The present work analyzes the monotypic Taenidium barretti ichnofabric developed in abandoned channels and floodplains to understand its paleoecological significance in highly seasonal fluvial systems.The data come from the Cretaceous paleosols of the Marília Formation(Bauru Basin,SE Brazil),in which the T.barretti ichnofabric and rhizoliths represent the biotic record.Beetle larvae are considered to be the most likely T.barretti tracemakers in these paleosols.The intensity and recurrence of bioturbation suggest that the windows of opportunity for beetle population growth were very short and controlled by the progressive substrate desiccation after flood cessation.The characteristics of the trace fossil assemblage suggest soil colonization by r-strategic organisms triggered by flooding events that provided the resource inputs necessary for their living during rainy seasons.The results also show that in fluvial systems with high discharge variations,the boundaries between a“pre-desiccation suite”and a“desiccation suite”in the Scoyenia ichnofacies can be diffuse due to the short duration of colonization windows and progressive terrestrialization of floodplains and channels.Therefore,the frequency of flooding events,the lowered water table,and the rapidity of substrate desiccation constrained colonization by other burrowers and may be considered as firstorder factors controlling the generation of a monotypic T.barretti ichnofabric.Thus,the monotypic Taenidium ichnofabric in the Scoyenia ichnofacies context is an ichnomarker of brief windows for colonization in highly seasonal environments.
Diego Luciano NascimentoRenata Guimaraes NettoAlessandro BatezelliFrancisco Sergio Bernardes LadeiraDaniel Sedorko
Over the last 30 years,terrestrial ichnofossil research was mostly focused on three ichnofacies:the Scoyenia ichnofacies,characterizing transitional fluvio-lacustrine environments,the Mermia ichnofacies,characterizing fully lacustrine settings,and the Termitichnus ichnofacies,characterizing terrestrial environments,especially,paleosol deposits.Specially in China,many terrestrial ichnofossils,including at least 24 ichnogenera from fluvial deposits and 59 ichnogenera from lacustrine deposits,have been found in Mesozoic and Cenozoic basins.Most of them belong to the common elements of the three ichnofacies and consist of feeding,grazing,crawling,dwelling,and resting traces and rhizoliths.Based on the composition,occurrence and distribution characteristics of trace fossils from terrestrial sedimentary basins of China,36 ichnoassemblages have been proposed.However,the most common 12 ichnoassemblages,in which six are found in fluvial sedimentary environments from the Upper Cretaceous of the Sichuan Basin in western China and Xixia Basin in western Henan Province,include:(1)Scoyenia-Rusophycus ichnoassemblage generated in the floodplain along channels and the shallow water swales or lakes(such as oxbow lakes);(2)Skolithos-Arenicolites ichnoassemblage produced in the high energy sandy bar(heart bar or point bar)of the channel environment;(3)Gastruichnus-Palaeophycus ichnoassemblage developed in the channel-levee environment;(4)Beaconites-Taenidium ichnoassemblage occurring in the embankment(crevasse splay)sedimentary environment;(5)Beaconites-Rhizolithos ichnoassemblage appearing in the floodplain sedimentary environment,and(6)Beaconites-Scoyenia ichnoassemblage distributed in the periodically exposed overbank lake(extremely shallow lacustrine)sedimentary environments.The other six ichnoassemblages are found in lacustrine sedimentary environments:(1)Scoyenia-Skolithos ichnoassemblage always developed in periodically exposed,extremely shallow lakeshore and interdistributary bay of the lake delta plain under drought or sem