Abstract
AbstractThe establishment of permanent animal communities on land was a defining event in the history of evolution, and one for which the ichnofauna and facies of the Tumblagooda Sandstone of Western Australia have been considered an archetypal case study. However, terrestrialization can only be understood from the rock record with conclusive sedimentological evidence for non-marine deposition, and original fieldwork on the formation shows that a marine influence was pervasive throughout all trace fossil-bearing strata. Four distinct facies associations are described, deposited in fluvial, tidal and estuarine settings. Here we explain the controversies surrounding the age and depositional environment of the Tumblagooda Sandstone, many of which have arisen due to the challenges in distinguishing marine from non-marine depositional settings in lower Palaeozoic successions. We clarify the terminological inconsistency that has hindered such determination, and demonstrate how palaeoenvironmental explanations can be expanded out from unambiguously indicative sedimentary structures. The Tumblagooda Sandstone provides a unique insight into an early Palaeozoic ichnofauna that was strongly partitioned by patchy resource distribution in a littoral setting. The influence of outcrop style and quality is accounted for to contextualize this ichnofauna, revealing six distinct low-disparity groups of trace fossil associations, each related to a different sub-environment within the high-ichnodisparity broad depositional setting. The formation is compared with contemporaneous ichnofaunas to examine its continued significance to understanding the terrestrialization process. Despite not recording permanent non-marine communities, the Tumblagooda Sandstone provides a detailed picture of the realm left behind by the first invertebrate pioneers of terrestrialization.
Highlights
The colonization of subaerial and non-marine landscapes by animals and plants was a defining, singular and irreversible innovation during the co-evolution of Earth and its biosphere (Vecoli et al 2010; Vermeij & Grosberg, 2010; Kenrick et al 2012)
The uncertainty that surrounds the unit is in urgent need of clarification because: (1) examples of its trace fossils are regularly used to ascertain the stratigraphic ranges of non-marine ichnotaxa (e.g. Netto, 2006) and ichnofacies (e.g. Krapovickas et al 2016); (2) the apparent existence of diverse non-marine trace fossils with a revised Ordovician age has been used in support of equivocal claims of trace fossils in pre-Silurian non-marine strata elsewhere; and (3) regardless of age or environment, the Tumblagooda Sandstone hosts one of the most diverse and abundant collections of arthropod trace fossils in the entire rock record, and the reasons for its ichnological richness have not been fully explored
We present new sedimentological (Section 3) and ichnological (Section 4) evidence that demonstrates a marine influence throughout much of the Tumblagooda Sandstone, in particular those strata with the highest diversity and disparity of trace fossils
Summary
The colonization of subaerial and non-marine landscapes by animals and plants was a defining, singular and irreversible innovation during the co-evolution of Earth and its biosphere (Vecoli et al 2010; Vermeij & Grosberg, 2010; Kenrick et al 2012). Trackways and other biological constructions are intensive properties of the sedimentary rock record and so are not diminished by the same taphonomic filters as the body fossil record is (Holland, 2016; Davies et al 2020), having the potential to reveal a more accurate census of early terrestrial communities (when tracemakers can be inferred) They provide tangible evidence of ancient, in situ organism–sediment interactions within physical habitats that cannot be examined by any other means (Buatois et al 1998; Minter et al 2016a, b, 2017; Buatois & Mángano, 2018). We conclude by comparing the new information from the Tumblagooda Sandstone with other lower Palaeozoic successions, to demonstrate that, rather than being a crucible of terrestrialization, the unit is better understood as hosting an archetypal littoral ichnofauna that was imparted shortly before the full arthropod invasion of the land
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