Ichnofauna record cryptic marine incursions onto a coastal floodplain at a key Lower Mississippian tetrapod site

Ichnofossils from a Lower Mississippian (Tournaisian) coastal plain succession with a rich vertebrate fauna, including early tetrapods, record repeated, short-lived marine interactions that influenced floodplain lake development and ecosystems. The ichnofauna contrasts with the more typical Scoyenia Ichnofacies of other Devonian-Carboniferous tetrapod sites. In the Tournaisian Ballagan Formation of the Scottish Borders, Chondrites is identified in 128 horizons within a 500-m succession and is associated with lesser occurrences of phycosiphoniform burrows, Diplocraterion, Rhizocorallium and Zoophycos?. Chondrites and phycosiphoniform burrows commonly occur within dolostones, interpreted as part of a saline-hypersaline lake facies association, containing a non-marine to marginal marine macrofauna of bivalves, ostracods, Spirorbis, Serpula and sarcopterygian fish. Marine scolecodonts are reported from 18 horizons of diverse lithology, four of which co-occur with Chondrites. The single-colonisation, single-tier, high ichnofabric index and thin beds (mean 10 cm thick) that characterise Chondrites horizons indicate: 1) adverse environmental conditions; 2) rapid colonisation of the sediment; and 3) short-lived bioturbation episodes. Other ichnotaxa identified are Monocraterion and Asterosoma?, which occur exclusively within the overbank or fluvial facies associations, and are here interpreted to have lived in freshwater conditions. The Chondrites trace-makers are interpreted as having originated in shallow-marine waters and were transported into floodplain lakes by storm surge events, although a few examples suggest more significant transgressions. Ichnofossils are much more common than marine body fossils or scolecodonts, and thus record cryptic marine incursions that might otherwise remain unidentified. This study contributes to our understanding of the complex mosaic of environments present when terrestrial tetrapods first appeared.