grains in order to define internal and external group boundaries. Triprojectacites can be informally subdivided by body, sculpture, and aperture types. I recognize 14 of these morphological types, some of which have not previously been assigned to triprojectates. In some cases these 14 groups correspond to previously described genera such asAquilapollenites Rouse,Triprojectus Mtchedlishvili,Mancicorpus Mtchedlishvili, andIntegricorpus Mtchedlishvili. In addition I regard four other groups roughly corresponding to the previously described pollen genera: (1)Cranwellia Srivastava; (2)Pentapollenites Krutzsch; (3)Morinoipollenites Zhao & Wang andJianghanpollis Zhao & Wang; and (4)Loranthacites Mtchedlishvili; as requiring additional investigation to document their triprojectate features. These morphological types can, and often do, correspond to previously published genera.