Dynamics of Silurian Plants as Response to Climate Changes

Abstract

The most ancient macroscopic plants fossils are Early Silurian cooksonioid sporophytes from the volcanic islands of the peri-Gondwanan palaeoregion (the Barrandian area, Prague Basin, Czech Republic). However, available palynological, phylogenetic and geological evidence indicates that the history of plant terrestrialization is much longer and it is recently accepted that land floras, producing different types of spores, already were established in the Ordovician Period. Here we attempt to correlate Silurian floral development with environmental dynamics based on our data from the Prague Basin, but also to compile known data on a global scale. Spore-assemblage analysis clearly indicates a significant and almost exponential expansion of trilete-spore producing plants starting during the Wenlock Epoch, while cryptospore-producers, which dominated until the Telychian Age, were evolutionarily stagnate. Interestingly cryptospore vs. trilete-spore producers seem to react differentially to Silurian glaciations—trilete-spore producing plants react more sensitively to glacial cooling, showing a reduction in species numbers. Both our own and compiled data indicate highly terrestrialized, advanced Silurian land-plant assemblage/flora types with obviously great ability to resist different dry-land stress conditions. As previously suggested some authors, they seem to evolve on different palaeo continents into quite disjunct specific plant assemblages, certainly reflecting the different geological, geographical and climatic conditions to which they were subject.

Cryogenian Glacial Habitats as a Plant Terrestrialisation Cradle – The Origin of the Anydrophytes and Zygnematophyceae Split

“…Current data support the hypothesis that in the Ordovician, there were already diversified tiny, possibly polysporangiate, cryptophyte-type floras. During the following Silurian, cryptospores producing plants decline while trilete producers prevail ( Pšenička et al, 2021 ). It is important to note that cryptophytes sensu Edwards ( Edwards et al, 2014 ) are extinct plants producing cryptospores, not to be confused with Cryptophyceae algae.…”

“…Therefore the very first adaptations of Streptophyta to the dry land occurred much earlier, most probably in the Precambrian ( Hedges et al, 2018 ; Servais et al, 2019 ; Shinohara and Nishitani, 2021 ; Strassert et al, 2021 ; Su et al, 2021 ). The first Silurian flora documented by macrofossils (e.g., Cooksonia, Baragwanathia) suggests an undocumented evolution and diversification of land plants preceding the earliest known fossils ( Pšenička et al, 2021 ). We will discuss the potential importance of Cryogenian acritarchs for understanding the evolution of land plants in section “Zygnematophyceae Diverged From Multicellular Anydrophytes Later in the Cryogenian, Adapting to Ice–Dominated Surface” and the “Discussion.”…”

“…The authors concluded that this extended glaciation period was an evolutionary stimulant rather than just a bottleneck ( Hoshino et al, 2017 ). We can thus draw a simplified framework where fossil record shows solid evidence for already complex multicellular land plants (Embryophyta) in the Ordovician/Silurian ( Pšenička et al, 2021 ). Molecular data models extend the possible timing for the origin of the anydrophytes and phylogenetic split between Zygnematophyceae and Embryophyta to either the Mesoproterozoic to early Neoproterozoic ( Su et al, 2021 ), the Cryogenian ( Strassert et al, 2021 ), or the Ediacaran/Cambrian ( Morris et al, 2018a ; see also Figure 1 ).…”

Early Silurian (mid-Sheinwoodian) palynomorphs from the Loděnice-Špičatý vrch, Prague Basin, Czech Republic

“…There is a dramatic exponential increase in diversity of trilete spores from small numbers in the Late Ordovicianearly Silurian, through the remainder of the Silurian (from the Homerian) and into the Devonian (Wellman et al 2013, Pšenička et al 2021. Many cryptospores disappear in the Rhuddanian to Sheinwoodian, and almost no new taxa appear during the Aeronian and the Telychian (Steemans 1999).…”

“…It is possible to recognise two key global plant events during the Silurian as demonstrated mainly by palynological data, e.g. Steemans (2000), Wellman et al (2013) and Pšenička et al (2021). Wellman et al (2013) used more than 1500 occurrences of cryptospores and trilete spores from all palaeocontinents for qualitative and quantitative analysis.…”