Middle Permian trace fossil assemblages from the Carnarvon Basin of Western Australia: Implications for the evolution of ichnofaunas in wave-dominated siliciclastic shoreface settings across the Permian-Triassic boundary

Abstract

Guadalupian (middle Permian) ichnoassemblages are described from the Mungadan Sandstone Formation (Kennedy Group), Merlinleigh sub-basin, Carnarvon Basin of Western Australia. In this formation, we recognize five facies (pebbly sandstone, intensely bioturbated sandstone, medium- to coarse-grained cross-stratified sandstone, fine-grained sandstone, and planar laminated siltstone), assigned to two facies associations, interpreted as the deposits of wave-dominated upper shoreface and lower shoreface settings. Ichnoassemblages, which contain 23 ichnospecies in 18 ichnogenera, are assigned to the Skolithos and Cruziana ichnofacies, as well as mixed types, and include representatives of cubichnia, repichnia, pascichnia, fodinichnia, domichnia, and agrichnia, indicating diverse trace-making behaviours. High burrow-density with monospecific occurrences of Skolithos indicate R-selected life strategies and burrow production over a short time period. The anomalous occurrence of Paleodictyon implies that the trace-making organisms of graphoglyptids could inhabit resource-rich shoreface environments as well as resource-limited deep sea niches. To better understand ichnological evolution across the Permian-Triassic (P-Tr) boundary, a database of Permian to Middle Triassic ichnoassemblages was also compiled for wave-dominated shoreface settings (WDSS) globally, and three ichno-ecological measures (ichnodiversity, ethological category, and ichnodisparity) were evaluated. Ichnodiversity shows a stepwise decline in ichnotaxa through the Permian, a pronounced increase in the Early Triassic, and a second phase of decline towards the Middle Triassic. Ethological data exhibit a different trend, with mean and median values dramatically dropping to a late Permian nadir, followed by a significant increase across the P-Tr boundary, culminating in an Early Triassic acme. The globally depauperate nature of late Permian ichnoassemblages may reflect sampling bias or decreased preservation potential resulting from abundant benthos and well-developed mixed layer in the seafloor. The presence of anomalously diverse Early Triassic ichnoassemblages may be due to the WDSS habitat providing a ‘habitable zone’ for trace-making organisms to survive when most marine environments were stressful in the aftermath of the P-Tr mass extinction. We emphasise that caution is needed in interpreting ichnodiversity variations across the P-Tr mass extinction event, as it may reflect shifting of trace-making behaviours of the same animals, or replacement of different animals with the same behaviours. • A new Guadalupian ichnofauna comprising 18 ichnogenera is described; • The Carnarvon ichnofauna represents the Skolithos and Cruziana Ichnofacies; • Paleodictyon occurs in wave-dominated siliciclastic shoreline (WDSS) settings; • The depauperate of ichnotaxa in the late Permian reflect sampling bias or decreased preservation potential of ichnofossils. • The diverse ichnoassemblages in WDSS settings during the Early Triassic results from ‘habitable zone’.