Middle Permian Alatoconchid Bivalves from Central Hubei, South China: Paleobiogeographic and Paleoecological Implications

Alatoconchid bivalves are among the most distinctive giant reef-associated organisms of Permian tropical seas and are widely regarded as important indicators of Guadalupian shallow-marine carbonate ecosystems. Although abundant occurrences have been documented from the Upper Yangtze and Dian-Qian-Gui regions of South China, no confirmed records had previously been reported from the Middle Yangtze region, leaving an important paleobiogeographic gap in their known distribution. Here we report the first discovery of alatoconchid bivalves from the Middle Permian Maokou Formation in Jingmen City, central Hubei Province, based on newly investigated fossil assemblages from the Duodao and Jingshan sections. The recovered specimens mainly occur as fragmented but comparatively well-preserved shell remains embedded within bioclastic limestones and shell-rich biostromal deposits. Despite varying degrees of diagenetic recrystallization, several specimens preserve diagnostic morphological features, including broad wing-like flanges, dorsoventrally compressed shells, U-shaped marginal folds, and a characteristic double-layered shell microstructure composed of outer prismatic and inner foliated layers. The measurable shell fragments are generally smaller than many previously reported alatoconchids, although similar-sized specimens have also been documented elsewhere in South China and Japan, suggesting that local environmental conditions, ontogenetic stages, or preservational factors may have influenced shell size variation. The alatoconchid-bearing strata are associated with diverse benthic fossil assemblages, including crinoids, fusulinids (Verbeekina sp.), brachiopods, gastropods, bryozoans, and calcareous algae, indicating deposition within a warm, shallow, oxygenated carbonate-platform environment during the Guadalupian. The abundance of crinoid remains and carbonate bioclasts suggests relatively clear, nutrient-rich marine conditions favorable for suspension-feeding benthic communities. Taphonomic observations, including limited abrasion, tightly embedded shell fragments, and locally articulated specimens, indicate relatively rapid burial and limited post-mortem transport under low-energy depositional conditions. The occurrence of alatoconchid biostromes within thick-bedded limestones containing chert nodules further reflects stable carbonate-platform sedimentation in central Hubei during the Middle Permian. This discovery significantly expands the known distribution of Alatoconchidae within South China and demonstrates that the Middle Yangtze carbonate platform also served as a favorable habitat for these giant bivalves within the Paleo-Tethys realm. The new occurrence fills an important paleobiogeographic gap between previously documented Upper Yangtze and Lower Yangtze localities and provides additional evidence for the widespread development of tropical carbonate ecosystems across South China during the Guadalupian. Furthermore, the absence of alatoconchids in younger strata supports the hypothesis that this group was severely affected by environmental deterioration associated with the Capitanian mass extinction, potentially linked to Emeishan volcanism, oceanic warming, acidification, and widespread marine anoxia. These findings contribute to a better understanding of Middle Permian marine ecosystem evolution, carbonate-platform ecology, and the biotic responses of giant benthic organisms to major environmental crises.