Oligochaetes, despite their important role in terrestrial ecosystems and a tremendous biomass, are extremely rare fossils. The palaeontological record of these worms is restricted to some cocoons, presumable trace fossils and a few body fossils the most convincing of which are discovered in Mesozoic and Cenozoic strata. The Olenekian (Lower Triassic) siliciclastic lacustrine Petropavlovka Lagerstätte of the southern Cis-Urals yields a number of extraordinary freshwater fossils including an annelid. The segmented body with a secondary annulation of this fossil, a subtriangular prostomium, a relatively thick layered body wall and, possibly, the presence of a genital region point to its oligochaete affinities. Other fossil worms which have been ascribed to clitellates are reviewed and, with a tentative exception of two Pennsylvanian finds, affinities of any pre-Mesozoic forms to clitellate annelids are rejected. The new fossil worm allows tracing of a persuasive oligochaete record to the lowermost Mesozoic and confirms a plausibility of the origin of this annelid group in freshwater conditions.

Key words: Annelida, Clitellata, Oligochaeta, Mesozoic, Lagerstätte, Russia.

Dmitry E. Shcherbakov [dshh@narod.ru], Borissiak Palaeontological Institute, Russian Academy of Sciences, Profsoyuznaya St 123, Moscow 117647, Russia. Tarmo Timm [tarmo.timm@emu.ee], Centre for Limnology, Estonian University of Life Sciences, 61117, Rannu, Tartumaa, Estonia. Alexander B. Tzetlin [atzetlin@gmail.com], Department of Invertebrate Zoology, Faculty of Biology, Lomonosov Moscow State University, Leninskie Gory 1(12), Moscow 119234, Russia. Olev Vinn [olev.vinn@ut.ee], Institute of Ecology and Earth Sciences, University of Tartu, Ravila 14A, 50411, Tartu, Estonia. Andrey Y. Zhuravlev [ayzhur@mail.ru] (corresponding author), Department of Biological Evolution, Faculty of Biology, Lomonosov Moscow State University, Leninskie Gory 1(12), Moscow 119234, Russia.