Scanning Electron Microscopy and Synchrotron Radiation X-Ray Tomographic Microscopy of 330 Million Year Old Charcoalified Seed Fern Fertile Organs

Abstract

Abundant charcoalified seed fern (pteridosperm) pollen organs and ovules have been recovered from Late Visean (Mississippian 330 Ma) limestones from Kingswood, Fife, Scotland. To overcome limitations of data collection front these tiny, sometimes unique, fossils, we have combined low vacuum scanning electron microscopy on uncoated specimens with backscatter detector and synchrotron radiation X-ray tomographic microscopy utilizing the Materials Science and TOMCAT beamlines at the Swiss Light Source of the Paul Scherrer Institut. In combination these techniques improve upon traditional cellulose acetate peel sectioning because they enable study of external morphology and internal anatomy in multiple planes of section on a single specimen that is retained intact. The pollen organ Melissiotheca shows a basal parenchymatous cushion bearing more than 100 sporangia on the distal face. Digital sections show the occurrence of pollen in some sporangia. The described ovule is new and has eight integumentary lobes that are covered in spirally arranged glandular hairs. Virtual longitudinal sections reveal the lobes are free above the pollen chamber. Results are applied in taxonomy and will subsequently contribute to our understanding of the former diversity and evolution of ovules, seeds, and pollen organs in the seed ferns, the first seed-bearing plants to conquer the land.