Soft-bodied Organisms Research Articles

The Emu Bay Shale: A unique early Cambrian Lagerstätte from a tectonically active basin.

The Emu Bay Shale (EBS) of South Australia is anomalous among Cambrian Lagerstätten because it captures anatomical information that is rare in Burgess Shale-type fossils, and because of its inferred nearshore setting, the nature of which has remained controversial. Intensive study, combining outcrop and borehole data with a compilation of >25,000 fossil specimens, reveals that the EBS biota inhabited a fan delta complex within a tectonically active basin. Preservation of soft-bodied organisms in this setting is unexpected and further underscores differences between the EBS and other Cambrian Lagerstätten. Environmental conditions, including oxygen fluctuations, slope instability, high suspended sediment concentrations, and episodic high-energy events, inhibited colonization of the lower prodelta by all but a few specialist species but favored downslope transportation and preservation of other largely endemic, shallow-water benthos. The EBS provides extraordinary insight into early Cambrian animal diversity from Gondwana. These results demonstrate how environmental factors determined community composition and provide a framework for understanding this unique Konservat-Lagerstätte.

Organic geochemistry, sedimentology and palaeontology of the Khatyspyt Formation, Arctic Siberia: Towards an integrated view of Ediacaran biofacies

The terminal Ediacaran Khatyspyt Lagerstätte (ca. 550–544 Ma) of Arctic Siberia has been a prime target for geobiological research. Previous evidence suggested that Ediacaran macroscopic soft-bodied organisms could be highly sensitive to sedimentary processes and various environmental factors such as water column stratification and seawater redox-conditions. By integrating organic geochemistry, sedimentology, and palaeontology of the Khatyspyt Formation, we identified three biofacies. The most proximal Longifuniculum biofacies consists of outer- to mid-ramp debris flow deposits and is characterised by a high taxonomic diversity and biomarker proxies pointing to a non-stratified non-euxinic water column. The most distal Aspidella biofacies comprises outer-ramp thin-bedded calcareous turbidites and is also marked by a high taxonomic diversity, although the associated biomarker proxies provide evidence for a stratified euxinic environment. Transient between those is the Nenoxites biofacies, consisting of outer- to mid-ramp debris flow deposits and characterised by a low taxonomic diversity, with biomarker proxies indicating redox instability. This systematic pattern suggests that the distribution of Ediacaran organisms was influenced by a heterogenous redox landscape. More specifically, the highest diversity of benthic soft-bodied organisms, including the iconic Charnia masoni, appears in stratified euxinic environments, while the highest diversity of macroalgae is found in non-stratified settings. The occurrence of a complex Ediacaran community in a stratified euxinic environment suggests that anoxia might have driven ecological differentiation of organisms, and that heterogeneous and dynamic redox landscapes were far more significant in early animal evolution than hitherto appreciated.

Ediacaran marine animal forests and the ventilation of the oceans

The rise of animals across the Ediacaran-Cambrian transition marked a step-change in the history of life, from a microbially dominated world to the complex macroscopic biosphere we see today.1,2,3 While the importance of bioturbation and swimming in altering the structure and function of Earth systems is well established,4,5,6 the influence of epifaunal animals on the hydrodynamics of marine environments is not well understood. Of particular interest are the oldest "marine animal forests,"7 which comprise a diversity of sessile soft-bodied organisms dominated by the fractally branching rangeomorphs.8,9 Typified by fossil assemblages from the Ediacaran of Mistaken Point, Newfoundland,8,10,11 these ancient communities might have played a pivotal role in structuring marine environments, similar to modern ecosystems,7,12,13 but our understanding of how they impacted fluid flow in the water column is limited. Here, we use ecological modeling and computational flow simulations to explore how Ediacaran marine animal forests influenced their surrounding environment. Our results reveal how organism morphology and community structure and composition combined to impact vertical mixing of the surrounding water. We find that Mistaken Point communities were capable of generating high-mixing conditions, thereby likely promoting gas and nutrient transport within the "canopy." This mixing could have served to enhance local-scale oxygen concentrations and redistribute resources like dissolved organic carbon. Our work suggests that Ediacaran marine animal forests may have contributed to the ventilation of the oceans over 560 million years ago, well before the Cambrian explosion of animals.

Electro-fluid-dynamics (EFD) of soft-bodied organisms swimming through mucus having dilatant, viscous, and pseudo-plastic properties

The sperm propelling mechanism has been proposed as a possible resource for soft micro-robots in confined spaces, with potential applications in biomedical engineering. Human sperm cells essentially swim through the non-Newtonian liquid (cervical mucus) to reach their target. Thus, sperm cells swimming through non-Newtonian fluids is not vital only for physiology, but also for the fabrication of swimming micro-robots. Inspired by these remarkable applications, we examine the basic mechanics of spermatozoa motility using an undulating sheet model. This undulating sheet is bounded between two rigid walls which is self-propeling in the negative axial direction. The liquid around the spermatozoa is taken as Carreau fluid with electro-osmotic properties. The application of the lubrication approximation results in the reduction of momentum equations. The resulting ODE is solved numerically via the finite difference method and MATLAB’s built-in routine bvp5c. The unknowns that are present in the boundary conditions are refined by the root-finding algorithm. Power losses, cell speed, flow rate, velocity of the fluid, and streamline pattern are visualized by graphs. The findings of this study have important implications for the designing and optimization of electrically controlled microswimmers.

New Canadian amber deposit fills gap in fossil record near end-Cretaceous mass extinction

Amber preserves an exceptional record of tiny, soft-bodied organisms and chemical environmental signatures, elucidating the evolution of arthropod lineages and the diversity, ecology, and biogeochemistry of ancient ecosystems. However, globally, fossiliferous amber deposits are rare in the latest Cretaceous and surrounding the Cretaceous-Paleogene (K-Pg) mass extinction.1,2,3,4,5 This faunal gap limits our understanding of arthropod diversity and survival across the extinction boundary.2,6 Contrasting hypotheses propose that arthropods were either relatively unaffected by the K-Pg extinction or experienced a steady decline in diversity before the extinction event followed by rapid diversification in the Cenozoic.2,6 These hypotheses are primarily based on arthropod feeding traces on fossil leaves and time-calibrated molecular phylogenies, not direct observation of the fossil record.2,7 Here, we report a diverse amber assemblage from the Late Cretaceous (67.04± 0.16 Ma) of the Big Muddy Badlands, Canada. The new deposit fills a critical 16-million-year gap in the arthropod fossil record spanning the K-Pg mass extinction. Seven arthropod orders and at least 11 insect families have been recovered, making the Big Muddy amber deposit the most diverse arthropod assemblage near the K-Pg extinction. Amber chemistry and stable isotopes suggest the amber was produced by coniferous (Cupressaceae) trees in a subtropical swamp near remnants of the Western Interior Seaway. The unexpected abundance of ants from extant families and the virtual absence of arthropods from common, exclusively Cretaceous families suggests that Big Muddy amber may represent a yet unsampled Late Cretaceous environment and provides evidence of a faunal transition before the end of the Cretaceous.

The Cabrières Biota (France) provides insights into Ordovician polar ecosystems

Early Palaeozoic sites with soft-tissue preservation are predominantly found in Cambrian rocks and tend to capture past tropical and temperate ecosystems. In this study, we describe the diversity and preservation of the Cabrières Biota, a newly discovered Early Ordovician Lagerstätte from Montagne Noire, southern France. The Cabrières Biota showcases a diverse polar assemblage of both biomineralized and soft-bodied organisms predominantly preserved in iron oxides. Echinoderms are extremely scarce, while sponges and algae are abundantly represented. Non-biomineralized arthropod fragments are also preserved, along with faunal elements reminiscent of Cambrian Burgess Shale-type ecosystems, such as armoured lobopodians. The taxonomic diversity observed in the Cabrières Biota mixes Early Ordovician Lagerstätten taxa with Cambrian forms. By potentially being the closest Lagerstätte to the South Pole, the Cabrières Biota probably served as a biotic refuge amid the high-water temperatures of the Early Ordovician, and shows comparable ecological structuring to modern polar communities.

Deep CNN-Based Static Modeling of Soft Robots Utilizing Absolute Nodal Coordinate Formulation.

Soft continuum robots, inspired by the adaptability and agility of natural soft-bodied organisms like octopuses and elephant trunks, present a frontier in robotics research. However, exploiting their full potential necessitates precise modeling and control for specific motion and manipulation tasks. This study introduces an innovative approach using Deep Convolutional Neural Networks (CNN) for the inverse quasi-static modeling of these robots within the Absolute Nodal Coordinate Formulation (ANCF) framework. The ANCF effectively represents the complex non-linear behavior of soft continuum robots, while the CNN-based models are optimized for computational efficiency and precision. This combination is crucial for addressing the complex inverse statics problems associated with ANCF-modeled robots. Extensive numerical experiments were conducted to assess the performance of these Deep CNN-based models, demonstrating their suitability for real-time simulation and control in statics modeling. Additionally, this study includes a detailed cross-validation experiment to identify the most effective model architecture, taking into account factors such as the number of layers, activation functions, and unit configurations. The results highlight the significant benefits of integrating Deep CNN with ANCF models, paving the way for advanced statics modeling in soft continuum robotics.

Shadows of the past: An Ediacara-style taphonomic window in advent of Cambrian bioturbators

One of the hypotheses, explaining a sudden vanishing of the Ediacaran biota at the Ediacaran-Cambrian transition, correlates this phenomenon with a disappearance of specific taphonomical factors providing fossilization of soft-bodied organisms (‘Cheshire Cat’ hypothesis). The terminal Ediacaran-Lower Cambrian fossil record does reveal a progressive development of bioturbation and associated decrease of microbial mats, which played a crucial role in the late Precambrian ecosystems. But the tempo of the evolution of organism-sediment interactions significantly varied between different zones of marine basins. Hence, if the ‘Cheshire Cat’ hypothesis is correct, the sedimentary environments barely (if any) colonized by the burrowing eumetazoans might provide the unique interplay of conditions for habitat and Ediacara-style preservation of soft-bodied organisms even in the early Cambrian. Herein we present the new carbon isotope data calibrating the Terreneuvian record in the Ura Uplift of South Siberia and also report fossilized soft-bodied organisms morphologically-close to the Ediacaran holdfasts preserved in association with microbially-induced sedimentary structures in the Cambrian Stage 2 carbonate-siliciclastic Nokhtuisk Formation. Although the late Terreneuvian age, this marginal marine succession is poorly bioturbated and hosts the soft-bodied fossils in the intervals undisturbed by the burrowing organisms. Our study demonstrates the possibility of Ediacara-style preservation of soft-bodied organisms in specific habitat and taphonomic conditions until being replaced by the Cambrian endobenthos.

Towards Stage Subdivision of the Vendian

Abstract —The way towards the stage subdivision of the Vendian starts with the assemblage of a complete composite geological section of this interval of the Earth’s history, a succession of geological bodies each reflecting a certain ecosystem state. A Vendian composite section of the Siberian Platform, which is a succession of regional to sub-global scale geological bodies reflecting particular states or unidirectional transformations determined by episodic expansion of relatively oxygen-rich environments onto the shelf and alternating with large-scale oceanic anoxia and euxinia events is proposed as a reference for constructing the stage subdivision of the Vendian. The redox instability had to be accompanied by changes in nutrient availability and could not but affect the course of macroevolution and macroecology. The geological record of the Vendian of the Siberian Platform is also marked by episodic increase in alkalinity of the World Ocean. At least five such alkalinity events could be provisionally identified in the composite section of the Vendian of the Siberian Platform. A hypothesis is proposed suggesting that the alkalinity events could control the appearance and disappearance in the geological record of a distinctive suite of sedimentary structures in carbonates, the discrete nature of early diagenetic cementation of aluminosilicoclastic sediments, and fossilization of soft-bodied organisms in the Vendian.

Taphonomy of early life (2.1 Ga) in the francevillian basin (Gabon): Role of organic mineral interactions

The taphonomy of early soft-bodied organisms in Palaeoproterozoic sediments is not yet clearly understood, even though some locations where these fossils are found present all the conditions for exceptional fossil preservation. Indeed, the degree of fossil preservation has received attention, but better knowledge of the environmental conditions and associated taphonomic processes is also essential. In the Gabonese Francevillian Basin, the discovery of macrofossils (2.1 Ga) of multicellular organisms in black shales is an outstanding example of this degree of preservation. Indeed, the biological diversity, as a wide variety of fossil morphologies are observed, is associated with two major taphonomic processes – moulding (lenticular-shaped forms) or early pyritization, while the fossil host rocks were not deeply buried and were affected only by weak to moderate diagenesis. However, usually, the mechanisms of this preservation remain difficult to assess, as the original taphonomic processes are impacted by diagenesis and still misunderstood. In this way, by closely observing fossil mineralization in four morphotypes of macrofossils and associated host rocks from mineralogical and textural points of view, this work aims to provide some keys to a taphonomic comprehension of soft-bodied organism preservation. After the deposition of dead organisms on the clayey sediment, an illitization process, which depends on the availability of dissolved K driven by bacterial activity, started from the first stages of preservation by moulding the lenticular-shaped forms and proceeded in the pores of the other macrofossils after their pyritization. At the fossil level, the intensity of illitization is controlled by the mode of preservation and the evolution of the associated permeability. In the nonpyritized lenticular-shaped specimens, illitization was not achieved, preserving I-S mixed-layer minerals, while in the pyritized forms, the illitization degree was more extensive. In comparison, I-S mixed-layer minerals are almost absent in pyritized abiotic concretions. A second process, which occurred later, consists of general chloritization from fluid circulation. Our detailed results show that each specimen behaved like a microsystem with its own physico-chemical and mineralogical evolution during preservation/diagenesis. This finding allows us to propose a conceptual model of the taphonomic history, describing the fossilization stages for each type of specimen.

Preservation of Soft-Bodied Organisms in Early Cambrian Carbonates

Abstract —Preservation of soft-bodied organisms as casts is common in the Ediacaran fossil record and extremely rare in Cambrian rocks. Among the factors, limiting fossilization of soft tissues, there is bioturbation–sediment disturbance by burrowing organisms. It is the emergence of burrowing metazoans and associated sediment bioturbation that is thought to be one of the major causes for the disappearance of Ediacaran soft-bodied organisms from the fossil record around ~540 Ma. Here, we study an assemblage of fossils preserved as casts in the Fortunian carbonates of the Olenek Uplift (northeastern Siberian Platform) in association with a typical Fortunian ichnoassemblage. The overall morphology and preservation of the fossils reveal that they comprise microbially induced sedimentary structures and soft-bodied holdfasts. The latter are vaguely reminiscent of some Ediacaran holdfasts, though it is unlikely that they include their phylogenetic descendants. Three-dimensional preservation of the studied fossils was caused by authigenic crystallization of calcite and its further early-diagenetic dolomitization. Our study confirms the critical importance of specific environmental conditions ensuring preservation of soft-bodied organisms as three-dimensional molds and casts. This unique interplay of environmental factors became rare in the Cambrian Period, which was caused by intensification and expansion of bioturbation in marine basins.

Biomonitoring of radioactive contamination using benthic invertebrate communities in Manzala Lake, Egypt

Manzala Lake, the largest in Egypt’s Nile Delta, has significant human activity. The Lake’s ecological condition has recently been impacted by a significant increase in agricultural, industrial, and urban wastewater discharge. Avaluable tool for monitoring the water quality of Manzal aLake is the sensitivity of invertebrate species to various types of pollution, such as radioactive contamination. Activity concentrations of radionuclides Radium-226 (226Ra),Thorium-232 (232Th), Potassium-40 ( 40K), and Caesium-137(137Cs) were measured in water, sediments and benthic invertebrate samples in 2020. The benthic community’s spatial distribution and the radionuclides’ bioaccumulation were evaluated to determine possible relationships.Thirty taxa of benthic invertebrates were recognised.The data illustrated that the mean activity concentratio nof radionuclides in water was in the order of 40K > 232Th >226Ra > 137Cs, which changed into 40K > 226Ra > 232Th > 137Csin the sediment and benthic invertebrates. Gastropoda and Ostracoda are the dominating groups of benthos in the lake and are related to the highest concentrations of radionuclides. The benthos species with shells dominated at the sites with the highest activity concentration of 226Ra, while soft-bodied organisms dominated in sites with the highest average 137Cs activity in these samples.

Multivariate mapping of ontogeny, taphonomy and phylogeny to reconstruct problematic fossil taxa.

Exceptionally preserved fossils of soft-bodied organisms provide unique evidence of evolutionary history, but they are often contentious; different approaches frequently produce radically different reconstructions of taxa and their affinities. Conflict arises due to difficulties in disentangling the three non-independent factors that underlie all morphological variation within and between fossils: ontogeny, taphonomy and phylogeny. Comparative data from extant organisms can be extremely powerful in this context, but is often difficult to apply given the multi-dimensionality of anatomical variation. Here, we present a multivariate ordination method using discrete morphological character data from modern taxa at different ontogenetic and taphonomic stages (semaphoront and 'semataphonts'). Analysing multiple axes of morphological variation simultaneously allows us to visualize character combinations that are likely to exist in fossil specimens at intersecting stages of growth and decay, and thus constrain interpretation of fossils. Application to early vertebrates finds variation in fossil specimens to be accounted for by all three axes: primarily decay in Mayomyzon, ontogeny in Priscomyzon and phylogeny in 'euphaneropoids' and Palaeospondylus. Our demonstration of empirical multi-factorial variation underscores the power of multivariate approaches to fossil interpretation, especially non-biomineralized taxa. As such, this conceptual approach provides a new method for resolving enigmatic taxa throughout the fossil record.

Ediacara-Type Biota in the Upper Precambrian of the Timan Range (Dzhezhim–Parma Hill, Komi Republic)

Macroscopic fossils as composite molds and casts of Ediacara-type soft-bodied organisms were found in the Dzhezhim Formation of the Timan Range for the first time. Among them, representatives of palaeopascichnids, arboreomorphs, chuariomorphids, microbial colonies, and trace fossils were identified. This finding of such a large number of various Ediacaran fossils on the Timan Range not only develops our understanding of their paleogeographic range but also clarifies the age limits of the deposition of the Dzhezhim Formation, the stratigraphic position of which in the Upper Precambrian section was controversial.

Ages of the Ediacaran Volyn-Brest trap volcanism, glaciations, paleosols, Podillya Ediacaran soft-bodied organisms, and the Redkino-Kotlin boundary (East European Craton) constrained by zircon single grain U-Pb dating

Combined single zircon crystal CA-ID-TIMS U-Pb dating, micropaleontological study, and quantitative XRD analysis were used to correlate sections of the Podillya-Moldova Ediacaran succession with sections of the Volyn Large Igneous Province (Volyn LIP). Five U-Pb zircon ages were obtained from Volyn LIP rocks, and four from three distinct Podillya bentonite horizons. Biostratigraphic determinations were made for one section in Volyn, and two sections in Podillya. It was concluded that Volyn LIP activity had two phases. A first phase of mafic and felsic magmatism was characterized by eruption of numerous basalt lava flows, and lasted from c. 580 to 567 Ma; a second phase of pyroclastic felsic magmatism, with no lavas, lasted from 567 to c. 545 Ma. The two phases can be distinguished in the quantitative mineral composition of rocks from the Kobryn 1K borehole in western Belarus. In the central area of the Volyn LIP, near Brest, rocks of the felsic phase were removed by erosion before deposition of the Kotlin subgroup. The Podillya Ediacaran succession, from the top of the Hrushka formation to the top of the Kalyus beds, (i.e. the Mohyliv-Podilsky group), hosts soft-bodied organisms. This succession corresponds to the felsic phase of the Volyn LIP. The age of the Redkino-Kotlin subgroup biostratigraphic boundary, at the base of the Nagoryany formation, is constrained at 555–545 Ma, close to the end of the Volyn LIP activity. The newly discovered glaciation in Volyn is dated as c. 567 Ma. The period of development of kaolinitic paleosols on top of the Volyn LIP effusive rocks is dated at c. 570–550 Ma.

A diverse Ediacara assemblage survived under low-oxygen conditions

The Ediacaran biota were soft-bodied organisms, many with enigmatic phylogenetic placement and ecology, living in marine environments between 574 and 539 million years ago. Some studies hypothesize a metazoan affinity and aerobic metabolism for these taxa, whereas others propose a fundamentally separate taxonomic grouping and a reliance on chemoautotrophy. To distinguish between these hypotheses and test the redox-sensitivity of Ediacaran organisms, here we present a high-resolution local and global redox dataset from carbonates that contain in situ Ediacaran fossils from Siberia. Cerium anomalies are consistently >1, indicating that local environments, where a diverse Ediacaran assemblage is preserved in situ as nodules and carbonaceous compressions, were pervasively anoxic. Additionally, δ238U values match other terminal Ediacaran sections, indicating widespread marine euxinia. These data suggest that some Ediacaran biotas were tolerant of at least intermittent anoxia, and thus had the capacity for a facultatively anaerobic lifestyle. Alternatively, these soft-bodied Ediacara organisms may have colonized the seafloor during brief oxygenation events not recorded by redox proxy data. Broad temporal correlations between carbon, sulfur, and uranium isotopes further highlight the dynamic redox landscape of Ediacaran-Cambrian evolutionary events.

Polar gigantism and remarkable taxonomic longevity in new palaeoscolecid worms from the Late Ordovician Tafilalt Lagerstätte of Morocco

ABSTRACT The Late Ordovician Tafilalt Biota of the Moroccan Anti-Atlas includes a diverse range of soft-bodied organisms, including palaeoscolecids, paropsonemid eldonioids, graptolites and cheloniellid arthropods, as well as a rich assemblage of mineralised taxa, among them conulariids, trilobites and echinoderms, often found as articulated skeletons. The new fossil locality, not far from the original Bou Nemrou site, has produced two new palaeoscolecid taxa, the new genus and species Anguiscolex africanus and the new species Wronascolex superstes. They are preserved as compression fossils in fine-grained mudstones, where the original phosphatic sclerites have been diagenetically substituted by pyrite and later weathered to iron oxides, giving them a characteristic rusty colour. This area of Gondwana was located adjacent to the Late Ordovician South Pole and both Anguiscolex africanus gen. et sp. nov. and Wronascolex superstes sp. nov. present a degree of polar gigantism, which has been suggested for other taxa in such high palaeolatitudes such as bryozoans, conulariids, trilobites and radiodonts. Lastly, the occurrence of Wronascolex extends the distribution of this typically Cambrian taxon into the Late Ordovician, indicating a total range for the genus exceeding 60 million years, more than any other palaeoscolecid genus described to date. urn:lsid:zoobank.org:pub:6FD5F086-FBC6-465C-B5B7-D29EF9A6B8FB

Description of Acheronauta gen. nov., a possible mandibulate from the Silurian Waukesha Lagerstätte, Wisconsin, USA

The Silurian Waukesha Lagerstätte of Wisconsin, USA, hosts a fauna of distinctive soft-bodied organisms, including a range of graptolites, worms, trilobites and uncommon arthropods, but otherwise lacks most of the shelly fauna typical of contemporaneous deposits. While several species have been described, much of the Waukesha biota remains understudied. Here, we examine numerous specimens of a previously unnamed and undescribed vermiform arthropod that bears a cephalic carapace, a suite of specialized head appendages, a long, segmented trunk, and a battery of small swimming appendages. The taxonomic and phylogenetic evaluations presented herein establish this taxon as a novel genus and species, Acheronauta stimulapis gen. et sp. nov., plausibly positioned within the basal mandibulates. Our phylogenetic analyses suggest that a new arthropod clade may be warranted, comprising Acheronauta gen. nov., Captopodus, and the thylacocephalans. https://zoobank.org/urn:lsid:zoobank.org:pub:B572831B-1A8A-4E4E-9258-34D91AA22A74nk

Ventral Morphology of the Non-Trilobite Artiopod Retifacies abnormalis Hou, Chen & Lu, 1989, from the Early Cambrian Chengjiang Biota, China.

Simple SummaryThis study reveals the detailed ventral morphology of the enigmatic Cambrian non-biomineralized euarthropod Retifacies abnormalis from the early Cambrian of South China through the use of micro-computer tomography and three-dimensional modelling. The ventral morphology of R. abnormalis includes five pairs of cephalic appendages in three forms (one uniramous antenna pair, three uniramous appendage pairs, one biramous appendage pair), biramous trunk appendages with multilamellar exites, a variable number of pygidial appendage pairs (five or six) and a multi-articulated tailspine with two pairs of short lobe-like accessories. The new anatomical data inform the ecology and evolution of R. abnormalis in the broader evolutionary context of trilobite-like euarthropods known from sites of exceptional preservation. R. abnormalis possessed a higher degree of appendage differentiation along the body than initially thought, in parallel to similar discoveries of other trilobitomorphs such as Naraoia spinosa, Pygmaclypeatus daziensis, and Sinoburius lunaris. This discovery provides additional support to the hypothesis that early trilobitomorphs were ancestrally characterized by heteronomous ventral appendages with various degrees of functional specialization for feeding and respiration. The artiopodans represent a diverse group of euarthropods with a typically flattened dorsal exoskeleton that covers numerous pairs of biramous ventral appendages, and which are ubiquitous faunal components of the 518-million-year-old Chengjiang Lagerstätte in South China. Despite their abundance, several Chengjiang artiopodans remain poorly known, such as the large euarthropoda Retifacies abnormalis, Hou, Chen & Lu, 1989, which is distinguished by the presence of mesh-like ornamentation on its dorsal exoskeleton. Although only a few ventral details were described in a single study in 25 years, it has been frequently featured in phylogenetic analyses that explore the relationships between Cambrian euarthropods. Here, we employ micro-CT and fluorescent microphotography to investigate the exceptionally preserved ventral morphology of R. abnormalis and explore its phylogenetic implications through maximum parsimony and Bayesian inference. Detailed morphology revealed here better supports R. abnormalis as a sister group to the diminutive artiopod Pygmaclypeatus daziensis, also known from Chengjiang, and strengthens the close relationship of these taxa that have been suggested by previous studies as early-branching representatives of Trilobitomorpha. Cephalic appendages suggest this animal might be a scavenger, possibly feeding on soft-bodied organisms. Different pairs of pygidial appendages suggest an anamorphic post-embryonic ontogeny, which adds to the understanding of the developmental mode of Cambrian artiopods, and further supports the statement that post-hatching segment addition occurred in the ancestor of Euarthropoda.

THE ASSOCIATION OF BENTHIC MACROBIOTA OF THE LATE EDIACARAN OF PODILLIA IN THE DEPOSITS OF THE MOHYLIV-PODILSKYI GROUP IN THE QUARRY NEAR THE DNIESTER HPP

The maximum level of Ediacaran-type biota diversity in the Volyn-Podilskyi sedimentation basin existed in the time interval that corresponds to the Lomoziv and Yampil Members of the Mohyliv Formation. Studies of the isotopic age showed that this period ended 557 million years ago. Environmental conditions that prevented the existence of classical Ediacaran biota prevailed during the accumulation of the Lyadova Member. A less numerous biotic association of the transitional type with individual representatives of the Ediacara-type association is observed in the deposits of the Bernashivka Member. Paleoecological conditions in the sedimentation basin were reconstructed on the basis of the study of the section in the quarry near the Dniester HPP. More than a thousand specimens of fossils of soft-bodied organisms and tracefossils were collected there. More than three dozen species were discovered in Podilla for the first time. The study of a significant number of new species continues. It was established that the biotic association of the Mohyliv Formation was dominated by problematic frondomorphic and spherical organisms, which sometimes reached significant sizes.The existence of probable ancestors of several groups of biota of the Phanerozoic type:Cnidaria, Porifera, Lophotrochozoa, Mollusca and Metaphita were found in the Late Ediacaran shallow water ecosystem. Thefossil remains of the earliest probable members of the phylum Chordata of the subphylum Tunicata are collected here. These arethe most likely ancestors of vertebrates according to data from several research groups. It was established that some Ediacaran organisms used life strategies typical for representatives of later biota: combinations of different types of reproduction, activeresponse to external environmental factors, etc. New methods of fossil research were proposed and important results were obtained in the process of their use. It was established that quite sharp changes in the taxonomic composition of biotic associations in the section of sediments are caused by facies conditions and evolutionary processes. This makes it possible to use the fossilized remains of the Ediacaran biota for the needs of biostratigraphy and correlation with the same age deposits in other regions.