Disaster microconchids from the uppermost Permian and Lower Triassic lacustrine strata of the Cis-Urals and the Tunguska and Kuznetsk basins (Russia)

Abstract

We describe aggregative microconchid (Lophophorata) tubes from the uppermost Permian (upper Changhsingian) and Lower Triassic (Olenekian) lacustrine and fluvial strata of the Tunguska and Kuznetsk basins and the southern Cis-Urals, Russia. These attach to clam shrimp carapaces, bivalve shells, terrestrial plant fragments and a horseshoe crab head shield, and also form their own monospecific agglomerations. Planispiral tubes of a wide size range (0.1–2.5 mm) create dense settlements on these firm substrates, which likely comprise multiple generations of the same species. These finds confirm that this extinct lophophorate group was inhabiting non-marine continental basins during latest Permian and earliest Triassic time, when they were major suspension feeders in such limnic ecosystems. Microconchids dispersed extensively and rapidly in the aftermath of the Permian–Triassic mass extinction into both marine and continental basins at low and moderately high latitudes, which were notably different in salinity, temperature, depth and redox conditions. This confirms that small lightly calcified microconchids were a genuine disaster eurytopic group, whose expansion may have been promoted by low predator pressure and low competition for substrate.

New austrolimulid from Russia supports role of Early Triassic horseshoe crabs as opportunistic taxa

“…The red beds yield the lungfish Ceratodus , temnospondyl amphibians, and procolophonid and erythrosuchid reptiles ( Shishkin et al, 1995 ; Novikov, 2018 ). By comparison, the grey lens contain a different set of fossils: abundant, but fragmentary vascular plants, numerous insects (mainly isolated wings of various roaches, beetles, hemipterans, and rare dragonflies, grylloblattids, and orthopterans), microconchids, rare millipedes, and a microdrile oligochaete ( Hannibal & Shcherbakov, 2019 ; Shcherbakov et al, 2020 ; Shcherbakov, Vinn & Zhuravlev, 2021 ). Clam shrimp and ostracods recorded in the grey bed occur in surrounding red beds as well.…”

“…Minute microconchids that encrusted plant stems, horseshoe crab exuvia, and other available firm substrates represent the major suspension feeders in the Petropavlovka ecosystem. These extinct lophophorates were genuine disaster taxa—eurytopic stress-tolerators that flourished in the aftermath of the end-Permian extinction in both marine and continental basins all over the world ( Shcherbakov, Vinn & Zhuravlev, 2021 ). Dense accumulations of primarily pyrite dodecahedra are common on the plant stem fragments and attached microconchid tubes.…”

“…Anterior most section of left genal spine noted. Two tentaculitoid tubeworms noted on left side of prosoma (round structures; Shcherbakov, Vinn & Zhuravlev, 2021 ).…”

“… (A) Map showing locality of Petropavlovka III (red star). Dotted line represents boundaries of tectonic regions, modified from Shcherbakov, Vinn & Zhuravlev (2021) . (B) Stratigraphic log of Petropavlovka II–IV sections showing location of horseshoe crab-bearing lens (modified from Tverdokhlebov, 1967 ).…”

“…The lens contains abundant plant megafossils including sphenophytes and gymnosperms ( Dobruskina, 1994 ). In 2018–2019 numerous diverse insects wings, millipedes, horseshoe crabs, microconchids, and a microdrile oligochaete were collected in the lens, along with seed fern pinnules and lycophyte fragments ( Hannibal & Shcherbakov, 2019 ; Shcherbakov et al, 2020 ; Shcherbakov, Vinn & Zhuravlev, 2021 ).…”