The Triassic pollen genus Camerosporites: New morphological and ultrastructural data, revised taxonomy and paleobiogeographical aspects

Abstract

This paper presents new morphological and ultrastructural data on Camerosporites pollen obtained from a range of microscopy techniques, and uses these (and other) data to emend the taxonomy of the genus, and review aspects of the paleobiology of its parent plants and their significance in conifer evolution. Camerosporites, a circumpolloid genus probably produced by cheirolepids, was most abundant and widely distributed in low to middle latitudes during the Carnian before declining and becoming extinct in the latest Triassic or perhaps Early Jurassic. The Cheirolepidiaceae, a large and diverse family of early conifers, are characterized by their unique combination of pollen attributes including a rimula, polar tenuitates, exinal threads, verrucae, tetrads/dyads, and exinal and sculptural variability. Confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) images show most C. secatus pollen grains have a clear proximal rimula and some a probable distal rimula. No convincing specimens with two rimulae were seen. Images from focused ion beam scanning electron microscopy (FIBSEM) show that the verrucae are essentially hollow chambers with thickened outer walls and floored by an irregular reticulate-like network of fine, elongated to granular exinal elements. These images suggest C. secatus pollen grains have relatively “primitive” ultrastructure for a circumpolloid – homogenous nexine, (some) granulae in the infratectum, and an indistinct or absent tectum. Camerosporites was able to thrive in the periodically humid Carnian, clearly demonstrating the vegetative organs of its parent plants were able to adapt to a wide range of paleoenvironments. Nevertheless, Camerosporites may have been out-competed for increasingly scarce water resources in the drying Norian-Rhaetian by other producers of circumpolloid pollen and early conifers with distinct pollination strategies possibly related to siphonogamy. We synonymized C. pseudoverrucatus and C. spissus with C. secatus as they are hard to differentiate consistently given their variability and intergrading attributes. We retained C. verrucosus and, tentatively, C. reductiverrucatus and C. hengyangensis. C. reductiverrucatus can plausibly be interpreted as reworked and degraded C. secatus rather than a different species.