PALEOBOTANICAL APPLICATIONS OF SYCHROTRON X-RAY TOMOGRAPHIC MICROSCOPY: ENHANCED VISUALIZATION AND UNDERSTANDING OF EOCENE PLANT LAGERSTÄTTEN

There are many important Eocene plant lagerstätten, including the Clarno Nut Beds (Oregon), Princeton Chert and Appian Way (British Columbia), London Clay (UK) and Messel World Heritage Site (Germany) that all preserve fruits and seeds with cellular detail. However, challenges in fully understanding diversity, evolution, paleoenvironments, and biogeography of these floras arise from several factors, such as the rarity of some morphotypes, difficulties in comparing different modes of preservation, and lack of modern comparative data. X-ray tomography provides a novel, non-destructive method for reexamining fossils, providing 3D data with both internal anatomy and external morphology. Taxonomically informative internal structures (e.g., resin canals, locules) can be visualized. Comparative data for modern fruits and seeds can be quickly obtained and studied in multiple planes of section. These datasets can also be virtually dissected, mimicking taphonomic processes like abrasion and decomposition, to reveal hidden or cryptic morphologies. Some of the many benefits of using SRXTM for fossil plants can be illustrated with a few case studies. (1) Synchrotron X-ray tomographic microscopy (SRXTM) and CT can confirm or determine affinities of fossils. Polycarpella from the London Clay is a sedge. Arillate seeds from Messel reveal a suite of features consistent with Strelitziaceae. (2) Seeds from the UK thought to be sedges were subsequently found in situ in distinctive discoid fruits from Messel that have been identified as Cyclanthus, a monotypic tropical genus found today in Central and South America. Virtual taphonomy of modern Cyclanthus seeds shows that fossilization resulted in an altered morphology that was taxonomically misleading. (3) Caricoidea sedge fruits are widespread in the Cenozoic of Europe, but their affinities within the sedge family were unclear. Exceptionally preserved fruiting heads from Messel, studied with SRXTM, revealed affinities with subfamily Mapanioideae. These are understory sedges growing in tropical forests, and therefore the Eocene fossils do not represent the more open vegetation occupied by many sedges today. These examples show the potential of SRXTM for providing new data for systematics, environmental interpretations, and investigating taphonomy.