Lichen fossils display diverse adaptations

Over 150 new lichen fossils contained in amber are reported in a paper published online this week in Nature Plants, which validates the prevailing view that central Europe was humid and covered with temperate forests during the Palaeogene, 47-24 million years ago. The discovery of the 152 new fossils increases the total number of known fossil lichens 11-fold, from 15 to 167.

Lichens are organisms that form through a symbiotic relationship in which fungi host one or more types of green algae and/or cyanobacteria. The fungi benefit from the algae/cyanobacteria because they produce energy through photosynthesis, and the algae/cyanobacteria benefit from the protective environment and water provided by the fungi. The discovery of fossilized lichens has been extremely rare, with only 15 unambiguous fossils described, until now.

Jouko Rikkinen and colleagues show that the new lichen fossils, found in two sites and dating from the European Palaeogene period, display a variety of structures and growth forms related to this highly specialized symbiosis. Many of the adaptations pertain to water absorption, reproduction, and gas exchange to aid photosynthesis, evolving to fit an intricate balance of hydration, photosynthesis and respiration in their particular environment. These structures further establish a prevailing view that central Europe was not covered with dense tropical forests during this time period, but instead with temperate forests that received more light and gave rise to these diverse forms.