Molecular and mineral biomarker record of terrestrialization in the Rhynie Chert

Abstract

The colonization of the terrestrial surface by land plants involved the evolution of a complex ecosystem of plants, animals, fungi, algae and bacteria, within a mineral framework. The record of this advance is highly fragmentary and uncertain. However, a wealth of fossil evidence is preserved in the Lower Devonian Rhynie Chert lagerstätte, which is consequently considered to be the world's oldest preserved terrestrial ecosystem. The physical record of fossils in the Rhynie Chert is accompanied by a chemical record of molecular biomarkers and mineral diagenesis. The biomarkers include components derived from vascular and other higher plants, and decomposers fungi and bacteria. The chert contains the earliest record of the plant diterpane ent-beyerane, identified in this study. The biomarker profile is distinct from that in Lower Devonian organic-rich shales at Strathpeffer, 100 km to the west, where plants are not preserved and the environment was hypersaline. In addition to organic biomarkers, the chert contains mineralogical characters which imply biological activity, including pyrite framboids, strongly leached monazite and garnet, and pitted micas similar to grains altered by modern fungi. The molecular and mineral biomarkers combine with the fossil record and phylogenetic data in the toolbox available to document plant colonization and symbiosis on the terrestrial surface.