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The brachiopod species Dalmanella kegelensis Alichova, 1953 is reported in the geological literature of the Baltic region as a species characteristic of the Keila Regional Stage at the Sandbian–Katian transition, and a nominal taxon of a regional biozone. The spatial distribution of D. kegelensis forms a belt around the deeper parts of the palaeobasin. The easternmost occurrences of D. kegelensis have been recorded at the westernmost periphery of the Moscow Basin. The selection of the holotype of the species D. kegelensis from a locality in western Estonia caused some confusion in understanding Alichova’s species, since the description of the species is mainly based on brachiopods collected from northwestern Russia. The earlier attempt to revise the taxonomy of D. kegelensis group brachiopods was unsustainable, while assigning these brachiopods to the genus Horderleyella is no longer feasible. However, two subspecies were identified, which are discussed here as two separate species, kegelensis and oanduensis, belonging to two different genera: one species belongs to the new genus Alichovella in the family Draboviidae, and the second one, following Alichova’s classification, to the genus Dalmanella in the family Dalmanellidae. These species possibly represent somewhat different ages prior to the beginning of the Guttenberg carbon isotope excursion and supposedly differ in their positions on the onshore-offshore transect.

The taxonomy of the Baltic Ordovician orthoidean brachiopods of the genus Cyrtonotella Schuchert & Cooper, 1931 is discussed, two neotypes are suggested and emended descriptions of species and subspecies are presented. This taxonomic revision of Cyrtonotella-like brachiopods highlights their differences from the family Nanorthidae Havlíček, 1977 in which they have been included up to now. The well-preserved Baltic representatives of the genus Cyrtonotella display several morphological features, which are unique to this group of brachiopods and may have a family-level importance. The studied brachiopods belong to the shallow-water faunal associations of the Baltic Basin, including its easternmost parts (the Moscow Basin). These brachiopods disappear during the faunal crisis at the Sandbian-Katian transition. The global distribution of Cyrtonotella is poorly known due to taxonomic problems. New data on the Baltic species, which specify their stratigraphic and spatial distribution in the Baltic Basin, is a step towards reducing the taxonomic confusion among Cyrtonotella species.

In this study, the first trimerellide brachiopods from Estonia are described. They occur in the uppermost Katian (Upper Ordovician) shallow shelf environments and represent some of the largest Ordovician brachiopods. The trimerellides described are assigned to the genus Gasconsia Northrop, 1939, which disappears before the Hirnantian and reappears in the lowermost Wenlock after a long interregnum. In the Baltic Basin, the new data from Estonia expand the distribution area of Gasconsia from the Scandinavian facies belt in the west to the Estonian shallow shelf in the east. Despite poor preservation, the specimens studied revealed some structures in shell morphology important for species level identification. The strati­graphic position of Gasconsia is analyzed relative to the Katian-Hirnantian (Pirgu-Porkuni regional stages) boundary and the occurrence of the late Katian brachiopod Holorhynchus in Estonia. The improved data on the distribution of Gasconsia in time and space and a short discussion on brachiopod gigantism are presented.

The widespread growth of reefs formed by a framework of biogenic constructors and frame-lacking carbonate mounds began on Baltica during Ordovician time. Previously, Ordovician reef and mound development on Baltica was considered to be sporadic and local. A review of all known bioherm localities across the Baltic Basin reveals a more consistent pattern. Ordovician bioherms grew in a wide E–W-aligned belt across the Baltic Basin and occur in several places in Norway. Substantial reef development began simultaneously across the region during the late Sandbian – early Katian interval and climaxed during the late Katian Pirgu age. The current spatiotemporal distribution of bioherms is a result of interdependent factors that involve original drivers of reef development such as relative sea level, climate during the time of deposition and effects of post-depositional erosion. Oceanographic conditions were likely more favourable during times of cooler global climates, low sea level and glacial episodes. At the same time, the likelihood that bioherms are preserved from long-term erosion is higher when deposited during low sea level in deeper parts of the basin. A main factor controlling the timing of the reef and mound evolution was Baltica's shift toward palaeotropical latitudes during Late Ordovician time. The time equivalence between initial reef growth and the Guttenberg isotope carbon excursion (GICE) suggests that global climatic conditions were important.

The latest Sandbian brachiopods and chitinozoans were studied in the Kõrgessaare and Haapsalu drill cores of Estonia. The brachiopod fauna shows a gradual renewal through the Keila Regional Stage (RS), differently from the rather persistent association of chitinozoans. An exception is the uppermost part of the stage, which differs in two sections in the taxonomic composition of chitinozoans and the occurrence of two species-level taxa of the Dalmanella kegelensis brachiopod group. D. kegelensis sensu lato has been considered an index taxon of the biozone in the Keila RS. It links the brachiopod faunas of North Estonia to those in NW Russia. In the latter region, the dolomitic and siliciclastic lagoonal and peritidal deposits overlying the strata with brachiopods of the D. kegelensis group are considered the youngest part of the Keila RS.

Four new Hirnantian species of orthide brachiopods, Sigmelasma peepi, Mendacella aerinensis, Drabovia? minuta and Tyronella siugensis are described from the type section of the Porkuni Regional Stage in the Porkuni quarry, northeastern Estonia. These species occur in the reef-related shallow-water bituminous limestone (Siuge Member of the Arina Formation). Sigmelasma peepi and Tyronella siugensis represent the families Wangyuiidae and Tyronellidae, respectively, which were hitherto unknown in the Baltic Ordovician. Additionally, a harknessellid Reuschella sp. is described from the stromatoporoid-coral reefs (Torevere Member) and skeletal grainstone (Vohilaid Member) of the same locality and formation. Together these new finds extend our knowledge of the latest Ordovician brachiopod fauna in the Baltic region, showing higher than previously expected diversity of the Porkuni Stage and distinctness of reef-related brachiopods in the shallow shelf environment. The new species are not present in the Hirnantia brachiopod fauna in deeper-water environments of the Central East Baltic.

The two lowermost Darriwilian (Middle Ordovician) brachiopod species that were previously included in the genus Cyrtonotella are re-studied and compared with the type species Cyrtonotella semicircularis (Eichwald) from the St Petersburg Region. Based on the new data, the classification of the important group of orthids, widely distributed in the Sandbian Kukruse to Keila regional stages in the East Baltic, is improved. The lower Darriwilian specimens, previously considered to be Cyrtonotella semicircularis, are ascribed to the genus Leoniorthis as the new species Leoniorthis rubeli, which occurs in the St Petersburg Region and in northern Estonia. The other species Cyrtonotella pakriensis Rubel is included in the new genus Rogorthis, which is restricted to the sandy limestones of the Pakri Formation (Kunda Stage) in northern Estonia. The type species of the genus Cyrtonotella, C. semicircularis (Eichwald), probably occurs in the upper Darriwilian Aseri Regional Stage in the St Petersburg Region, and is the first appearance of that genus.

Sequence stratigraphic analysis of the Nabala–Porkuni regional stages (RSs; upper Katian–Hirnantian) of Estonia clarifies the Late Ordovician evolution of the Estonian Shelf–Livonian Basin. The integration of depositional facies, biostratigraphy, carbon isotope chemostra- tigraphy, karst surfaces, and hiatuses indicates seven sequences: (1) Nabala (Paekna and Saunja Formations (Fms)); (2) Vormsi (Kõrgessaare, Tudulinna, and Fjäcka Fms); (3) Lower Pirgu (Moe and Jonstorp Fms); (4) Middle Pirgu (most of the Adila and Halliku Fms, and Jelgava and Parovėja Fms); (5) Upper Pirgu (Kabala Member (Mb), part of the Halliku Fm); (6) Lower Porkuni Sequence (most of the Ärina Fm); and (7) part of the Upper Porkuni–Juuru Sequence (Kamariku Mb and Saldus Fm). A lowstand systems tract is only identified in the uppermost sequence. Transgressive units are marked by onlapping depositional packages. Highstands consist of one or more shallowing-upward shelf packages (notably in the Vormsi, Lower Pirgu, and Middle Pirgu sequences). The sequences record the progradation of shallow-to-middle ramp facies as sediment infilled the northern edge of the Livonian Basin, leading to an open shelf (Porkuni RS). Eustasy was the major factor in sequence boundary formation with larger amplitude sea level oscillations associated with Hirnantian (Porkuni) glaciations. A shift to more strongly differentiated ramp facies at the Nabala–Vormsi transition coincides with the initial collision of Baltica and Avalonia.

The brachiopod species Dalmanella kegelensis Alichova, 1953 is reported in the geological literature of the Baltic region as a species characteristic of the Keila Regional Stage at the Sandbian-Katian transition, and a nominal taxon of a regional biozone. The spatial distribution of D. kegelensis forms a belt around the deeper parts of the palaeobasin. The easternmost occurrences of D. kegelensis have been recorded at the westernmost periphery of the Moscow Basin. The selection of the holotype of the species D. kegelensis from a locality in western Estonia caused some confusion in understanding Alichova's species, since the description of the species is mainly based on brachiopods collected from northwestern Russia. The earlier attempt to revise the taxonomy of D. kegelensis group brachiopods was unsustainable, while assigning these brachiopods to the genus Horderleyella is no longer feasible. However, two subspecies were identified, which are discussed here as two separate species, kegelensis and oanduensis, belonging to two different genera: one species belongs to the new genus Alichovella in the family Draboviidae, and the second one, following Alichova's classification, to the genus Dalmanella in the family Dalmanellidae. These species possibly represent somewhat different ages prior to the beginning of the Guttenberg carbon isotope excursion and supposedly differ in their positions on the onshore-offshore transect.

The composition, diversity and distribution of various groups of organisms in the late Llandovery (Telychian) and early Wenlockian (Sheinwoodian) interval of the Pahapilli 675 and Kõrkküla 863 sections (western Estonia) were studied. Due to the small size, fragmentation and disarticulation of the specimens, the studied paleontological material could, in the major part, be tentatively considered as “biodetritus” or “bioclast material” occurring in the carbonate and siliciclastic lithologies. The recognized conodont biozonation provides a detailed stratigraphic framework for this study. The brachiopod fauna is represented by two different Dicoelosia–Skenidioides communities, comprising diverse associations of non-brachiopod fauna, whose composition changes over time. Successive associations of trilobites are based on dominating taxa among odontopleurids, encrinurids, calymenids and aulacopleurids. The distribution and taxonomic composition of Silurian echinoderms (cyclocystoids, bothriocidarid echinoids, pisocrinid crinoids) in the East Baltic area are documented for the first time. New data on bryozoans, corals, sponges and other fossils are provided, improving the knowledge of the whole biota of the open shelf and transition to basinal environments. The distribution of faunas is affected by different changes in the ecosystem. The highest diversity was recorded in the Upper Pterospathodus a. amorphognathoides Conodont Zone, before the main faunal turnover caused by the Ireviken Event, which was associated with rapid sea-level fall and climate changes. The faunal recovery started at the early Wenlockian in the second half of the Ireviken Event, above the base of the Lower Kockelella ranuliformisConodont Zone.