DOI: 10.5061/dryad.95x69p8gg
title: A probable oligochaete from an Early Triassic Lagerstätte of the southern Cis-Urals and its evolutionary implications

description: 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.
description: 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 <em>Pronaidites carbonarius</em> Kušta,
 1888 from the Middle Pennsylvanian (Moscovian) Radnice Formation (Kušta
 1888; Fritsch 1907: pl. 4: 1–3; Štamberg and Zajíc 2008)
 and <em>Lumbricopsis permicus</em> 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.
description: The Dataset includes: Appendix S1. Figure
 1. <em>Pronaidites carbonarius </em>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.
 <em>Lumbricopsis permicus</em> 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).