The dicynodonts are an emblematic group of herbivorous therapsids that survived the Permo-Triassic (P-Tr) crisis. Laotian dicynodonts from stratigraphically constrained beds, recently dated using the U-Pb zircon method, yield new insights into terrestrial faunas of Southeast Asia during the latest Permian and earliest Triassic. Summarily described, they were originally attributed to the genus Dicynodon. We provide a new phylogenetic analysis for Laotian dicynodonts, based on three well-preserved skulls, indicating that they belong to two new taxa: Counillonia superoculis, gen. et sp. nov., and Repelinosaurus robustus, gen. et sp. nov. Our phylogenetic analysis of Dicynodontia indicates that (1) Counillonia is closely related to some ‘Dicynodon’-grade taxa and (2) Repelinosaurus is a kannemeyeriiform. The phylogenetic affinities of these new Laotian dicynodonts allow discussion of the survivorship of multiple lineages (Kannemeyeriiformes and ‘Dicynodon’-grade dicynodontoids) across the P-Tr crisis. The Laotian dicynodonts also shed new light on the paleobiogeography of Southeast Asia from the late Paleozoic to the early Mesozoic, particularly the timing of collisions between the Indochina, the South China, and the North China blocks. The presence of dicynodonts in Laos, most likely in the Early Triassic, thus implies that the connection between the Indochina Block and the South China Block occurred no later than the latest Permian or earliest Triassic (i.e., when the dicynodonts provide direct evidence for a connection).

The availability of nutrients in seawater, such as dissolved phosphorus (P), is thought to have regulated the evolution and activity of microbial life in Earth's early oceans. Marine concentrations of bioavailable phosphorus spanning the Archean Eon remain a topic of debate, with variable estimates indicating either low (0.04 to 0.13 μM P) or high (10 to 100 μM P) dissolved P in seawater. The large uncertainty on these estimates reflects in part a lack of clear proxy signals recorded in sedimentary rocks. Contrary to some recent views, we show here that iron formations (IFs) are reliable recorders of past phosphorus concentrations and preserved a primary seawater signature. Using measured P and iron (Fe) contents in Neoarchean IF from Carajás (Brazil), we demonstrate for the first time a clear partitioning coefficient relationship in the P-Fe systematics of this IF, which, in combination with experimental and Archean literature data, permits us to constrain Archean seawater to a mean value of 0.063 ± 0.05 μM dissolved phosphorus. Our data set suggests that low-phosphorus conditions prevailed throughout the first half of Earth's history, likely as the result of limited continental emergence and marine P removal by iron oxyhydroxide precipitation, supporting prior suggestions that changes in ancient marine P availability at the end of the Archean modulated marine productivity, and ultimately, the redox state of Earth's early oceans and atmosphere. Classification: Physical Sciences, Earth, Atmospheric and Planetary Sciences