Lycopsid forests in the early Late Devonian paleoequatorial zone of Svalbard

Christopher M Berry,John E.A Marshall

doi:10.1130/g37000.1

Christopher M Berry, John E.A Marshall

Open Access

https://doi.org/10.1130/g37000.1

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Abstract

The Middle to early Late Devonian transition from diminutive plants to the first forests is a key episode in terrestrialization. The two major plant groups currently recognized in such “transitional forests” are pseudosporochnaleans (small to medium trees showing some morphological similarity to living tree ferns and palms) and archaeopteridaleans (trees with woody trunks and leafy branches probably related to living conifers). Here we report a new type of “transitional” in-situ Devonian forest based on lycopsid fossils from the Planteklofta Formation, Munindalen, Svalbard. Previously regarded as very latest Devonian (latest Famennian, 360 Ma), their age, based on palynology, is early Frasnian (ca. 380 Ma). In-situ trees are represented by internal casts of arborescent lycopsids with cormose bases and small ribbon-like roots occurring in dense stands spaced ∼15–20 cm apart, here identified as Protolepidodendropsis pulchra Hoeg. This plant also occurs as compression fossils throughout most of the late Givetian–early Frasnian Mimerdalen Subgroup. The lycopsids grew in wet soils in a localized, rapidly subsiding, short-lived basin. Importantly, this new type of Middle to early Late Devonian forest is paleoequatorial and hence tropical. This high-tree-density tropical vegetation may have promoted rapid weathering of soils, and hence enhanced carbon dioxide drawdown, when compared with other contemporary and more high-latitude forests.

Highlights

The evolution of trees in the Middle and LateDevonian was a key stage in the biological evolution of the Earth system (e.g., Le Hir et al, 2011) affecting the atmosphere, pedogenic processes, sediment transport, and terrestrial microenvironments
Archaeopteridaleans became pre-eminent in the Late Devonian, but were present in the Givetian (Cornet et al, 2012) including probable in-situ roots (Mintz et al, 2010)
While studying the paleobotany and palynology of the Devonian Mimerdalen Subgroup on Spitsbergen in Svalbard (Fig. 1C), we reinvestigated reports and made new discoveries of in-situ lycopsids formerly attributed to Archaeosigillaria from the Plantekløfta Formation in Munindalen (Fig. 1B), previously considered to be of latest Devonian age (Dallmann et al, 2004; Piepjohn and Dallmann, 2014)

Summary

Introduction

The evolution of trees in the Middle and LateDevonian was a key stage in the biological evolution of the Earth system (e.g., Le Hir et al, 2011) affecting the atmosphere, pedogenic processes, sediment transport, and terrestrial microenvironments. Spore assemblages from individual cality AF1, upright trunk with slightly flared mudstone clasts in forest bed conglomerates base. J,K: Locality AF2 Sandstone cast base range from Early Devonian to Givetian in age.

Results

Conclusion