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The evolutionary events during the Ediacaran–Cambrian transition (~541 Ma) 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 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 Ma. X-ray microtomography reveals three-dimensionally preserved traces ranging from 50–600μm in diameter, indicative of small-bodied, meiofaunal tracemakers. Burrow morphologies suggest they were created by a nematoid-like organism that utilised 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., Field costs for L.A.P. were supported by an undergraduate travel grant from St. Anne’s College, University of Oxford. Fieldwork costs for M.D.B. were supported by CNPq-Conselho Nacional Desenvolvimento Científico e Tecnológico- Brazil (Proc. 451245/2012-1). This project was supported by an NERC Isotope Geoscience Facilities Steering Committee grant (project IP-1560-0515). J.M.L., P.C.B., R.T., G.A.C.C., C.Q.C.D. and M.L.A.F.P. were supported by grant numbers 2009/02312-4, 2010/02677-0, 2013/17835-8 and 2016-06114-6, São Paulo Research Foundation (FAPESP), Brazil. A.G.L. and L.A.P. are supported by the Natural Environment Research Council (grant numbers NE/L011409/2 and NE/L501554/1, respectively). R.J.G. is a Scientific Associate at the Natural History Museum, London, and a member of the Interdisciplinary Centre for Ancient Life (UMRI). D.M. recognizes the support of an NSERC discovery grant.

Topical application of a standard dose of noradrenaline (NA) to the anesthetized rat mesentery evoked a vasoconstrictor response the latency of which was increased in a dose-dependent way by previous addition of PAF-acether or histamine but not by Lyso-PAF. Lyso-PAF, however, blocked the antagonistic effect of PAF-acether to NA, the blockade being equivalent to that observed with PAF-antagonists (BN 52021 or WEB 2086) either injected iv, or applied topically. In contrast, the antagonistic action of histamine towards NA was not affected by Lyso-PAF or PAF-antagonists. Since platelet aggregation and leukocyte accumulation did not occur, PAF-acether appears to interact directly with microvessels to block vasoconstrictor stimuli, an effect antagonized by PAF-antagonists and the metabolite/precursor, Lyso-PAF.