Double fossilization in eukaryotic microorganisms from Lower Cretaceous amber

Ana Martín-González,Jacek Wierzchos,Juan-Carlos Gutiérrez,Jesús Alonso,Carmen Ascaso

doi:10.1186/1741-7007-7-9

Abstract

BackgroundMicrofossils are not only useful for elucidating biological macro- and microevolution but also the biogeochemical history of our planet. Pyritization is the most important and extensive mode of preservation of animals and especially of plants. Entrapping in amber, a fossilized resin, is considered an alternative mode of biological preservation. For the first time, the internal organization of 114-million-year-old microfossils entrapped in Lower Cretaceous amber is described and analyzed, using adapted scanning electron microscopy in backscattered electron mode in association with energy dispersive X-ray spectroscopy microanalysis. Double fossilization of several protists included in diverse taxonomical groups and some vegetal debris is described and analyzed.ResultsIn protists without an exoskeleton or shell (ciliates, naked amoebae, flagellates), determinate structures, including the nuclei, surface envelopes (cortex or cytoplasmic membrane) and hyaloplasm are the main sites of pyritization. In protists with a biomineralized skeleton (diatoms), silicon was replaced by pyrite. Permineralization was the main mode of pyritization. Framboidal, subhedral and microcrystalline are the predominant pyrite textures detected in the cells. Abundant pyritized vegetal debris have also been found inside the amber nuggets and the surrounding sediments. This vegetal debris usually contained numerous pyrite framboids and very densely packed polycrystalline pyrite formations infilled with different elements of the secondary xylem.ConclusionEmbedding in amber and pyritization are not always alternative modes of biological preservation during geological times, but double fossilization is possible under certain environmental conditions. Pyritization in protists shows a quite different pattern with regard to plants, due to the different composition and cellular architecture in these microorganisms and organisms. Anaerobic sulphate-reducing bacteria could play a crucial role in this microbial fossilization.

Highlights

Microfossils are useful for elucidating biological macro- and microevolution and the biogeochemical history of our planet
Little detailed work has been published on pyrite in fossils, three grades of biological preservation by pyritization have been recognized [5,9]: 1) permineralization, involving pyrite precipitation in cellular cavities or cell walls made of poorly biodegradable components such as cellulose and chitin; 2) formation of mineral coats, which is usually involved in the preservation of very degradable biological components
We have studied in detail, by scanning electron microscopy (SEM)-BSE associated to energy dispersive X-ray spectroscopy (EDS) microanalysis, the degree of biological conservation and the mechanism/s involved in the preservation of the eukaryotic microorganisms embedded in this ancient amber

Summary

Introduction

Microfossils are useful for elucidating biological macro- and microevolution and the biogeochemical history of our planet. A major problem for understanding the origin of life, microbial evolution and phylogeny is the lack of microbial fossils It is especially evident when we consider the available well-preserved record of pluricellular organisms, animals and plants [1]. Little detailed work has been published on pyrite in fossils, three grades of biological preservation by pyritization have been recognized [5,9]: 1) permineralization, involving pyrite precipitation in cellular cavities or cell walls made of poorly biodegradable components such as cellulose and chitin; 2) formation of mineral coats, which is usually involved in the preservation of very degradable biological components. The main difference between these three modes of preservation by pyritization is the extent of mineral precipitation

Methods

Results

Discussion

Conclusion