Morphological Evolution and Ecological Adaptation of the Triassic Bivalve Genus Unionites Yan Zhong, Hang Chu, Hong-Wei Kuang, Peng

The extinction event at the Permian-Triassic transition led to the collapse of the Paleozoic ecosystems, followed by the gradual establishment of modernized ecosystems. In the meantime, the marine benthic ecosystems shifted from brachiopod-dominated to bivalve-dominated ecosystems, and the Triassic was the critical period for studying this transition. Genus Unionites, one of the most distinctive Triassic bivalve groups, spans the entire Triassic and witnesses significant geological processes such as biotic recovery, radiation, and eco-structural transformation. Originating prior to the mass extinction and quickly occupying ecological niches afterwards, Unionites became an important component of marine benthic ecosystems until its extinction in the end of the Triassic. As a representative of shallow endobenthic bivalves, it is highly sensitive to paleoenvironmental changes, and the evolution of its morphological adaptation characteristics can serve as an indicator for tracing major bio-environmental events during the Triassic. Based on first-hand fossil materials from 13 Lower Triassic, 2 Middle Triassic, and 2 Upper Triassic sections in South China (totaling 17 sections), combined with a global literature review of the genus and its synonyms, we systematically determined the basic biological and ecological characteristics of each reported species and clarified their classification positions. Additional fieldwork was conducted at the Upper Triassic (Carnian) Longchang section in Zhenfeng County, Guizhou Province. Results: (1) Fossil identification revealed Unionites canalensis and U. fassaensis in the Lower Triassic, U. spicatus in the Middle Triassic, and U. griesbachi, U. guizhouensis, and Unionites sp. in the Upper Triassic. (2) Among 49 previously reported species within Unionites and its synonyms, we recognize 38 valid species. Its systematic position is assigned to Anthracosiidae, Cardiida. (3) The genus ranges from the Wuchiapingian (Late Permian) to the Rhaetian (Late Triassic). Diversity evolution shows: origination in the Late Permian, rapid growth in the Induan, stable growth in the Olenekian-Anisian, setback in the Ladinian, rapid radiation in the Carnian, and gradual decline in the Norian-Rhaetian until extinction. Lithofacies transition: Induan-Olenekian balanced in carbonate and clastic facies; Anisian-Ladinian carbonate-dominated; from the Carnian onward. (4) Morphometric analysis (PCA) of 62 morphological features indicates that morphological space expanded in the Induan and Anisian, contracted in the Norian and Rhaetian. Seven key features (shell length, height, length/height ratio, apex angle, angles between anterior/posterior edges, beak position) show increasing length/height ratio and apex angle through the Triassic, with the beak position moving forward. The Carnian is a critical turning point. (5) Functional-morphological analysis reveals enhanced filtering and burrowing efficiency and increased endobenthic depth in the Carnian. The shell length/height ratio is key for endobenthic depth, followed by apex angle and ventral margin. Unionites exhibited a broad adaptive strategy during the Induan-Olenekian (co-evolution in both facies), adapted mainly to carbonate shallow marine environments during the Anisian-Ladinian, and shifted to high-energy clastic environments during the Carnian-Rhaetian. (6) These adaptive changes were driven by a combination of biotic (predation pressure) and abiotic factors (temperature, sea-level change, and substrate changes).