The Fremouw Formation of Antarctica: Updated vertebrate fossil record and reevaluation of high-latitude Permian–Triassic paleoenvironments

Permian–Triassic rocks of the Transantarctic Basin provide an unparalleled record of high latitude paleoenvironments and terrestrial vertebrate faunas. Here we summarize the taxonomic and paleoecological implications of the approximately 1300 vertebrate fossils collected since 1968, as well as report on new geologic field observations made during the 2017–18 austral field season. The Fremouw Formation records a vertebrate assemblage taxonomically most similar to that of the Karoo Basin of South Africa, with 10 genera shared in common. However, temnospondyls form a much greater percentage of tetrapod occurrences in the Fremouw Formation, suggesting favorable conditions for these ectothermic fossil amphibians at high latitudes. Lower Triassic small reptiles (viz. Procolophon, Prolacerta) occur in slightly higher proportions than in the Karoo, but their taxonomic diversity is likely undercounted. Seven stratigraphic columns of the upper Buckley and lower–middle Fremouw formations detail fluvial depositional environments in the central Transantarctic region, recording a shift from wet swamp lands to drier floodplains, most similar to Gondwanan basins in Australia. Fremouw Formation paleosols primarily consist of Protosols, which indicate poor soil forming conditions likely due to low precipitation and high sediment supply from crevasse splays. Mineralogy from X-ray diffraction, review of igneous intrusives, and Buckley Formation coal characterization demonstrate post-pedogenic diagenetic alteration that casts doubt on the results of previous stable isotopic studies of these paleosols. Tetrapod fossils first appear in the Fremouw Formation, which has been taken as evidence for immigration to the Antarctic portion of southern Pangea around the time of the end-Permian mass extinction. However, this may be due to higher soil pH, increased base saturation, lower moisture content, and more rapid burial conditions in the Fremouw than the underlying Buckley Formation that favored bone preservation.