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

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Abstract
The commonest arthropod trace fossils from Triassic aquatic red beds are the bilobate tracesIsopodichnusand trackways known as‘Merostomichnites’ triassicus. These trace fossils were probably produced by notostracan branchiopods, similar toTriops.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. OneIsopodichnusichnofauna associated with flute moulds (Dumfries-shire, Scotland) shows a normal age-structured population of arthropods responding with strong rheotaxis within shallow fluvial channels. The secondIsopodichnusassemblage 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 ofTriops cancriformisderived from the analysis of three Triassic body fossil faunas and literature on living populations. Taxonomic consideration favours retention of the nameIsopodichnusbut the trackways should be included inAcripesMatthew. Brief review of late PalaeozoicIsopodichnusassemblages which appear to predate known notostracan fossils is inconclusive as regards both identifying producers or infallible means of separation fromCruzianaassemblages.