The composition of altered volcanic ash of the Late Ordovician Kinnekulle bed was studied in geological sections of the Baltic Paleobasin. The composition of altered ash varies with paleosea depth from northern Estonia to Lithuania. The ash bed in shallow shelf limestones contains an association of illite-smectite (I-S) and K-feldspar, with the K2O content ranging from 7.5 to 15.3%. The limestone in the transition zone between shallow- and deep-shelf environments contains I-S-dominated ash with K2O content from 6.0 to 7.5%. In the deep-shelf marlstone and shale, the volcanic ash bed consists of I-S and kaolinite with a K2O content ranging from 4.1 to 6.0%. This shows that authigenic silicates from volcanic ash were formed during the early sedimentary-diagenetic processes. The composition of the altered volcanic ash can be used as a paleoenvironmental indicator showing the pH of the seawater or porewater in sediments as well as the sedimentation rate.

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.

Apart from a single study of the early Katian δ13C chemostratigraphy in two regions in China, no investigations of the Sandbian and Katian chemostratigraphy have been published from anywhere in Asia. A recent study of the conodont biostratigraphy of the classical Ordovician succession on Langkawi Islands, peninsular Malaysia, showed the presence there of strata coeval with those having the Guttenberg Carbon Excursion (GICE) on the Yangtze Platform. In an effort to establish for the first time the presence of this widespread δ13C excursion in southern Asia, a series of samples from the upper part of the Kaki Bukit Formation was isotopically analysed. This resulted in the discovery of a conspicuous δ13C excursion with peak values of ~ 2 ‰ above the baseline values. The excursion is located just above the Baltoniodus alobatus Subzone and near the level of the first appearance of Hamarodus europaeus, hence the same stratigraphic position as the GICE on the Yangtze Platform. Using the GICE, the Malaysian study interval is closely correlated with the GICE intervals at three localities representing an approximately 23 000 km long transect from Malaysia across Baltoscandia to central North America. This shows the usefulness of δ13C chemostratigraphy to clarify previously obscure stratigraphic relationships between geographically very widely separated localities.

In the recently completed and formally ratified new series and stage classification of the Ordovician System, the base of the Middle Ordovician Series coincides with the base of the global Dapingian Stage. In the Global Stratotype Section and Point (GSSP) of this stage, which is located at Huanghuachang in southern China, the base of the Dapingian Stage is defined as the level of first appearance of the conodont Baltoniodus triangularis. The fact that this species, along with some other taxa present at the Dapingian GSSP, occurs in many sections in Baltoscandia makes it possible to recognise with considerable precision the level of this global stage boundary in Sweden, Estonia, northwestern Russia, and Denmark. In several, but not all, regions, especially in the East Baltic, the global stage boundary coincides with the base of the regional Volkhov Stage and can be tied to the base of the Megistaspis polyphemus Trilobite Zone. The regionally somewhat different relationships between the position of the global stage boundary and a very widespread hardground complex are probably due to the occurrence of local and/or regional unconformities in the upper Floian–lower Dapingian interval. Although biostratigraphically important graptolites are present in the study interval in some Baltoscandic sections, the precise graptolite correlation of the base of the Dapingian Stage remains somewhat unclear, although it appears to be near, or at, the base of the Isograptus victoriae victoriae Zone (Ca 2).

Microzarkodina is a genus of mainly Middle Ordovician conodonts that has its centre of distribution in Baltoscandia, and much less commonly occurs in southern China, Australia, Argentina and Laurentia. In Baltica a series of species, Microzarkodina russica n. sp., M. flabellum, M. parva, M. bella, M. hagetiana and M. ozarkodella, established themselves successfully. The succession of species ranges from just below the base of the Middle Ordovician (M. russica) to the upper part of the Middle Ordovician (M. ozarkodella). The species are frequently used for biostratigraphical purposes. The largely contemporaneous species Microzarkodina bella and M. hagetiana probably both evolved from M. parva and mostly occurred in separate areas. Microzarkodina ozarkodella probably evolved from M. hagetiana. This present investigation is based on a total of 94,208 elements, collected from 20 sections and one drill-core site in Sweden, one drill-core site and one outcrop in Estonia and two sections in the St Petersburg area in Russia. The Microzarkodina apparatus probably consisted of 15 or 17 elements: four P, two or four M and nine S elements. The S elements include different Sa, Sb1, Sb2, and Sc element types.

The Volkhov Regional Stage (Middle Ordovician) in the East Baltic preserves diverse fossil assemblages dominated by epibenthic suspension feeders. Brachiopods, ostracodes, conodonts, echinoderms and bryozoans are the main components of palaeocommunities obtained from clay horizons in the Putilovo section (St Petersburg region, Russia), whereas trilobites, machaeridians, hyolithids, graptolites, benthic foraminifers and gastropods are rare or occur sporadically. Brachiopod bioclasts volumetrically dominate the debris of the studied sediments. Quantitative faunal data are used to assess species diversity patterns, as expressed by the species richness (total number of species in the standardized sample size) and by the evenness or equitability of the community. The numerical abundance of particular taxa in each standardized sample was used to evaluate the density of the fauna. The communities in the Volkhov Stage in Putilovo Quarry reveal a remarkable stability throughout the studied interval and are characterized by high density (1000–6000 specimens per 100 g), relatively moderate species richness (10–15 species) and a moderately variable equitability (0.3–0.7) for the dominant fossil groups (conodonts, ostracodes and brachiopods). Ostracodes significantly increase in numbers within the upper part of the section. This confirms a shallowing of the basin during the late Volkhov interval. Small-scale variability of the diversity estimates does not correlate with the small-scale sea-level changes reconstructed for this part of the basin. It may be connected with error in diversity measurement, or the result of undetected environmental parameters. Variability estimates for different faunal groups are poorly correlated because particular groups have different environmental tolerances.

The Lower Ordovician (Oeland Series) type species of Lamanskya, L. splendens Moberg & Segerberg 1906, was described originally as an articulate brachiopod, but it is here redescribed and referred to the lingulid Family Elkaniidae. L. splendens ranges from the late Tremadoc to the early Arenig in Baltoscandia (Norway, Sweden, Estonia, and the St Petersburg district of Russia). Aulonotreta kuraganica Andreeva 1972 from the Lower Ordovician of the Ural Mountains is also redescribed and referred to Lamanskya. The genus Dictyobolus Williams & Curry 1985, from the Lower Ordovician of Ireland, is a junior synomym of Lamanskya. All species of the genus are strongly biconvex, and have a well-developed ornamentation of elongate lenticular pits. The problematic Genus Volborthia von Moller 1870 is also considered tentatively to belong within the Elkaniidae.

Agnostid trilobites are described from the Lower Ordovician (Oelandian Series) of Öland, Östergötland, Västergötland and Närke, southern Sweden. In addition, a few agnostids are described from the Ceratopyge Limestone (upper Tremadoc) of Scania (Skåne), the southernmost province of Sweden. 24 species (of which Galbagnostus nericiensis is new), assigned to four genera, are described or discussed. Species of Geragnostus and Arthrorhachis dominate numerically. Also recorded locally are species of Galbagnostus and Leiagnostus. Most species have a markedly restricted vertical distribution, and many species appear to have a sufficiently short range to be of value in stage- and zone-level correlation. Agnostids are represented almost throughout the Lower Ordovician of southern Sweden, but they have not been recorded from the Tremadocian “Dictyonema” and Ceratopyge Shales, nor from the upper Holen Limestone (upper Kundan Stage). In the Ordovician of Baltoscandia, agnostids are largely restricted to the Central Baltoscandian Confacies Belt; they are very rare or absent in the adjacent belts to the east and in the graptolitic shales of the Scanian Confacies Belt.