Early basal angiosperm Jixia from the Lower Cretaceous of Northwest China: evidence for the radiative expansion of the Jehol Biota

The late Early Cretaceous witnessed the rapid radiation and extensive expansion of early angiosperms in terrestrial ecosystems, though the underlying mechanisms remain poorly understood. Here, we describe Jixia jiuquanensis sp. nov., a new early angiosperm species from the Lower Cretaceous Zhonggou Formation of the Jiuquan Basin, Northwest China. The fossil specimens are preserved as large, simple leaves with deeply twice-pinnately dissected lobes. Phylogenetic analyses place Jixia within the ANA-grade of basal angiosperms, likely related to Austrobaileyales, as a sister group to Trimeniaceae (the sole genus Trimenia). However, its distinct morphology suggests it may represent an extinct lineage rather than a member of the crown group Trimeniaceae. Leaf morphology, including the presence of oil cells and glandular teeth on the leaf surface, suggests that Jixia was a rapidly growing herbaceous angiosperm adapted to low-energy, shaded riparian or lacustrine environments near forests. The fossil record of Jixia, spanning Northeast and Northwest China as well as the Russian Far East, not only suggest a diversification and wide distribution within the Jehol Biota during the Early Cretaceous but also provide key insights into the evolutionary radiation of flowering plants in East Asia.