Fossil record and new aspects of evolutionary history of Calcium biomineralization and plant waxes in fossil leaves

Abstract

Calcium oxalate (CaOx) is one of the most common bio-minerals in extant plants. It plays a major role in a better understanding of plant taphonomy, environmental effects and evolution of calcium minerals in fossil plants. Although CaOx is common in extant leaves of many plant groups, the fossil record of CaOx in fossil leaves is almost completely unknown, due to the imperfect preservation of CaOx bio-minerals and the disintegration of CaOx crystals during fossilization. Recognition of the remains of CaOx in fossil leaves can be challenging. The record of casts of CaOx in fossil leaves from basal plants, gymnosperms and angiosperms has been established to reconstruct the evolutionary history of CaOx from the Devonian to the Neogene. <br /> The present thesis aims to introduce the different distribution patterns of CaOx traces (individual crystals and druses) in fossil leaves and to compare the substituted or residual chemical components in their casts in fossil leaves from basal to higher plant groups. In order to achieve this goal, both fossil and modern (for extant groups) leaves of seed ferns, pteridophytes (ferns), gymnosperms (cycadales, ginkgophytales, conifera) and angiosperms have been examined. Fossil samples were borrowed from different sites and ages from natural history museums in Germany. Extant samples were collected in the Botanical Garden, University of Bonn. Light microscopy (LM) and Scanning Electron Microscopy (SEM) were utilized to clarify the distribution of CaOx casts in both fossil and fresh leaves. <br /> The results show that CaOx is almost non-existent in modern ferns. In seed ferns, the casts of former CaOx crystal druses were observed under a carbon layer in a regular distribution pattern with a size of approximately 20 µm. CaOx in cycads and ginkgophytes appeared as aggregated forms or druses in the phloem. Druses in ginkgophyte measured up to 100 µm. Conifera show a different trend: crystals occur as individuals and more or less small size (10-12 µm) under the cuticle. In angiosperms CaOx appears commonly and in diverse forms. <br /> In conclusion, the presence of the CaOx as an important chemical factor in plants is reported from primitive seed ferns to the modern gymnosperms and angiosperms. The distributional pattern of CaOx in each major group of plants is different and can be affected by environmental factors and physiological aspects in plants.