Acritarchs of the Mesozoic of Ukraine
Authors: Shevchuk Olena , Ivanchenko Kateryna 
Published: September 2022

Introduction. Acritarchs are one of the orthostratigraphic groups of microfossils that are widely used in Proterozoic and Paleozoic biostratigraphy. In the Mesozoic period there is a decrease in this group, and this is due to certain reasons.

Formulation of the problem. Acritarchs are studied by palynologists from samples of Mesozoic sediments in combination with other representatives of organic bone microplankton, primarily with dinocysts. In the practice of Ukrainian micropaleontologists, the role of such a group as acritarchs, which may be unique in paleoecological reconstructions of the environment, is underestimated.

History of the study of acritarchs. None of the researchers studied the group of acritarchs in the Mesozoic deposits of Ukraine. In scientific works it was noted only about the presence of these forms in the description of palynological complexes of Jurassic, Cretaceous and other times.

Brief description of the group. Acritarchs are unicellular, non-colonial, organic microfossils.

Formulation of the purpose of the article. The aim of the study was to focus on such a little-studied group for the Mesozoic as acritars and to prove its role and significance for stratigraphic and paleoecological constructions.

Materials and methods. The research material was samples of rocks of the Middle, Upper Jurassic and Cretaceous deposits, selected separately from 93 sections, but from all major tectonic structures of Ukraine: Peninsky zone of the Carpathians, Volyn-Podolsk plate, western and eastern slopes of the Ukrainian Shield, Priazovsky array of the Ukrainian shield, Dnieper-Donetsk basin, Donbas, South Ukrainian monocline (Black Sea basin), Crimea, North-Azov depression and Azov shaft (Ukrainian part of the Sea of Azov).

Presentation of the main material of the study. Acritarchs Jurassic and Cretaceous belong to 10 genera, including 11 species. The most common species found in both Jurassic and Cretaceous sediments of Ukraine are acritarchs Micrhystridium fragile and Fromea sp. Jurassic complexes are slightly richer than chalk in terms of percentage and are represented mainly by Micrhystridium spp., Micrhystridium flagile, M. longum, Veryhachium brevitrispinum, Wilsonastrum sp., Baltisphaeridium sp. Cretaceous: Micrhystridium spp., Micrhystridium fragile, M. longum, Baltisphaeridium breviciliatum, B. aff. capillatum, B. annelieae, B. accinctum, Acanthodiacrodium sp., Solisphaeridium inaffectum, Comasphaeridium sp., Comasphaeridium aff. brachyspinosum, Veryhachium spp., Veryhachium singulare, Leiofusa stoumonensis, Fromea sp., Ascostomocystis sp. The article presents photo tables of images of Jurassic and Cretaceous acritarchs.

Conclusions. For the first time in Ukraine, acritarchs were found in samples from Jurassic and Cretaceous sediments and attention was focused on such a little-studied group for the Mesozoic. Their certain role and significance for stratigraphic and paleoecological constructions are proved, their species composition and vertical distribution in sections of Mesozoic sediments are studied. The regularities of the distribution of acritarchs in the same age layers are established. Analyzing the Jurassic and Cretaceous microfossils studied from Mesozoic sediments from 93 sections of different regions of Ukraine, we can say that the trend of disappearance of acritarchs during the Mesozoic is weakly observed. Jurassic forms of acritarchs are up to 5% in the complex, Cretaceous - up to 4%. The next stage of work should be the study of acritarch Jurassic and Cretaceous deposits of all regions of Ukraine for the purposes of the overall picture of the reproduction of paleoecological conditions in Ukraine during the Jurassic and Cretaceous period.