Cryptic tubular network trace fossils evidence life in Middle Triassic arid palaeosols of South-West England

Mark Howson, Maurice E Tucker, Fiona F Whitaker

Research output: Contribution to conferenceConference Abstract

Abstract

Evidence of life in the terrestrial Middle Triassic strata of South-West England is rare or unrecorded. We describe and interpret novel tubular network structures, consistent with a biogenic origin, that are inferred to be of Anisian to Carnian age. These occur at Portishead on the North Somerset coast within basal continental sediments of the Dolomitic Conglomerate, the marginal facies unit of the Triassic Mercia Mudstone Group. Photomicrographs, thin-section petrography and three-dimensional images developed from high-resolution micro-focus computed tomography (μ-CT) allow study of form and complexity, the effect of post-depositional changes and, ultimately, an understanding of the formative organisms.

The fossils occur in 5–30 cm subangular reworked clasts and comprise dense networks of unfilled burrows and tubular constructions (‘tubules’). These occur within a friable and porous palaeosol with a grain size of 10 – 30 μm, a mineralogy of residual hydrated iron oxides, possibly clays, partly cementing carbonate, and sporadic post-depositional baryte replacement. The presence of relict marine fossils indicates derivation from weathered Carboniferous Limestone.

Most of the tubules (Type I) have a circular-section where 90% of the internal diameters are 0.1–0.35 mm. They have calcite linings, at minimum 0.01 mm thick, but in voids or friable substrate, typically thickened to 0.2 mm with overlapping calcite layers that may include evidence of repair. They have slight internal circumferential 0.02 mm ripples, are randomly orientated in three-dimensions and include several sub-types. The majority are ‘irregular maze’ networks of meandering tubules with Y-shaped junctions typically 1 mm apart with local anastomosis. These are linked at T-shaped branches to fewer ‘galleries’ that may be relatively straight for at least 15 mm, of 0.2–0.5 mm diameter with calcite linings thickened up to 0.3 mm. Rare irregular rounded chambers with up to 0.4 mm linings, are up to 4 mm long, 1 mm across, accessed by narrower tubules, and usually located adjacent to apparently resistant, un-burrowed substrate. Many of these features suggest collaborative and possibly eusocial behaviour.

Conspicuous but less numerous Type II burrows, of diameter 1–2 mm, are unlined with irregular ovoid cross-sections and randomly orientated. They intersect Type I indiscriminately, with evidence of breakage, and may be attributable to a second, unrelated, possibly predatory organism. Some Type I lined tubules have apparently been re-constructed within older Type II burrow cavities.

The palaeosol clasts containing the tubules were part of a partially cemented regolith developed during subaerial weathering of limestone hills in an arid subtropical continental climate. They record biological activity within the regolith, with synchronous calcite cementation, prior to erosion. The friable palaeosol was eroded and transported after rainfall along and down a palaeo-gully where it was buried as clasts in a sandy-pebbly dolomitic matrix. Following torrential rain, these sediments were partly re-eroded but then buried by 1–4 m thick fanglomerate deposits.

This depositional archive provides a rare window on a Middle Triassic terrestrial ichnofacies. Many questions await research, with speculation on formative organisms, including plants, fungi, annelids, micro-molluscs and/or arthropods, even insects with early eusocial traits.
Original languageEnglish
Pages207
Publication statusPublished - 21 Jun 2021
Event35th IAS Meeting of Sedimentology - Prague, Czech Republic, Prague, Czech Republic
Duration: 21 Jun 202125 Jun 2021
https://www.iasprague2021.com/

Conference

Conference35th IAS Meeting of Sedimentology
Abbreviated titleIAS Prague 2021
CountryCzech Republic
CityPrague
Period21/06/2125/06/21
Internet address

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