INVESTIGATING 'HORNY' TRIPROJECTATE POLLEN

Triprojectates are a biostratigraphically crucial group of angiosperm pollen in studies of Upper Cretaceous to Eocene strata of northern latitudes. This group has a circumpolar distribution and is a key component of Late Cretaceous floras in the Western Canadian Sedimentary Basin and Canadian Arctic. Species belonging to the triprojectate group are morphologically variable and complex and have been classified into eighteen genera. Three of these genera (Parviprojectus, Pseudoaquilapollenites and Novemprojectus) contain species that are known to have subpolar projections or 'horns'. The development of horns is a morphological feature that appears to be unique to the triprojectates and as such inspired a subsequent investigation of 'horny' pollen occurrences and their functional morphology.

Large populations of Parviprojectus trialatus occur in Campanian strata of the Little Bear Formation from the Mackenzie Valley, Northwest Territories and of the Horseshoe Canyon Formation from central Alberta. These populations reveal considerable variation in size, sculpture and shape of the polar domes, which can be rounded or flattened with or without horns. The selection pressure for the shape of the polar domes seems to be independent of latitude as preliminary results suggest the percentage of smooth domes, flattened domes and horns are fairly consistent between populations from Alberta and Northwest Territories. We had originally predicted that the flattened dome was an intermediate morphology between the rounded dome and horned morphologies; however, during the investigation it became clear that horns can develop on domes of either shape. Horned specimens of P. trialatus from the Northwest Territories were more frequently associated with a rounded dome compared to specimens from Alberta, which almost exclusively developed horns on a flattened dome. The two resulting types of horns likely achieved the same function despite slight differences in shape.

The plants that produced triprojectate pollen are interpreted as entomophilous (insect pollinated). We propose that the development of horns on P. trialatus increases the ability of the pollen to latch onto its pollinator; therefore, the functional morphology of these horns is consistent with entomophily.