Neopterygii is the largest clade of ray-finned fishes, including Teleostei, Holostei, and their closely related fossil taxa. This clade was first documented in the Early Carboniferous and underwent rapid evolutionary radiation during the Early to Middle Triassic. This article describes a new perleidid neopterygian species, Teffichthys elegans sp. nov., based on 13 well-preserved specimens from the lower Daye Formation (Dienerian, Induan) in Guizhou, China. The new species documents one of the oldest perleidids, providing insights into the early diversification of this family. The results of a phylogenetic analysis recover Teffichthys elegans sp. nov. as the sister taxon to Teffichthys madagascariensis within the Perleididae. T. elegans sp. nov. shares three derived features of Perleididae: the length of the anteroventral margin of the dermohyal nearly half the length of the anterodorsal margin of the preopercle; the anteroventral margin of the preopercle nearly equal to the anterior margin of the subopercle in length; and the anteroventral margin of the preopercle one to two times as long as the anterodorsal margin of the preopercle. It possesses diagnostic features of Teffichthys but differs from T. madagascariensis by the following features: presence of three supraorbitals; six pairs of branchiostegal rays; relatively deep anterodorsal process of subopercle; absence of spine on the posterior margin of the jugal; and pterygial formula of D26/P14, A22, C36/T39-41. The Perleidiformes are restricted to include only the Perleididae, and other previously alleged ‘perleidiform’ families (e.g., Hydropessidae and Gabanellidae) are excluded to maintain the monophyly of the order. Similar to many other perleidids, T. elegans sp. nov. was likely a durophagous predator with dentition combining grasping and crushing morphologies. The new finding also may indicate a relatively complex trophic structure of the Early Triassic marine ecosystem in South China.

Finely preserved fossil assemblages (lagerstätten) provide crucial insights into evolutionary innovations in deep time. We report an exceptionally preserved Early Triassic fossil assemblage, the Guiyang Biota, from the Daye Formation near Guiyang, South China. High-precision uranium-lead dating shows that the age of the Guiyang Biota is 250.83 +0.07/–0.06 million years ago. This is only 1.08 ± 0.08 million years after the severe Permian-Triassic mass extinction, and this assemblage therefore represents the oldest known Mesozoic lagerstätte found so far. The Guiyang Biota comprises at least 12 classes and 19 orders, including diverse fish fauna and malacostracans, revealing a trophically complex marine ecosystem. Therefore, this assemblage demonstrates the rapid rise of modern-type marine ecosystems after the Permian-Triassic mass extinction.

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The biotic recovery following the Permian/Triassic boundary mass extinction was influenced by several secondary extinctions during the Early Triassic, of which the late Smithian crisis is the most severe known for some nekto-pelagic organisms such as ammonoids. The Smithian-Spathian transition is characterized by successive global biotic and environmental changes, including a dramatic positive carbon isotopic excursion, oceanic anoxia and a cooling event beginning in the late Smithian. However, the tempo, modalities and the causal relationships among these various events remain poorly constrained. Here we synthesized paleontological, sedimentological, and geochemical data from three sections (Motianling, Nafang, and Shanggang) within the Nanpanjiang Basin, representing platform slop to basinal deposits spanning the Smithian/Spathian boundary. High-resolution analyses of thin sections show a sudden decrease in richness and abundance of fossil grains at the middle/upper Smithian boundary in all three sections, coinciding with the onset of a positive shift in δ 13 Ccarb. It also slightly precedes the beginning of the late Smithian cooling. At Shanggang, a change in dominance from bivalve-rich to ostracod-rich strata is also recorded at the middle/upper Smithian boundary. Overall, our results therefore indicate that a first major biotic crisis and turnover happened during the beginning of the late Smithian in the Nanpanjiang Basin, rather than around the Smithian/Spathian boundary. Complementary analyses on pyrite framboid size indicate that these observed biotic changes are concurrent with oxygen depletion in the studied sections, supporting the hypothesis that oceanic anoxia also played an important role in the late Smithian crisis in combination with climate cooling and oceanic acidification.

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