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.

A single incomplete worm specimen was collected in the Lower Triassic Petropavlovka Formation from the locality Petropavlovka III in 2018 during field work of the Arthropoda Laboratory (PIN). The fossil is a single fragment of three-dimensionally preserved worm body wall on a bedding surface of a greenish-grey polymictic micro-wavy-laminated siltstone. The images of the fossil are obtained with a Leica M165C stereomicroscope coupled to a Leica DFC425 digital camera, and a TESCAN VEGA variable-pressure and environmental SEM using backscattered electron detector in PIN. An elemental analysis of uncoated and unpolished sample including the fossil and adjacent matrix was per- formed with a quantitative energy dispersive X-ray INCA microanalyser coupled to a TESCAN VEGA SEM, at an accelerating voltage of 20 keV, in PIN. Besides, quantitative elemental composition data were obtained from the host sediment powder by a wavelength dispersive X-ray fluo-rescence SPEKTROSKAN-MAKS-GV spectrometer, using high resolution LiF(200) analysing diffraction crystal at an accelerating voltage of 40 keV, in the Geological Faculty, MSU.

In addition, new photographs of vermiform fossils Pronaidites carbonarius Kušta, 1888 from the Middle Pennsylvanian (Moscovian) Radnice Formation (Kušta 1888; Fritsch 1907: pl. 4: 1–3; Štamberg and Zajíc 2008) and Lumbricopsis permicus Fritsch, 1907 described by Fritsch (1907: pl. 4: 7) from the Cisuralian (Sakmarian) lacustrine Prosečné Formation of the Bohemian Massif from (Zajíc 2014), which have been kindly provided by Vojtěch Turek and Lenka Váchová (National Museum, Prague, Czech Republic), are shown in the Appendix.

The Dataset includes:

Appendix S1. Figure 1. Pronaidites carbonarius Kušta, 1888; holotype, Me102, National Museum, Prague; Rakovník, Bohemian Massif, Czech Republic; Carboniferous, Pennsylvanian, Moscovian Stage, Radnice Formation. A1, general view. A2 and A3, enlargements of boxed regions in A1, arrows depict possible chaetal bundles.

Appendix S1. Figure 2. Lumbricopsis permicus Fritsch, 1907; specimen P1391, National Museum, Prague; Bítouchov, Bohemian Massif, Czech Republic; Lower Permian (Cisuralian), Sakmarian Stage, Prosečné Formation. A1, general view, water cover. A2 and A3, enlargements of boxed regions in A1.

Appendix S2. Figure 1. Petropavlovka III section, Orenburg region, Russia; Lower Triassic, Olenekian Stage, Petropavlovka Formation.Thin sections of the fossil-bearing matrix. A, section across the sediment thickness. B, section along the bedding plane, dark material represents plant remains, square transparent element at the bottom left – a quartz grain.

Appendix S2. Figure 2. Microdrile oligochaete specimen PIN 5640/212, Petropavlovka III section, Orenburg region, Russia; Lower Triassic, Olenekian Stage, Petropavlovka Formation. An elemental analysis of the uncoated and unpolished sample surface including the fossil and adjacent matrix, performed with a quantitative energy dispersive X-ray INCA microanalyser coupled to a TESCAN VEGA SEM, at an accelerating voltage of 20 kV.

Appendix S2. Table 1. Host rock elemental composition, obtained by a wavelength dispersive X-ray fluorescence SPEKTROSKAN-MAKS-GV spectrometer using high resolution LiF(200) analysing diffraction crystal at an accelerating voltage of 40 keV, of the Lower Triassic Petropavlovka Formation from the fossil-bearing lens of Petropavlovka III locality, Orenburg region, Russia. Principal elements (mean values, %).

Appendix S2. Table 2. Host rock elemental composition , obtained by a wavelength dispersive X-ray fluorescence SPEKTROSKAN-MAKS-GV spectrometer using high resolution LiF(200) analysing diffraction crystal at an accelerating voltage of 40 keV, of the Lower Triassic Petropavlovka Formation from the fossil-bearing lens of Petropavlovka III locality, Orenburg region, Russia. Auxiliary elements (mean values, ppm).