Palaeoecology of an Early Permian playa lake trace fossil assemblage from Castle Peak, Texas, USA

Abstract

The Early Permian Choza Formation of the Clear Fork Group at Castle Peak in Texas, USA, contains a diverse and remarkably abundant trace fossil assemblage comprising exquisitely preserved arthropod trackways ( Diplichnites gouldi types A and B, Diplichnites isp. A and B, Lithographus isp. and cf. Kouphichnium isp.), striated trails ( Cruziana problematica) and isolated resting traces ( Rusophycus carbonarius and R. furcosus), surface or shallow subsurface grazing trails or burrows ( Gordia indianensis, Helminthoidichnites tenuis, Treptichnus isp., a nodular trail and unilobate epichnial trails), backfilled deposit-feeding burrows ( Planolites beverleyensis), meniscate backfilled burrows ( Taenidium barretti), horizontal branching networks, and tetrapod trackways ( Dromopus palmatus, Erpetopus willistoni, Varanopus curvidactylus and cf. Amphisauropus isp.). Trace fossils formed in, and around the margins of, a shallow, ephemeral (playa) lake within an alluvial plain, and provide evidence of temporary communities, comprising arthropods, most notably notostracan branchiopod crustaceans, as well as euthycarcinoids, myriapods, insects, and tetrapods, including protorothyridid, captorhinid and araeoscelid reptiles and rare amphibians. Periodic influxes of water into the alluvial plain filled various depressions, creating temporary habitats and flourishes of life and activity. The trace fossil assemblage is typical of the Scoyenia ichnofacies, and is similar to other assemblages from a range of different transitional subaqueous to subaerial settings, indicating that the distribution of trace fossils is not inextricably linked to the depositional setting itself, but rather to the environmental factors that arise. Such diverse trace fossil assemblages probably reflect relatively long-lived ephemeral water bodies, whereas more impoverished examples probably represent shorter-lived ephemeral water bodies. Assemblages such as Castle Peak, with abundant delicate arthropod trackways, probably reflect low energy conditions, conducive to the presence of an abundant vagile epifauna and the preservation of their trackways, whereas assemblages with open vertical burrows tend to lack arthropod trackways and probably reflect higher energy conditions. Trace fossil assemblages dominated by arthropod and tetrapod trackways may represent fleeting preservation windows following sheetfloods on floodplains.