Cyst size trends in the genus Leiosphaeridia across the Mulde (lower Silurian) biogeochemical event

Abstract

The upper Wenlock epoch (Homerian age) of the Silurian period was an interval of intense changes in biotic composition, oceanic chemistry and sea level, which also witnessed a double-peaked positive stable carbon isotopic excursion. These biotic and environmental perturbations are thought to have originated due to reorganizations of the ocean system and high-amplitude sea level fluctuations. However, the evolutionary responses of the size of the micro-phytoplankton, which would help comprehension of the oceanographic mechanisms of these global perturbations, are currently unknown. Therefore, in this contribution we present morphometric data on the size changes of cysts of the dominant acritarch genus Leiosphaeridia during the middle and upper parts of the Homerian, which includes the lundgreni and Mulde bioevents, from the deep shelf facies of the eastern Baltic Basin (western Lithuania, Viduklë-61 core). Three parameters were measured for size distributions. Those are namely: average size, range of sizes, and power law exponent, which measures degree of "heavy-tailedness" and thus complexity of the distribution of cyst sizes. The average of the cyst sizes increased in the post-lundgreni interval of the Homerian, which points to the fundamental shift in the acritarch communities. The uncovered trends in cyst size ranges and power law exponents of their cyst size distributions revealed their close correspondence to the 4 th order sea level fluctuations. Probable paleoclimatic and paleoecological mechanisms for this connection are presented. • Key words: Acritarchs, green algae, Silurian, Mulde Event, size evolution, Eastern Baltic. Lithuania; andrej.spiridonov@gf.vu.lt, agne.aleksiene@gmail.com, sigitas.radzevicius@gf.vu.lt The Silurian period was one of the least stable time intervals throughout the entire Phanerozoic (Melchin et al. 2012). It is characterized by a set of conodont and graptolite extinctions, extirpations (regional disappearances), as well as general turnover events

Dynamic ecophenotypy in the Silurian Monograptidae (Graptolithina)

“…This is particularly relevant due to the presence of annuli in specimens of Colonograptus praedeubeli from the low-palaeolatitude Canadian High Arctic (Lenz & Kozłowska-Dawidziuk 1998) and the apparent absence of annuli in contemporaneous high-palaeolatitude material from Bolivia (Maletz et al 2003), indicating that the formation of annuli may have been unique to tropical monograptids. Moreover, the low-stand at the end of the lundgreni Zone and beginning of the parvus Zone was associated with the extreme dominance of very small plankton, which also confirms the dominance of oligotrophic and low-productivity conditions at low-stands/ lower values of natural gamma curves in the Baltic Basin (Spiridonov et al 2017b). Only one genus (Leiosphaeridia) was studied for variation in size by Spiridonov et al (2017b), due to it possessing a simple spherical cyst shape, which eases calculations.…”

“…Moreover, the low-stand at the end of the lundgreni Zone and beginning of the parvus Zone was associated with the extreme dominance of very small plankton, which also confirms the dominance of oligotrophic and low-productivity conditions at low-stands/ lower values of natural gamma curves in the Baltic Basin (Spiridonov et al 2017b). Only one genus (Leiosphaeridia) was studied for variation in size by Spiridonov et al (2017b), due to it possessing a simple spherical cyst shape, which eases calculations. Although just a part of the overall assemblage, Leiosphaeridia absolutely dominated the microphytoplankton communities, constituting approximately 85 % of all preserved individual organic microfossils (19,091 out of 22,338 counted individuals; reported in Venckutė-Aleksienėet al 2016).…”

“…Moreover, the low-stand at the end of the lundgreni Zone and beginning of the parvus Zone was associated with the extreme dominance of very small plankton, which also confirms the dominance of oligotrophic and low-productivity conditions at low-stands/lower values of natural gamma curves in the Baltic Basin (Spiridonov et al . 2017b). Only one genus ( Leiosphaeridia ) was studied for variation in size by Spiridonov et al .…”

“…This model is created from the high resolution (∼0.1 m) natural gamma ray log of the Viduklė-61 core (Radzevičius et al 2014) and the extraction of statistically significant frequencies using phase-preserving Gaussian filtering (Radzevičius et al 2017). The major pattern of sea level change is widely recognised to be of global significance and is associated with the positive late Homerian δ 13 C excursions and major biotic perturbations (Cramer et al 2011;Venckutė-Aleksienėet al 2016;Spiridonov et al 2017b).…”

Graptolite turnover and δ13Corg excursion in the upper Wenlock shales (Silurian) of the Holy Cross Mountains (Poland)

“…Although it is debatable whether the Mulde Event had a significant impact on conodont communities (e.g., Radzevičius et al 2014c;Jarochowska et al 2018), it has been determined that, at least in Baltica, the community structure and abundance fluctuation patterns radically changed as a consequence of the events (Spiridonov 2017;Spiridonov et al 2017a). The impact on the microphytoplankton communities of the Mulde Event is not so clear and the changes in their taxonomic composition and the size distribution probably were more related to sea level changes (Porębska et al 2004;Venckutė-Aleksienė et al 2016;Spiridonov et al 2017b).…”