First record of Brachyphyllum (Cheirolepidiaceae) from the Lower Jurassic of the Ordos Basin, China: Paleogeographic and paleoclimatic implications linked to T-OAE

A new cheirolepidiaceous conifer species, Brachyphyllum ordosensis sp. nov., is described from the Lower Jurassic Fu'xian Formation in the Ordos Basin, northern China. This represents the first confirmed record of this genus Brachyphyllum from the Lower Jurassic of China, significantly expanding its paleogeographic distribution. Brachyphyllum originated in the Late Triassic, expanded rapidly during the Early Jurassic, peaked in the Early Cretaceous, and gradually declined in the Late Cretaceous. It was widely distributed across the Northern Hemisphere with sparse records in the Southern Hemisphere, and predominantly thrived in tropical arid zones, serving as a reliable indicator of hot climates. The flourishing and expansion of Brachyphyllum during the Toarcian Oceanic Anoxic Event (T-OAE) and Cretaceous OAEs 1a and 1b reflect its adaptive response to these extreme climatic events. Using the stomatal ratio method based on well-preserved fossil cuticles, the paleoatmospheric CO 2 concentration of the Toarcian in the Ordos Basin is reconstructed as 1280.05 ppmv (Carboniferous standardization), indicating a significant rise in paleo-CO 2 levels. The elevated CO 2 concentration, coupled with contemporaneous negative carbon isotope excursion and mercury enrichment, is interpreted to be linked to the massive magmatism of the Karoo–Ferrar Large Igneous Province in southern Gondwana during the T-OAE, which released voluminous greenhouse gases. This discovery provides critical paleobotanical evidence for reconstructing Early Jurassic paleogeography and paleoclimate, and offers new insights into the coevolution of plants and environments during this critical geological interval.