ORDOVICIAN ODONTOPLEURID TRILOBITES FROM ESTONIA AND LATVIA

All available type and other odontopleurid material from Estonia and Latvia is described and figured. This includes the new species Acidaspis aviensis from the Upper Ordovician Porkuni Stage (F_II) and Middle and Upper Ordovician (D₁-F_Ia) material belonging to at least 3 new species of Diacanthaspis. However, because of the incompleteness of this latter material, no specific names are given to it.     

First report of craniide brachiopods in the Palaeozoic of Iran (Pseudocrania,Ordovician), and Early to Mid‐Ordovician biogeography of the Craniida

Abstract: Lower Ordovician faunas of Bohemia (Perunica), Baltica and North China include the oldest known representatives of the Order Craniida, but otherwise in Gondwana and associated terranes, the record of craniides is sparse. Pseudocrania insperata sp. nov. from the Lashkarak Formation of the Eastern Alborz Mountains is the first and as yet only record of the occurrence of craniides in the Middle Ordovician (Darriwilian) of Iran and temperate to high latitude peri‐Gondwana. Pseudocrania was known hitherto only from the Middle Ordovician of Baltoscandia and the Chu‐Ili terrane of Kazakhstan.     

New crinoids from the Baltic region (Estonia): fossil tip‐dating phylogenetics constrains the origin and Ordovician–Silurian diversification of the Flexibilia (Echinodermata)

This study documents previously unknown taxonomic and morphological diversity among early Palaeozoic crinoids. Based on highly complete, well preserved crown material, we describe two new genera from the Ordovician and Silurian of the Baltic region (Estonia) that provide insight into two major features of the geological history of crinoids: the early evolution of the flexible clade during the Great Ordovician Biodiversification Event (GOBE), and their diversification history surrounding the end‐Ordovician mass extinction. The unexpected occurrence of a highly derived sagenocrinid, Tintinnabulicrinus estoniensis gen. et. sp. nov., from Upper Ordovician (lower Katian) rocks of the Baltic palaeocontinent provides high‐resolution temporal, taxonomic and palaeobiogeographical constraints on the origin and early evolution of the Flexibilia. The Silurian (lower Rhuddanian, Llandovery) Paerticrinus arvosus gen. et sp. nov. is the oldest known Silurian crinoid from Baltica and thus provides the earliest Baltic record of crinoids following the aftermath of the end‐Ordovician mass extinction. A Bayesian ‘fossil tip‐dating’ analysis implementing the fossilized birth–death process and a relaxed morphological clock model suggests that flexibles evolved c. 3 million years prior to their oldest fossil record, potentially involving an ancestor–descendant relationship (via ‘budding’ cladogenesis or anagenesis) with the paraphyletic cladid Cupulocrinus. The sagenocrinid subclade rapidly diverged from ‘taxocrinid’ grade crinoids during the final stages of the GOBE, culminating in maximal diversity among Ordovician crinoid faunas on a global scale. Remarkably, diversification patterns indicate little taxonomic turnover among flexibles across the Late Ordovician mass extinction. However, the elimination of closely related clades may have helped pave the way for their subsequent Silurian diversification and increased ecological role in post‐Ordovician Palaeozoic marine communities. This study highlights the significance of studies reporting faunas from undersampled palaeogeographical regions for clade‐based phylogenetic studies and improving estimates of global biodiversity through geological time.     

Late Ordovician carbon isotope trend in Estonia, its significance in stratigraphy and environmental analysis

Carbon isotope changes during most of Late Ordovician time (from the mid-Caradoc Kinnekulle K-bentonite until the beginning of the Silurian) were investigated. As the corresponding sequence of rocks is stratigraphically nearly complete in Estonia, an attempt was made to use it to elaborate the general pattern of carbon isotope changes in the Late Ordovician. Complications were caused by several local or regional hiatuses in the middle and late Caradoc and Hirnantian. A total of 385 whole rock samples were studied from eight drill cores in northern and central Estonia. The following positive carbon isotope events were observed: (1) the mid-Caradoc excursion (peak δ¹³C value 2.2‰) in the uppermost part of the Keila Stage, also known in Sweden; (2) the first late Caradoc excursion (1.9‰) in the lower part of the Rakvere Stage; (3) the second late Caradoc excursion (2.4‰) in the upper part of the Nabala Stage; (4) the early Ashgill excursion (2.5‰) in the lowermost part of the Pirgu Stage; (5) the widely known large Hirnantian excursion (in Estonia the peak value reaches 6.7‰) in the Porkuni Stage. The study interval comprises a long (10 Ma) period characterized by low-magnitude carbon isotope changes and a following brief (2 Ma) interval with large changes. No obvious lithological preference for hosting the positive shifts was recorded. In principle, the δ¹³C values exceeding the background values may occur in all types of rocks present in a sedimentary basin. Several δ¹³C positive excursions (values 1.5‰ to 3‰) in the Mohawkian of North America are evidence that the minor Caradoc and early Ashgill δ¹³C positive shifts in Baltoscandia may have counterparts in Laurentia. If correctly correlated, these shifts may have global significance. The Hirnantian excursion is usually linked to a major glacial event, even if some carbon cycling mechanisms are not completely understood. The environmental causes suggested for the earlier minor shifts range from global climatic and glacial events to very local changes in basin regime and sea level. Our study supports the primary role of climatic or climatically triggered oceanic processes.     

Cryptostomid Bryozoans From The Sassito Formation,Upper Ordovician Cool‐Water Carbonates of the Argentinean Precordillera

Abstract: Two new bryozoan species are described from the Upper Ordovician Sassito Formation of the Argentinean Precordillera: Moyerella  spinata sp. nov. and Phylloporina  sassitoensis sp. nov. The bryozoans are found in cool‐water carbonates. The Silurian genus Moyerella is reported the first time in the Ordovician, showing palaeobiogeographic connections with Estonia and Siberia.     

Ontogeny of the Furongian (late Cambrian) trilobite Proceratopyge cf. P. Lata Whitehouse from northern Victoria Land,Antarctica, and the evolution of metamorphosis in trilobites

There were multiple origins of metamorphosis‐undergoing protaspides in trilobite evolution: within the superfamilies Remopleuridioidea, Trinucleoidea, and within the Order Asaphida. Recent studies have revealed that the protaspides of the Cambrian representatives of the Remopleuridioidea and the Trinucleoidea did not undergo metamorphosis. However, ontogeny of the Cambrian members of the Order Asaphida has remained unknown. This study documents the ontogeny of the Furongian asaphoidean ceratopygid trilobite, Proceratopyge cf. P. lata Whitehouse, from northern Victoria Land, Antarctica. Two stages for the protaspid phase, five developmental stages for the post‐protaspid cranidia, and ten stages for the post‐protaspid pygidia have been identified. Interestingly, the protaspis directly developed into a meraspis without metamorphosis. A new cladistic analysis resulted in a single most parsimonious tree, according to which the presence of the bulbous commutavi protaspis turns out to be a synapomorphy for Asaphidae + Cyclopygoidea, not a synapomorphy for the Order Asaphida as previously suggested. In addition, it is inferred that there was convergent evolution of indirectly‐developing commutavi protaspides during the Furongian and Early Ordovician. Metamorphosis‐entailing planktonic larvae evolved in many different metazoan lineages near the Cambrian–Ordovician transition, due to the escalating ecological pressure of the Great Ordovician Biodiversification Event. Since the bulbous commutavi protaspid morphology is thought to be an adaptation for a planktonic life mode, the convergent evolution of the indirect development in the three trilobite lineages at this period might have been a result of adaptation to the early phase of the Great Ordovician Biodiversification Event.     

Growth trajectories of some major ammonoid sub‐clades revealed by serial grinding tomography data

Molluscs such as ammonoids record their growth in their accretionary shells, making them ideal for the study of evolutionary changes in ontogeny through time. Standard methods usually focus on two‐dimensional data and do not quantify empirical changes in shell and chamber volumes through ontogeny, which can possibly be important to disentangle phylogeny, interspecific variation and palaeobiology of these extinct cephalopods. Tomographic and computational methods offer the opportunity to empirically study volumetric changes in shell and chamber volumes through ontogeny of major ammonoid sub‐clades in three dimensions (3‐D). Here, we document (1) the growth of chamber and septal volumes through ontogeny and (2) differences in ontogenetic changes between species from each of three major sub‐clades of Palaeozoic ammonoids throughout their early phylogeny. The data used are three‐dimensional reconstructions of specimens that have been subjected to grinding tomography. The following species were studied: the agoniatitid Fidelites clariondi and anarcestid Diallagites lenticulifer (Middle Devonian) and the Early Carboniferous goniatitid Goniatites multiliratus. Chamber and septum volumes were plotted against the septum number and the shell diameter (proxies for growth) in the three species; although differences are small, the trajectories are more similar among the most derived Diallagites and Goniatites compared with the more widely umbilicate Fidelites. Our comparisons show a good correlation between the 3‐D and the 2‐D measurements. In all three species, both volumes follow exponential trends with deviations in very early ontogeny (resolution artefacts) and near maturity (mature modifications in shell growth). Additionally, we analyse the intraspecific differences in the volume data between two specimens of Normannites (Middle Jurassic).     

Earliest ontogeny of Early Palaeozoic Craniiformea: compelling evidence for lecithotrophy

Popov, L.E., Bassett, M.G. & Holmer, L.E. 2012: Earliest ontogeny of Early Palaeozoic Craniiformea: compelling evidence for lecithotrophy. Lethaia, Vol. 45, pp. 566–573. The early ontogeny of Palaeozoic Craniiformea (Brachiopoda) remains controversial, with conflicting reports of evidence indicating lecithotrophic versus planktotrophic larval stages. Further compelling evidence for lecithotrophy in Palaeozoic craniiforms is described here. Newly obtained, well‐preserved Silurian specimens of craniiforms, including Craniops (Craniopsida), and Lepidocrania? and Orthisocrania (Craniida) from Gotland and the St. Petersburg region, form the basis for this study. The new material demonstrates that the characters of shell structure and shell formation provide evidence of early differentiation of an adult dorsal mantle, and the presence of a distinctive primary layer with a characteristic lath‐like pattern indicates that these craniiforms underwent a lecithotrophic larval stage, more or less identical to that of living. □Novocrania. Brachiopoda, Craniiformea, ontogeny, phylogeny, Early Palaeozoic.     

Early symbiotic rugosan endobionts in stromatoporoids from the Rhuddanian of Estonia (Baltica)

The earliest known symbiotic rugosan endobionts occur in stromatoporoids from the early Rhuddanian of Estonia. The stromatoporoid Ecclimadictyon nikitini from the Tamsalu Formation contains the rugosan Donacophyllum middendorffii endobiont. A stromatoporoid Clathrodictyon boreale from the Varbola Formation contains Streptelasma estonica and Bodophyllum sp. endobionts. There are up to three endobiotic rugosans per stromatoporoid host. Stromatoporoid hosts were beneficial for symbiotic rugosans as elevated substrates on a seafloor that offered a higher tier for feeding; they also offered enhanced substrate stability. Stromatoporoids of the end‐Ordovician mass‐extinction recovery fauna hosted a diverse fauna of symbiotic endobionts. There were few if any negative effects of this mass extinction on the symbiotic endobionts.     

Discovery of rhombifera (Echinoderms) in the Ordovician of Mongolia

Two new rhombiferan species Echinosphaerites mongolicus sp. nov. and Stichocystis altaicus sp. nov., described from a recently discovered Upper Ordovician locality in the Mongolian Altai, on the Chegertei River, suggest possible biogeographic links between this region and Gondwana and Baltica in the Ordovician. In the Upper Ordovician beds in the East Gobi Depression near Saishand Well, we identified the crinoid Ristnacrinus, previously recorded from the Ordovician of Estonia and Central Asia.     

Palaeoenvironmental aspects of Late Ordovician Sericoidea shell concentrations in an impact crater,Tvären,Sweden

Frisk, Å.M. & Harper, D.A.T. 2010: Palaeoenvironmental aspects of Late Ordovician Sericoidea shell concentrations in an impact crater, Tvären, Sweden. Lethaia, Vol. 44, pp. 383–396. Numerous studies have reported the presence of the small, thin‐shelled cosmopolitan rhynchonelliformean Sericoidea as being environmentally controlled and, together with its close relatives, characteristic of deep‐water, distal, clastic Ordovician and Silurian settings. Assemblages of Sericoidea have been analysed from post‐impact strata in a newly formed Late Ordovician impact crater. In the crater succession, colonization of benthic faunas can be monitored through the post‐impact limestone, demonstrating a number of environmental preferences. Consequently, the crater, as a result of its restricted area, provides an experimental arena for faunal distributions to be correlated with specific environments. The continuous infilling of the crater following its formation reveals a transition from argillaceous mudstones to carbonates deposited in deeper‐water environments to shallower regimes. Rhynchonelliformean brachiopods inhabited the crater depression very late after the impact and are entirely represented by the genus Sericoidea, occurring abundantly in the upper third of the existing crater infill. The deep‐water regime that existed in the depression during the initial interval of crater formation had been substantially reduced. Clearly Sericoidea‐bearing associations associated with shaly substrates did not merely favour and occupy deep‐water environments as previously suggested. The unfavourable conditions triggered by the impact and the inhospitable aftermath allowed Sericoidea to exploit a less‐crowded ecospace. This reorganization, following the catastrophe, from a deep‐water related ecological niche to considerable shallower settings suggests that Sericoidea was a pioneer colonist displaying an opportunist r‐strategy. The shell beds analysed are related to shallower water and this may, moreover, help unravel the dilemma of the general absence of Sericoidea in the deeper‐water Foliomena fauna. □Dalby Limestone, impact crater, Late Ordovician, opportunists, Sericoidea, Tvären.     

Upper Ordovician sequences of western Estonia

The Upper Ordovician (uppermost Caradoc-Ashgill) section of western Estonia consists of a series of seven open-shelf carbonate sequences. Depositional facies grade laterally through a series of shelf-to-basin facies belts: grain-supported facies (shallow shelf), mixed facies (middle shelf), mud-supported facies (deep shelf and slope) and black shale facies (basin). Locally, a stromatactis mud mound occurs in a middle-to-deep shelf position. Shallow-to-deep shelf facies occur widely across the Estonian Shelf and grade laterally through a transitional (slope) belt into the basinal deposits of the Livonian Basin.

Each sequence consists of a shallowing-upward, prograding facies succession. Sequences 1 (Upper Nabala Stage) and 2 (Vormsi Stage) record step-wise drowning of underlying shelf units (lower Nabala) that culminated in the deposition of the most basinal facies (Fjäcka Shale) in the Livonian Basin. Sequences 3–6 comprise the overlying Pirgu Stage and record the gradual expansion of shallow and middle-shelf facies across the Estonian Shelf. The Porkuni Stage (sequence 7) is bracketed by erosional surfaces and contains the shallowest-water facies of the preserved strata. The uppermost part of the section (Normalograptus persculptus biozone) is restricted to the Livonian Basin, and includes redeposited carbonate and siliciclastic grains; it is the lowstand systems tract of the lowest Silurian sequence 8. Sequence 7 and the overlying basinal redeposited material (i.e., the lowstand of sequence 8) correspond to the latest Ordovician (Hirnantian) glacial interval, and the bracketing unconformities are interpreted as the widely recognized early and late Hirnantian glacial maximums.

The sequences appear correlative to Upper Ordovician sequences in Laurentia. Graptolite biozones indicated that the Estonian sequences are equivalent to carbonate ramp sequences in the western United States (Great Basin) and mixed carbonate-siliciclastic sequences in the eastern United States (Appalachian Basin–Cincinnati Arch region). These correlations indicate a strong eustatic control over sequence development despite the contrasting tectonic settings of these basins.     

EARLY ORDOVICIAN CONODONTS FROM TARUTAO ISLAND,SOUTHERN PENINSULAR THAILAND

Abstract: Early Ordovician conodont faunas of the Thung Song Formation on Tarutao Island, southern peninsular Thailand, consist of 14 known species belonging to 17 genera, and eight undescribed species. Utahconus tarutaoensis and Filodontus tenuis are new species. Three conodont zones: the Rossodus manitouensis Zone, the Utahconus tarutaoensis Zone and the Filodontus tenuis Zone, in ascending order, are defined in the study sections. These are coeval with the interval from the Rossodus manitouensis Zone to the Acodus deltatus‐Oneotodus costatus Zone of the standard zonation in the North American Midcontinent. Based on the conodonts studied here and fossils previously reported from Tarutao Island, the Thung Song Formation is early Tremadocian to middle Arenig (Ibexian) in age. This formation is lithostratigraphically subdivided into the S1 to S5 members, and our study sections consist of the S1 to S3 members. These strata accumulated on a shelf in the Early Ordovician. The depositional environments of the limestones making up the S1 and S3 members were in deeper‐shelf conditions. Limestone and shale of the S2 member formed in a shallow‐water, high‐energy environment.     

The new chronostratigraphic classification of the Ordovician System and its relations to major regional series and stages and to δ13C chemostratigraphy

The extensive work carried out during more than a decade by the International Subcommission on Ordovician Stratigraphy has resulted in a new global classification of the Ordovician System into three series and seven stages. Formal Global Boundary Stratotype Section and Points (GSSPs) for all stages have been selected and these and the new stage names have been ratified by the International Commission on Stratigraphy. Based on a variety of biostratigraphic data, these new units are correlated with chronostratigraphic series and stages in the standard regional classifications used in the UK, North America, Baltoscandia, Australia, China, Siberia and the Mediterranean‐North Gondwana region. Furthermore, based mainly on graptolite and conodont zones, the Ordovician is subdivided into 20 stage slices (SS) that have potential for precise correlations in both carbonate and shale facies. The new chronostratigraphic scheme is also tied to a new composite δ¹³C curve through the entire Ordovician.     

UPPER ORDOVICIAN BRYOZOANS OF THE PIN FORMATION (SPITI VALLEY, NORTHERN INDIA)

Abstract: Twenty‐nine species of bryozoans from the Upper Ordovician–Lower Silurian Pin Formation (Spiti, India) have been identified. Eight of these are new: Trematopora minima, Ulrichostylus bhargavai, Ptilodictya exiliformis, Phaenopora ordinarius, Oanduellina himalayaica, Pesnastylus? vesiculosum, Ralfina? originalis and Pinocladia triangulata. The fossil record and facies analyses of the area investigated indicate shallow‐water conditions within the subtropical–tropical realm. The distribution pattern of fossils among the Ordovician/Silurian succession on the Northern Gondwana shelf and the influence of the Late Ordovician cooling phases on marine organisms are distinctive owing to a dramatic reduction in diversity globally. As far as the bryozoan taxa of Spiti are concerned, only one (Helopora fragilis) of the 29 species was recorded above the Ordovician/Silurian boundary. Observed bryozoan communities are very similar to faunas of Laurentia, the Baltic, Siberia and southern China of early–late Ordovician age.     

Eine neue fragliche kalkschalige Foraminif ere aus einem mittelordovizischen Geschiebe Norddeutschlands

Calcareous foraminifers were hitherto unknown from the Ordovician of Baltoscandia. From an erratic Backsteinkalk boulder of Northern Germany (Middle Ordovician) a new bodily preserved questionable species ofGlomovertella is described.Glomovertella belongs to the questionable order of foraminifers Reitlingerellida and was known hitherto only from the Lower Cambrian of Siberia and the Caradoc of Kazachstan. The members of the Reitlingerellida were, until now, only known by thin sections.     

The development and shell microstructure of the pseudodeltidium and interarea in thecideide brachiopods

Abstract: This study examines the development of the delthyrium, pseudodeltidium and interarea, their growth during the early juvenile stages of ontogeny and the extrapolation of morphology from adult shells to the probable juvenile state. Examination of the development and shell microstructure of the cardinalia of early juvenile thecideide brachiopod ventral valves from Jurassic, Cretaceous and Holocene specimens suggests that the delthyrium develops early in ontogeny and that the initial development of the pseudodeltidium precedes that of the interarea. Also, until the interarea is formed, the postero‐lateral flanks of the ventral umbo are palintropic. The development of the interarea can be seen to be a consequence of the lateral extension of the early juvenile hinge line. Initially, the pseudodeltidium consists solely of a thin plate of primary shell material. Comparison of the morphology of the pseudodeltidium of early juveniles with that of adults suggests that the initially curved lateral profile of the pseudodeltidium is retained, or even accentuated, in the ontogeny of lacazellines, but in all thecidellinines, with the exception of Pachymoorellina and Minutella, following the appearance of the interarea, the pseudodeltidium becomes flattened and often appears continuous with the interarea. However, we do not support any proposal that suggests that, in thecideides, only those forms in which the delthyrium is closed by a dorsally convex plate should be considered to possess a pseudodeltidium sensu stricto, mainly because of physiological differences and the prospect of possible taxonomic confusion in the future. Instead, we propose the term planodeltidium for a flat pseudodeltidium, typically developed in the thecidellinines, and rugideltidium for a convex pseudodeltidium, typically developed in the lacazellines, but also in Pachymoorellina and Minutella. Despite the presence of a rugideltidium, we believe the affinities of Minutella are more strongly with the thecidellinines and have included it in the new subfamily Minutellinae of the family Thecidellinidae.     

Conodontophorid biodiversification during the Ordovician in South China

Wu, R., Stouge, S. & Wang, Z. 2012: Conodontophorid biodiversification during the Ordovician in South China. Lethaia, Vol. 45, pp. 432–442. Analysis of the Ordovician conodontophorid diversity pattern for South China using normalized and total diversity measures reveals that diversity peaks occurred in the mid‐Tremadocian, mid‐late Floian, early Dapingian and mid‐Darriwilian periods. The conodontophorids radiated during the Floian, maintaining relatively high diversity into the early part of the Middle Ordovician until a significant diversity decrease occurred in the late Dapingian. A relatively low diversity level prevailed in the Late Ordovician. Three diversification intervals based on origination, extinction and turnover rates have been identified i.e. (1) Tremadocian to mid‐late Floian, (2) early Dapingian and (3) late Dapingian to early Darriwilian. Diversity curves for conodontophorids, brachiopods, graptolites, acritarchs and trilobites from South China are comparable during the Early Ordovician, although differences are apparent in the Middle and Late Ordovician. In South China, conodontophorid diversity reacted primarily to sea‐level changes during the Early and Middle Ordovician, when the peak of this biodiversification generally coincided with a transgression. Climate changes – especially the global cooling that occurred during the Late Ordovician glaciation – and sea‐water chemistry were also important controlling factors. □Biodiversification, conodonts, Ordovician, South China.     

Late Ordovician brachiopods from eastern North Greenland: equatorial offshore migration of the Red River fauna

Abstract: Late Ordovician rhynchonelliformean brachiopods, typical of the North American Red River fauna, are found sporadically in the Børglum River Formation of the Centrum Sø area, Kronprins Christian Land, eastern North Greenland. The geographical distribution of this characteristic brachiopod fauna is thus extended to the easternmost extremity of the Laurentian craton. The assemblage compares specifically with the Hiscobeccus brachiopod fauna, based on key taxa such as notably Hiscobeccus gigas (Wang, 1949), and indicates a late Katian age for this part of the succession. For the first time, this typically inland, shallow‐water fauna is found associated with genera like Bimuria, suggesting a transitional marginal facies with outer shelf benthos. The current study describes a Hiscobeccus fauna that lived at the seaward edge of its preferred habitat. Furthermore, an unpublished Hiscobeccus fauna, from the Børglum River Formation of Peary Land, central North Greenland, as well as several occurrences from the Kap Jackson and Cape Calhoun formations in various parts of Washington Land, western North Greenland, are described here. These show a distinct shift from older strata containing H. capax (Conrad, 1842) to younger strata exclusively yielding specimens of H. gigas. As H. gigas occurs in the upper part of the Cape Calhoun Formation in Washington Land, it indicates that the upper boundary of the Cape Calhoun Formation is considerably younger than previous estimates, reaching into the uppermost Katian (middle Cautleyan–Rawtheyan). The Cape Calhoun Formation correlates with the upper member of the Børglum River Formation and further demonstrates that the Hiscobeccus fauna was widespread in Laurentian marginal settings of North Greenland. Even though the Hiscobeccus fauna was pan‐continental during the late Katian (Richmondian), it possesses a strong provincial signal during the later Ordovician. The new occurrences indicate that this fauna extended to the north‐eastern margin of the Laurentian Craton. It lived in close association with cosmopolitan faunal elements that may have been the earliest sign of the succeeding invasion of migrants from Baltica that arrived later during the Hirnantian. The offshore migration of this atypical Hiscobeccus fauna likely demonstrates the path of warm‐water currents as the Centrum Sø locality was located at the equator during the Late Ordovician.     

Baltoscandian conodont biofacies fluctuations and their link to Middle Ordovician (Darriwilian) global cooling

The Middle Ordovician conodont genera that are suitable for palaeoenvironmental interpretations from the epicontinental Baltoscandian platform have been identified and evaluated to establish and describe conodont biofacies and their relationship to global cooling. The construction of biofacies was based on multivariate statistical analyses of more than 375 700 conodont specimens from 520 samples and 21 localities across Baltica. Three distinct, recurrent and laterally extensive conodont biofacies existed across the Baltoscandian platform of the Baltica continent during the Dapingian and early to middle Darriwilian stages (Middle Ordovician). A relatively shallow water conodont assemblage named the Baltoniodus–Microzarkodina Biofacies characterized the inner shelf localities in central Sweden, Estonia, Russia and Ukraine. In the distal shelf areas, patterns are more complex. Here, genera of the Periodon Biofacies characterized the shelf margin areas of the Scandinavian Caledonides facing the relatively warm Iapetus Ocean towards the north, whereas the Protopanderodus Biofacies dominated the distal shelf areas facing the cooler Tornquist Sea towards the south‐west. Although these three main biofacies continued to dominate during the succeeding Darriwilian stage, distinct changes in the distribution of biofacies took place during the transition from the Dapingian Stage to the Darriwilian. We argue that the biofacies change was triggered by a regressive event related to early Darriwilian cooling, and that the palaeoclimatological changes influenced the Baltic conodont faunas near the Tornquist Sea margin before those of the Iapetus margin (early vs middle Darriwilian).