Lichen preservation in amber: morphology, ultrastructure, chemofossils, and taphonomic alteration

C. Hartl,J. Heinrichs,A. R. Schmidt,C. Gröhn,U. Kaasalainen,J. Rikkinen,L. J. Seyfullah,N. Schäfer

doi:10.5194/fr-18-127-2015

Abstract

Abstract. The fossil record of lichens is scarce and many putative fossil lichens do not show an actual physiological relationship between mycobionts and photobionts or a typical habit, and are therefore disputed. Amber has preserved a huge variety of organisms in microscopic fidelity, and so the study of amber fossils is promising for elucidating the fossil history of lichens. However, so far it has not been tested as to how amber inclusions of lichens are preserved regarding their internal characters, ultrastructure, and chemofossils. Here, we apply light microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and Raman spectroscopy to an amber-preserved Eocene lichen in order to gain information about the preservation of the fossil. The lichen thallus displays lifelike tissue preservation including the upper and lower cortex, medulla, photobiont layer, apothecia, and soredia. SEM analysis revealed globular photobiont cells in contact with the fungal hyphae, as well as impressions of possible former crystals of lichen compounds. EDX analysis permitted the differentiation between halite and pyrite crystals inside the lichen which were likely formed during the later diagenesis of the amber piece. Raman spectroscopy revealed the preservation of organic compounds and a difference between the composition of the cortex and the medulla of the fossil.

Highlights

The fossil record of lichens is scarce, which is due to the fact that there are few taphonomic chances of conserving the minute structures that define a lichen (Rikkinen, 2003)
In the Early Devonian Cyanolichenomycites devonicus (Honegger et al, 2013) the symbiosis is established between an ascomycete mycobiont and cyanobacteria that are reminiscent of the extant genus Nostoc, whereas Chlorolichenomycites salopensis from the Early Cretaceous of Vancouver Island has green algae as photobionts (Matsunaga et al, 2013)
Our study revealed that amber inclusions of lichens can be morphologically, ultrastructurally, and chemically preserved

Summary

Introduction

The fossil record of lichens is scarce, which is due to the fact that there are few taphonomic chances of conserving the minute structures that define a lichen (Rikkinen, 2003). The oldest putative lichen fossil is 2.5 billion years old (Neoarchean) and consists of agglutinated hyphae around a central cord (Hallbauer et al, 1977); no photobionts were observed to conclusively prove a lichen nature (Matsunaga et al, 2013). 410-million-year-old Rhynie chert is a cyanolichen that has two different cyanobacteria as photobionts (Taylor et al, 1997; Karatygin et al, 2009). In the Early Devonian Cyanolichenomycites devonicus (Honegger et al, 2013) the symbiosis is established between an ascomycete mycobiont and cyanobacteria that are reminiscent of the extant genus Nostoc, whereas Chlorolichenomycites salopensis from the Early Cretaceous of Vancouver Island has green algae as photobionts (Matsunaga et al, 2013)

Methods

Results

Discussion

Conclusion