In an alternating marine and non-marine depositional setting, where and how are early Carboniferous tetrapods preserved?

2018-07-30T11:39:28Z (GMT) by Janet Elizabeth Sherwin
During the Tournaisian tetrapods were adapting to terrestrial environments. Three sites exposing the Ballagan Formation in the Northumberland and Tweed basins were logged and sampled in detail to determine the sedimentological processes and depositional environments preserving tetrapod and associated vertebrate fossils. The Coldstream site is interpreted as a marginal marine bay with fluvial input, whereas Coquetdale and Whitrope Burn are interpreted as marginal marine, lagoonal environments without direct fluvial input. Dolostone beds typical of the Ballagan Formation are present where there is a degree of restriction but absent in the more open marginal marine bay site. Packages of dolostones indicate marine transgressions: the repeated close vertical association of dolostones with siltstones with roots and desiccation cracks indicates a location close to base level subject to repeated changes in marine water depth. Vertebrate and most invertebrate fossil material was transported in silty flows into marginal marine basins. Early pyritisation or dolomitisation has preserved uncrushed fossils. At Coldstream tetrapod and lungfish material was transported directly into the bay. At Coquetdale and Whitrope Burn marginal marine species have been transported into and around the more restricted, lagoonal sites. At Whitrope Burn chondrichthyan teeth, tetrapod fragments and lungfish toothplates were transported within the lagoon in a density flow. Total organic carbon (TOC) values of generally <1% are typical of the Ballagan Formation. Bulk (-20 to -22‰) and specific (average 22.2‰) δ13C org. carbon isotope values compare well with other Ballagan successions but are less negative than published values for the early Carboniferous. The similarity of the bulk and specific values indicate dominance by terrestrial material. The preservation of tetrapod and other vertebrate fossils in the marginal shallow marine basins in this study contrasts with finds in the Tweed Basin, where most of the significant tetrapods have been preserved in alluvial overbank settings: floodplains or ephemeral floodplain lakes.

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