Lycopsid forests in the early Late Devonian paleoequatorial zone of Svalbard

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 Plantekløfta 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 Høeg. 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.

Phylogenomics indicates the “living fossil” Isoetes diversified in the Cenozoic

“…This assessment is based on shared synapomorphies including bipolar growth from a shoot like "rhizomorph" structure and secondary woody tissue [48]. Arborescent lycopsids are known from the Frasnian [122,123] (382.7-372.2 Ma), although the rhizomorph root structure could not be identified in these early fossils. However, discovery of a putatively homosporous arborescent lycopsid (the Isoetales are heterosporous) suggests that arborescence could be a convergent phenotype within the lycopods [124].…”

The antiarch fish Asterolepis orcadensis from the Scottish Middle Devonian

“…As regards the Spitsbergen form Asterolepis scabra, it is found in the Fiskløfta Member of the Tordalen Formation in the Mimer Valley. There has been some debate on the age of these deposits (see Berry and Marshall, 2015;Piepjohn and Dallmann, 2014) but Newman et al (2019) have shown without doubt that the Fiskeløfta Member is stratigraphically higher than the Rousay Flagstone Formation, notwithstanding the morphological differences between the two species pointed out above. The other species of Asterolepis can all be FIGURE 17.…”

Devonian–Mississippian collapse and core complex exhumation, and partial decoupling and partitioning of Eurekan deformation as alternatives to the Ellesmerian Orogeny in Spitsbergen

“…Ellesmerian contraction, which is believed to have initiated after the deposition of the Fiskekløfta Formation in the late-latest Givetian (i.e., latest Middle Devonian; Berry and Marshall, 2015) and terminated prior to the deposition of middle-late Famenian-Mississippian (Scheibner et al, 2012; 1080 https://doi.org/10.5194/se-2019-200 Preprint. Discussion started: 13 January 2020 c Author(s) 2020.…”