The calyx of a new genus and species, Kukrusecrinus stellatus from the Kukruse Regional Stage (Sandbian Upper Ordovician) of Northern Estonia, close to Colpodecrinus from the Bromide Formation (Sandbian) of North America, is described in detail. A fragment of Kukrusecrinus sp. is also described from the Ukhaku Regional Stage (Darriwilian). These echinoderm finds indicate the presence of biogeographical connections between Baltica and Laurentia in the second half of the Middle Ordovician (Late Darriwilian). The composition of the family Colpodecrinidae Sprinkle et Kolata, 1982 and its possible phylogenetic relationships with other Ordovician crinoids are considered, based on the reconstruction of the morphogenesis of unusual key features of the family.

A new species of small favositid corals of the genus Sutherlandia Cocke et Bowsher, 1968, found in the Gzhelian deposits of the Moscow Syneclise, is described. The combination of morphological characters, such as the shape and number of squamules, the size of the colony, calyxes, and pores, as well as the number and location of connecting pores, made it possible to identify a new species Sutherlandia gzheliensis sp. nov. It is shown that these corals settled mainly on fragments of other invertebrates. The collection contains many corals that are attached to brachiopod spines and fragments of bryozoan skeletons, as well as crinoid stems.

A new species and genus of colonial coral, Lindaphylon solovjevi , from the Upper Ordovician of Estonia is described in detail. The colony has a zonation similar to the zonation structure of the mesogleal skeleton of octocorals, since it consists of a narrow axis and a surrounding wide cortex of small calcite sclerites and a system of large solenia. Corallites present 12 sclerosepta in a wide shallow cup. A new family Lindaphylonidae and an order Lyndaphylonaceae are based on these features, and a new order placed in the subphylum Anthozoa as incertae sedis. A comparative study of skeleton formation and mesenteric apparatus symmetry in different groups of coral polyps is considered in detail.

Traces of burst oxygen bubbles in a cyanobacterial mat on the surface of carbonate layers of the Middle Cambrian (Amgian, Stage 5 of Series 3) on the left bank of the Lena River (Yakutia) are described and their origin is discussed. They appear to be similar to some Vendian (Ediacaran) fossils usually described as remains of metazoans (e.g., edrioasteroids).

Described two new species of cornutan stylophora of the genus Phyllocystis from the Volkhov Regional Stage (Middle Ordovician, Dapingian) of Baltica. This points to the biogeographic connection between Afro-European part of Gondwana and Baltica since the very beginning of the Middle Ordovician. Both species were confined to cold-water, shallow conditions with low water mobility. Analysis of the axial symmetry of the stylophora and the location of the hydropore indicates the absence of torsion in their ontogeny and the location of the ambulacral system to the right of their anteroposterior axis. To explain this structure, two alternative hypotheses have been proposed: 1) inversion inversion in the development of right and left coelomes; 2) inverted state of stylophora compared to other invertebrates, what makes them similar to the chordates in terms of the position of the dorsal and ventral sides.

Two new species of cornutan stylophora of the genus Phyllocystis are described from the Volkhov Regional Stage (Middle Ordovician, Dapingian) of Baltica. This indicates a biogeographic connection between the Afro-European part of Gondwana and Baltica since the very beginning of the Middle Ordovician. Both species were confined to cold-water, shallow basins with low water mobility. Analysis of the axial symmetry of stylophorans and the location of the hydropore indicates the absence of torsion in their ontogeny and the location of the ambulacral system to the right of their anteroposterior axis. To explain this morphology, two alternative hypotheses are proposed: (1) inversion in the development of right and left coeloms; (2) inverted state of stylophorans compared to other invertebrates, in which respect they resemble chordates in terms of the position of the dorsal and ventral sides.

Oxygen oases were the ecological niches where the evolutionary processes that led to the emergence of the first multicellular animals probably took place. These oases likely appeared in zones of maximum photosynthesis at depths of 10–30 meters in the pelagic zone and on the sea bottom, due to the delay in the release of oxygen from seawater into the atmosphere. The likelihood of the emergence of multicellularity among choanoflagellates, ancestral to Metazoa, is supported by their wide range of life forms, which through various morphogenetic pathways developed the main archetypes postulated by the hypotheses of Phagocytella, Gastraea, Synzoospores and their modifications.

The formation of pentaradial symmetry in the evolution of echinoderms was based on the possibility of the middle–left coelom to terminally forward growth along the anteroposterior axis and the appearance of a second growth vector along the left–right axis during the replication of the formed ambulacra. Both growth vectors were realized into the pentamerism of modern echinoderms due to the development of coelom asymmetry and subsequent torsion associated with the attachment of the larva to the ground by the anterior end of the body. In this process, the molecular genetic mechanisms of anteroposterior growth and left–right regulation, common to bilateria, and associated with the genes of the Wnt, BMP, Nodal signaling cascades, and Hox system genes, were probably used together. In the process of replication of channels extending from the ambulacral ring, the emerging ambulacral system was the organizer of the symmetry of the skeleton and the nervous and muscular systems. Replication in many fossil echinoderms ended on the three channels extending directly from the ambulacral ring. In crinoids, sea urchins, sea stars, brittle stars, and holothurians, the second stage of the formation of a more perfect five-ray symmetry of the ambulacral ring with five radial canals extending from it appeared, associated with a shift in ontogenesis of the branch point to the early stages of hydrocoel development.

The variability of the location of the mouth along the theca in different genera of solutans indicates the presence of torsion in their ontogeny and is explained by a paedomorphic delay or overdevelopment of this process. The plane of change in the location of the mouth with the feeding appendage completely covering it corresponds to the larval plane. The flattening of the theca of most solutans is a secondary result of the transition to lying on the substrate. Larvae or young individuals of different taxa lay down on the substrate on their right or left side. This explains the opposite direction to the ground of adult animals of some taxa. Solutans branched off from the main stem of echinoderms before the origin of pentaradial symmetry, but after the appearance of torsion in their ontogeny. In their level of organization, they appear to be neither directly related to hemichordates, nor to be strongly modified Blastozoa. The appearance of the torsion in the ontogeny of echinoderms phylogenetically precedes the emergence of pentamerism but does not necessarily lead to it. The inversion of the anterior Hox-genes is a consequence, but not the cause, of torsion and pentamerism.

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 K??rkk??la 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 ranuliformis Conodont Zone.

Paleontological data on the appearance and development of metazoan axial symmetry are compared with molecular genetic evidence to determine the axial body plan and growth vectors of bilaterian organisms by the coordinated functioning cascades of signaling pathways Wnt, BMP, Nodal, Hedgehog, and regulatory Hox and ParaHox gene clusters. The axial body symmetry in most bilaterians is associated with the uniaxial growth vector. Posterior body growth ensured by the progenitor stem cell pool of the growth zone localized at the preterminal end of the body became a cardinal evolutionary innovation in Bilateria. The conservatism of ancestral molecular mechanisms of bilaterian body plan specification is combined with their flexibility, which provided the possibility of evolutionary transformations of the axial pattern of the organism. Radical evolutionary changes with the formation of additional growth zones are reported in the representatives of Deuterostomia (Vertebrata and Echinodermata). In modern vertebrates, while the main ancestral axis of body growth is preserved, two pairs of additional growth zones (limb buds) emerged. They are considered paramorphic homologs (replicates) of the posterior part of the main body axis. The pentamerous pattern of the body plan and growth of recent echinoderms can be explained as a result of the multiplication of the ancestral organizing center of axial architectonics and its radialization during larval metamorphosis (or in early embryogenesis during lecithotrophic development) with a corresponding multiplication of growth zones localized at the ends of rays (ambulacra). It is consistent with paleontological data and the concept of echinoderm rays as paramorphic homologs of the main body axis of the bilaterian ancestors of echinoderms.

The main directions and results of the Paleontological Institute’s activities over the past 40 years are considered. The demand for paleontological research conducted by the Institute’s scientists has shifted from stratigraphy and geological surveying towards the study of the evolution of the biosphere and evolutionary developmental biology. It is shown that the rapidly developing biological aspect of paleontological research continues to be largely based on a detailed analysis of the stratigraphic and geographic distribution of fossil organisms.

The hybocrinid crinoid Trecrinus schmidti gen. et sp. nov. is described from the Middle Ordovician (Uhaku Regional Stage, Darriwilian) of the lower reaches of the Volkhov River. Three long arms in the A, D and C radii, and a low cup with a flattened base without a stem are the main characteristics of the new genus. Morphology of the new genus emphasizes the similarities and differences in the morphogenetic capabilities of the hybocrinid lineages, which simultaneously developed on the Baltica and Laurentia paleocontinents, separated from each other in Ordovician. In the pronounced tiering of the Valim benthic community, this new genus occupied the lowest bottom stage.

As a result of examination of the collection of holdfasts of stemmed echinoderms from the Ordovician of the Leningrad oblast, Russia, all holdfasts are divided into two large groups: “unbranched” and “branched.” Three morphotypes of branched holdfasts are identified and described in detail. Morphogenetic transitions between morphotypes and their relationship with the peculiarities of the evolution of stemmed echinoderms in the Ordovician evolutionary radiation were identified.

In myelodactylids, the coil of the mesostele and the crown with a proxistele coiled in the same plane in the opposite direction result from the two fundamental processes in the evolution of echinoderms, which happened when their free-floating ancestor settled at the bottom and became attached to the ground by the ventral side of the anterior end of the body. The coil of the mesostele emerged due to the tendency of the stem to curve from the ventral to the dorsal side in the larvae, after they became attached, by the ventral side of the preoral lobe, and the growth vector changed from horizontal to vertical. The curvature of the proxistele, together with the crown in the same larval plane but in the opposite direction, from the dorsal side to the ventral, appeared during the paedomorphic delay in the process of elevation (torsion) in the ontogeny of myelodactylids. The coiling of the mesostele and the curvature of the proxistele with the crown in the E‑BC plane determine the conformity of this plane to the larval plane in pentaradiate echinoderms, and the approximate conformity of the E ray to the dorsal side of the larva, and the interray BC to its ventral side. The morphology of myelodactylids shows that their stem in the feeding position stretched along slightly compacted ground, resting on cirri, and the crown extended at a slight acute angle over the stem downstream. In cases of danger, the stem coiled with its lower side inward, so that the crown was protected inside the coil by cirri. The coil of Zuravlicrinus milicinae gen. et sp. nov. from the Silurian of the Urals, and Valimocrinus terentyevi gen. et sp. nov. from the Ordovician of the Leningrad Region, coiled while growing around stems of other crinoids, and could not uncoil. Both genera, as well as Musicrinus Donovan, are known only from stem fragments and are tentatively assigned to myelodactylids. The finding of the genus Eomyelodactylus in China shows its spread far beyond North America. Findings of myelodactylids in Siberia show significant differences from other myelodactylids, allowing them to be assigned to a new genus Imagdacrinus gen .nov.

Based on new finds and additional evidence, the interpretation of the enigmatic echinoderm Bolboporites is confirmed; it is a blastozoan echinoderm most likely related to eocrinoids, in which juvenile holdfast, columnals, and reduced skeletal plates of the theca were fused into a single skeletal element, to which a biserial brachiole was attached.

This review summarizes literature data on phylogenomics, indicating genomic–morphogenetic correlations that determine the metazoan body plan and phenotypic complexity. Radical, macroevolutionary transformations of the bilaterian body plan, which correlate with duplications or losses of individual Hox genes of an ancestral cluster, or with its topological reorganization, are revealed. Regressive evolution is rather common in Metazoa; reduction and simplification of the genome and phenome occurred quite often in the evolution of metazoans. Since the system of cluster Hox genes has been shown to be the key regulatory node of genome–phenome relationships, regulating a wide range of developmental processes, as well as being the correlation node determining different evolutionary trajectories of various bilaterian taxa, it is a highly informative correlate and indicator of evolutionary changes of the body plan.

In this article the history of the Joint Russian‒Mongolian Paleontological Expedition (JRMPE) and its work and main results are described. The organization of the expedition (Soviet-Mongolian at that time) in 1969 was preceded by a hard and lengthy work to promote this idea in academic and state institutions. The joint expedition was planned as a complex one by both the tasks and the tactics of their implementation. As a result, the JRMPE became the largest expedition in the history of paleontology. The excavations carried out within its framework provided the most important scientific materials and significantly replenished the paleontological collections of the museums in Moscow and Ulaanbaatar. The JRMPE became the basis for the development of paleontology in Mongolia and the training of highly qualified scientific personnel for Russia and Mongolia.