Isopodichnus, related arthropod trace fossils and notostracans from Triassic fluvial sediments

The commonest arthropod trace fossils from Triassic aquatic red beds are the bilobate traces Isopodichnus and trackways known as ‘Merostomichnites’ triassicus. These trace fossils were probably produced by notostracan branchiopods, similar to Triops. Four arthropod ichnocoenoses from Lower Triassic fluvial sandstones have been analysed in terms of size frequency distribution, behavioural variation and relationship to sedimentary structures and depositional environment. One Isopodichnus ichnofauna associated with flute moulds (Dumfries-shire, Scotland) shows a normal age-structured population of arthropods responding with strong rheotaxis within shallow fluvial channels. The second Isopodichnus assemblage associated with ripple marks (Worcestershire, England) also shows strong rheotaxis but is bimodal in size and morphotype, possibly suggesting change in arthropod behaviour with age. Two ichnocoenoses of trackways with less pronounced rheotaxis associated with ripples (Cheshire, England) and flute moulds (Württemberg, Germany) were produced by larger arthropods than the resting traces. These arthropods probably possessed 6 to 9 pairs of walking limbs. The conclusions derived from these notostracan trace fossils are compared with data on palaeoecology, population size-frequency, morphology and behaviour of Triops cancriformis derived from the analysis of three Triassic body fossil faunas and literature on living populations. Taxonomic consideration favours retention of the name Isopodichnus but the trackways should be included in Acripes Matthew. Brief review of late Palaeozoic Isopodichnus assemblages which appear to predate known notostracan fossils is inconclusive as regards both identifying producers or infallible means of separation from Cruziana assemblages.