Despite being reported from various localities and stratigraphic intervals, knowledge of the siliceous sponges from the Cenozoic of Eastern Europe remains surprisingly limited. Studies assessing their diversity are almost exclusively in Russian and rather hard to obtain. The most comprehensive elaboration of the sponge spicules from the Paleogene of the East European Platform was published in 2003 and deals with material from Ukraine, Russia, Belarus, and Lithuania. However, the classification in that paper is purely artificial and extremely difficult to interpret according to modern biological criteria. A reassessment of this material is carried out, with the aim of revising all morphotypes of spicules, and identifying them to the lowest possible taxonomic level. Results suggest that the assemblage is much more diverse than previously thought, including members of 24 demosponge families (class Demospongiae), one homoscleromorph (class Homoscleromorpha), and at least one hexactinellid (class Hexactinellida). Our improved understanding of the diversity of Paleogene sponge fauna of the East European Platform will have implications for the interpretation of the past and future ecological and paleobiogeographic studies.

The present paper contains new results of complex studies on the geological structure, lithology, and fauna of the Middle Eocene siliceous rocks of the Ukrainian Shield Ingul megablock located in the vicinity of the villages of Tsybulevo, Verblyuzhka and Pervozvanovka of Kirovograd Oblast and Voronovka village of Mykolayiv Oblast. Based on new findings and analysis of malacofauna, the Middle Eocene age of the gaize-like sandstones of Verblyuzhka village was confirmed. For the first time, numerous remains of rock-forming organisms such as sponges (lithistid Demospongiae), coralline (Corallinaceae) and green algae (Dasycladales, Halimedaceae) have been found in the siliceous deposits of this region. In addition, biogenic buildups formed by organisms with carbonate (coralline and green algae, large foraminifera) and silicate (lithistid sponges) skeletons were found for the first time in the siliceous rocks of the Pervozvanovka deposit. The observed biogenic lepispheres in the studied rocks suggests that the latter have been formed with active participation of silica-producing bacterial communities. Based on the first finds of zeolites, as well as montmorillonite traces, it is forecast that "disguised" pyroclastic material is present in the gaize-like sandstones of Voronovka village. Analysis of the lithological features and the mineral and faunal composition of the studied rocks allow us to draw a conclusion about the multi-stage genesis of the Middle Eocene siliceous deposits in the central part of the Ukrainian Shield and the influence of Paleogene volcanism on their formation. It appears that volcanic activity and denudation of volcanic products caused a massive flow of SiO2 into the waters of the Middle Eocene paleobasin. This contributed to the widespread development of siliceous-skeleton organisms (in particular, silica-producing bacterial communities) that actively participated in the formation of siliceous rocks.

ВВЕДЕНИЕ На юге Украины олигоценовые отложения представлены темными песчано-глинистыми породами майкопской серии, чаще всего слабо-или бескарбонатными, насыщенными органикой. Осадки формировались в основном в условиях неблагоприятного газового режима, о чем свидетельствует преобладание в комплексах фораминифер представителей средне-и глубокозарывающейся инфауны (Pullenia, Melonis, Caucasina, Uvigerinella, Bolivina, Brizalina, Furssenkoina, Reophax, Haplophragmoides, Trochammina), а также наличие в той или иной степени пиритизированных остатков фораминифер, радиолярий, диатомовых водорослей, губок. Последние две группы, нуждающиеся для воспроизводства в нормальной циркуляции воды, вероятно, были наиболее чувствительными к аноксическим условиям придонных вод. Как следствие, олигоценовые отложения Южной Украины бедны остатками диатомей и спикул губок. Описано всего одно местонахождение диатомовых в Юго-Восточном Приазовье (Ольштынская, 2007; Ольштинська, 2013). Спикулы губок в олигоценовых отложениях Южной Украины также встречаются спорадически и, в целом, мало изучены. Их таксономический состав установлен М.М. Иваником в Юго-Западном Крыму, Северном Причерноморье (Иваник, 2003) и на Керченском полуострове (Иваник, Краева, 1983).

Porifera is a clade of globally distributed, early diverging metazoans that are important components of modern aquatic environments. Despite the fact that they were also abundant in the geological past, their fossil record is uneven. For example, little is known about sponge communities that inhabited the Tethyan areas during the Cenozoic. Here, we record an abundant and taxonomically diverse assemblage of middle–late Eocene sponges from east‐central Ukraine. The material consists of preserved whole‐body specimens as well as disassociated spicules. The taxonomic composition of this new assemblage, supplemented with previously published data obtained from sponge materials originating from the middle and upper Eocene of the East European Platform, documents a surprisingly rich sponge community that inhabited shallow waters (depths of 100 m) and comprised at least 30 demosponge species, including two new taxa, Paratetilla milanek Łukowiak and Theonella alexandriae Łukowiak, three hexactinellids and a single homoscleromorph. Additionally, we report the first fossil occurrence of Vetulina, a demosponge genus that currently is found near the Philippines and Australia and in the Caribbean Sea. Some of the sponge taxa recognized were previously noted from the upper Eocene of Australia and New Zealand. This is indicative of their wide distribution during the early Cenozoic and also shows that a non‐interrupted connection existed between the western Tethyan and peri‐Australian areas, explaining the patchy distribution of some sponge taxa in modern seas.