Arcellites punctatus sp. nov.: a new megaspore from the Early Cretaceous of Portugal studied using high resolution synchrotron radiation X-ray tomographic microscopy (SRXTM)

Abstract

A new species of megaspores, Arcellites punctatus sp. nov. and associated microspores, are described from the Early Cretaceous Torres Vedras, Catefica and Buarcos localities in Portugal. The new taxon is most abundant in the Torres Vedras plant assemblages where it co-occurs with several other types of megaspores, remains of aquatic and terrestrial angiosperms as well as other plant fossils. The megaspore body is smooth without appendages, finely perforated and with a rugose surface pattern. High resolution synchrotron radiation X-ray tomographic microscopy (SRXTM) analysis of the megaspore wall shows a three-layered structure. The outer layer is punctate-perforate. Perforations are densely spaced in the valleys between the ridges, while they are more scattered or lacking on the top of the ridges. The middle layer is granular-fibrous and of uneven thickness causing the rugose surface pattern. The inner layer is evenly thin and solid. The outer layer of the megaspore body continues into the elongated, six-parted and inverted cone-shaped acrolamella or neck. Lateral margins of acrolamella segments are laminar and crenulate. Microspores occur attached to the acrolamella of all specimens studied in scanning electon microscopy (SEM) and SRXTM. In general morphology, the megaspores are closely similar to megaspores of Arcellites rugosus from Canada that also lack body appendages, but the Portuguese megaspores are distinguished by their distinct punctate-perforate megaspore wall structure.