Mid-Devonian Archaeopteris roots signal revolutionary change in earliest fossil forests

Abstract

The origin of trees and forests in the Mid Devonian (393–383 Ma) was a turning point in Earth history, marking permanent changes to terrestrial ecology, geochemical cycles, atmospheric CO ₂ levels, and climate. However, how all these factors interrelate remains largely unknown. From a fossil soil (palaeosol) in the Catskill region near Cairo NY, USA, we report evidence of the oldest forest (mid Givetian) yet identified worldwide. Similar to the famous site at Gilboa, NY, we find treefern-like Eospermatopteris (Cladoxylopsida). However, the environment at Cairo appears to have been periodically drier. Along with a single enigmatic root system potentially belonging to a very early rhizomorphic lycopsid, we see spectacularly extensive root systems here assigned to the lignophyte group containing the genus Archaeopteris. This group appears pivotal to the subsequent evolutionary history of forests due to possession of multiple advanced features and likely relationship to subsequently dominant seed plants. Here we show that Archaeopteris had a highly advanced root system essentially comparable to modern seed plants. This suggests a unique ecological role for the group involving greatly expanded energy and resource utilization, with consequent influence on global processes much greater than expected from tree size or rooting depth alone. Using data from a Middle Devonian fossil soil, Stein et al. report root systems from the earliest intact forest to date, including cladoxylopsids, possibly stigmarians and Archaeopteris. Striking seed plant-like features of the latter indicate a special role for this clade in the profound changes in Earth global systems that took place at that time.

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ORCID for John E.a. Marshall: orcid.org/0000-0002-9242-3646

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