The evolutionary events during the Ediacaran-Cambrian transition (~541 Myr ago) are unparalleled in Earth history. The fossil record suggests that most extant animal phyla appeared in a geologically brief interval, with the oldest unequivocal bilaterian body fossils found in the Early Cambrian. Molecular clocks and biomarkers provide independent estimates for the timing of animal origins, and both suggest a cryptic Neoproterozoic history for Metazoa that extends considerably beyond the Cambrian fossil record. We report an assemblage of ichnofossils from Ediacaran-Cambrian siltstones in Brazil, alongside U-Pb radioisotopic dates that constrain the age of the oldest specimens to 555-542 Myr. X-ray microtomography reveals three-dimensionally preserved traces ranging from 50 to 600 μ m in diameter, indicative of small-bodied, meiofaunal tracemakers. Burrow morphologies suggest they were created by a nematoid-like organism that used undulating locomotion to move through the sediment. This assemblage demonstrates animal-sediment interactions in the latest Ediacaran period, and provides the oldest known fossil evidence for meiofaunal bilaterians. Our discovery highlights meiofaunal ichnofossils as a hitherto unexplored window for tracking animal evolution in deep time, and reveals that both meiofaunal and macrofaunal bilaterians began to explore infaunal niches during the late Ediacaran. NATuRE ECoLoGy & EvoLuTIoN Articles Nature ecology & evolutioNsupport for these suggestions is limited to purported body fossils of sponges 19 and demosponge biomarkers 20 . A considerable gap therefore remains between the fossil record of the late Ediacaran and molecular clock estimates for deep splits in the animal tree, for example the origin of Metazoa and Eumetazoa 3 . Assuming that contemporary molecular clock analyses yield accurate, if imprecise 18 , node ages for animal divergences, a small body size and concomitant limited fossilization potential 21 could reconcile these discordant records of animal evolution (but see ref. 22 ).The small body size of the ancestral bilaterian is supported by recent phylogenomic analyses of deep animal relationships, with acoel flatworms and xenoturbellids (Xenacoelomorpha) being a sister group to all remaining bilaterians (Nephrozoa) 23 , and smallbodied spiralian taxa (the 'Platyzoa') recognized as a paraphyletic grade with respect to macroscopic trochozoans 24 . This suggests that early bilaterians and spiralians were small bodied, possibly meiofaunal, and moved using ciliary gliding.Meiofauna comprises all organisms between 32 and 1,000 μ m in size that inhabit pore-water-rich sediments in freshwater to deepmarine environments 25 . Modern meiofaunal communities include animals, foraminifera and some ciliates, and contribute significantly to sediment bioturbation and bioirrigation 26,27 . The meiofauna can be divided into permanent members (that is, animals with organisms of a small size adapted and restricted to the meiofaunal, interstitial realm) and temporary meiofauna (for example, the ...

Mixed carbonate-siliciclastic deposits of the Tamengo Formation (terminal Ediacaran), record the rise of calcifying metazoans and the origin of exoskeletons in animals. To explore the relationships between environmental setting and the first appearance of calcified metazoans and their ecology, this study presents detailed sedimentological and stratigraphic data of eight sections that capture the final stages of the Ediacaran (550 to 543 Ma). This study combines stratigraphic characterization with detailed facies descriptions and evaluates lateral heterogeneity and overall ramp sedimentation integrated with fossil distribution. The Tamengo Formation represents a storm-dominated ramp. The outer to mid-ramp is composed of very finegrained siliciclastic rocks containing Corumbella body fossils and thinbedded mudstone/wackestone containing Cloudina. The mid-inner ramp is dominated by wackestone/packstone with abundant Cloudina skeletal debris and ooid packstone/grainstone shoal deposits. Locally, fragments of Corumbella and Cloudina are found on the same horizon, which is a result of their high accumulation rate, resulting from the reworking and mixing of epifaunal organisms. In spite of taphonomic biases, the general distribution of Corumbella and Cloudina across the unit suggests that these organisms have been transported across ramp and/or probably show a differential response of fauna preservation. When compared to other occurrences worldwide, this dataset indicates an already complex ecosystem in the Ediacaran, where these early animals were capable of adapting to specific environmental niches.

The Tamengo Formation (Corumbá Group) is an important Ediacaran stratigraphic unit in South America due to the presence of metazoan fossils and geochemistry data of carbonate rocks, with excellent geochronological delimitation (between 555–541 Ma) obtained by U–Pb dating on volcanic zircons. The present work shows three new species of macroalgae found as carbonaceous compressions and studied for their morphology and taxonomy. All new taxa are characterized as centric macroalgae; Tamengophyton espinosa sp. nov. is a fan-shaped alga with striated thalli, dichotomous branches, trichomes with perpendicular growth, and a connecting membrane. Ladariella hidria sp. nov. is formed by a set of striated and branched thalli in a cylindrical form with almond-shaped structures in the top. Ladariophyton veinosa sp. nov. is characterized by the main growth thallus and an enlarged longitudinal structure at the center. These new occurrences of macroalgae add to the largest life assemblages in the Neoproterozoic of South America, which contributes to documentation of the evolutionary history of macroalgae and the paleoecological settings of the Late Ediacaran.

Ichnofossils are sedimentary structures of biological origin directly associated with the behavior of the organism that produced them. Using specific tools between sedimentology and biology, it is possible to recognize the producer, its behavior, environmental factors that influenced the activities, or the impact that these activities caused through bioturbation. The objective of this work is to identify trace fossils from the Tamengo and Guaicurus formations, Corumbá Group, analyze the stratigraphic distribution of trace fossil associations, their link with recognized sedimentary facies and determine their influences and limitations. The Tamengo Formation presents intercalation of carbonate and siliciclastics, facies attributed to the upper Ediacaran, and holds records of metazoans, macroalgae, microfossils and ichnofossils. The Guaicurus Formation corresponds to the top of the Corumbá Group, it has homogeneous pelitic sedimentation with the record of macroalgae, microbial mats and ichnofossils. From analyzes with hand samples, petrographic slides, computerized microtomography scans and energy dispersive spectroscopy, it was possible to obtain data related to morphology, bioturbation index and chemical composition of trace fossils and microbial mats. Three ichnotaxons linked to locomotion and feeding activities were recognized, Planolites isp. nov. and Palaeophycus tubularis that characterize fodinichnia and domichnia ethologies and Multina minima with pascichnia ethologies. The bedding plane bioturbation is low, with a maximum value of 10%, and the bioturbation index in the laminations is level 2, the most common for this period. Sub-horizontal to slightly vertical trace fossils were found, indicating the beginning of colonization in infaunal ecospaces. EDS analyzes on trace fossils indicate a high value of Fe and C in the filling and pelitic film compared to the host rock. Microbial mats were found only in the Guaicurus Formation where the bioturbation is lower. The association of mats with ichnofossils is an indication of the preference of excavations in these places, exerting a control over the preservation of metazoans. The combination of the higher degree of bioturbation and hydrodynamic flow in the Tamengo Formation prevents the development of mats and the ability to live in community with the organisms, different from the Guaicurus Formation, where the depositional environment was predominantly calmer.

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