The Enzonalasporites group of Triassic pollen genera and species: New morphological and ultrastructural data, revised taxonomy and paleobiogeographical aspects

The ‘Enzonalasporites group’ comprises Enzonalasporites, Patinasporites, Vallasporites, Pseudenzonalasporites, Tulesporites, Daughertyspora and Zonalasporites pro parte, and their described species. There is little consensus on the taxonomy of this widely distributed group of mainly Late Triassic pollen despite their importance in biostratigraphy, paleoecology and conifer evolution. Last revised over forty years ago, their taxonomy is plagued by inadequate diagnoses and emendations, inconsistent or inaccurate terminology, and intergrading morphological attributes. In this paper we present new data and interpretations on the morphology, ornamentation and ultrastructure of key species, and use them together with analysis of published images and data to underpin a major review of the group's taxonomy and position in conifer evolution. Confocal laser scanning microscopy (CLSM) shows the raised ridges of rugulae on the proximal face of some pollen grains are impression marks, not trilete marks. Focused ion beam scanning electron microscope (FIBSEM) sectioning of some E. vigens and Patinasporites densus pollen grains shows that overlapping and intertwined rugulae create alveoli in a slightly inflated lateral exinal envelope, or “saccoid”. Dimensions of the corpus, saccoid and rugulae were measured using both SEM and transmitted light microscopy (LM) images. The challenge of intergrading attributes was managed by selecting morphologically stable criteria that can be used with routine LM, an approach which enables most specimens to be assigned taxonomically with confidence, while acknowledging that some may remain problematic. The hierarchy of criteria used in revising the taxonomy were: (1) corpus diameter/total width ratio, (2) dominant sculptural element, (3) presence or absence of impression marks, and (4) saccoid width and characteristics. The first two criteria help discriminate between genera, and the second two between species. After applying these criteria, emending diagnoses and synonymizing or transferring several species, three genera and seven species remain: E. vigens, E. ignacii, E. antonii n.c., Patinasporites densus, Pseudenzonalasporites cinctus n.c., Pseudenzonalasporites leschikii n.c., and Pseudenzonalasporites summus. The Enzonalasporites group reached maximum diversity, abundance and distribution during the Carnian, a stage characterized by several humid episodes, but the details of their paleobiogeographic distribution are complex. E. vigens and E. ignacii seem to have preferred xerophytic or halophytic conditions with occasional wetter episodes (e.g. monsoons), while Patinasporites densus seems to have preferred more hygrophytic conditions. The common thread between these species is their requirement for moisture during germination which required a pollination drop. Paleoenvironmental changes during the latest Triassic may have favored conifers which germinated using siphonogamy.