Cambrian Burgess Shale-type (BST) fossil biotas document nearly complete snapshots of the oldest Phanerozoic marine ecosystems1, 2, 3-4. However, the rarity of deposits bearing high-diversity BST biotas5 has restricted our understanding of the evolutionary and ecological dynamics of the Cambrian explosion. Here we report the Huayuan biota-a lower Cambrian (Stage 4, approximately 512 million years ago) BST Lagerst & auml;tte from an outer shelf, deep-water setting of the Yangtze Block in Hunan, South China. The Huayuan biota yields remarkable taxonomic richness, comprising 153 animal species of 16 phylum-level clades dominated by arthropods, poriferans and cnidarians, among which 59% of species are new. The biota is comprised overwhelmingly of soft-bodied forms that include preserved cellular tissues. The complex ecosystem contained diverse radiodonts and pelagic tunicates, filling a gap of high-diversity BST biotas from the Cambrian Stage 4. Critically, multivariate ordination based on a global dataset of Cambrian BST biotas places the Huayuan biota within a main transition of marine animal ecosystems between Cambrian Age 3 and Age 4. Network analysis reveals close faunal connections between the Huayuan and Burgess Shale biotas, indicating transoceanic dispersal. Dated shortly after the Sinsk event6, 7-8, the Huayuan biota illuminates differences in the impacts of this extinction in shallow- versus deep-water settings during the first Phanerozoic mass extinction and offers critical insights into the transformation of global ecosystems in the early Cambrian.