Preliminary Study of New Material of Branchiocaris from the Cambrian Stage 3–4 in South China and Its Implications for Paleogeography

Branchiocaris,a large bivalved arthropod, was a significant component of the early Cambrian fauna. Its body is typically partially or entirely enclosed by bivalved carapaces, and it is well - represented in the Chengjiang Biota. The type species,Protocaris pretiosaResser, 1929, has taxonomically reassigned to the genusBranchiocaris by Briggs D.E.G. To date, at least seven species have been described, includingBranchiocaris pretiosa,B.malongensis, B. xundianensis, B. guizhouensis, B.tylodes, B. yiliangensis and B.yunnanensis.This genus displays a broad geographic distribution, with occurrences reported in South China and North America. Stratigraphically, its temporal range extends from Cambrian Series 2, Stage 3, to the Miaolingian Drumian Stage. This taxon is usually characterised by large, wide carapaces with a subcircular outline and a slightly anterior abdomen. The posterior abdomen is moderately inflated and the hinge line is straight and elongated. Short, triangular, subequal dorsal spines project anteriorly and posteriorly from the junctions of the frontal and posterior margins with the dorsal margin, respectively. The morphology of these spines is a key feature that distinguishesBranchiocarisfrom other co-occurring large bivalved arthropods. The carapace has a relatively high height-to-length ratio, with the maximum height located slightly behind the mid-ventral line. Its surface is ornamented with a reticulated pattern, while the free margin lacks spine-like structures. Carapace length ranges from 10 mm to 51 mm and height from 12 mm to 41 mm. Owing to the scarcity of well-preserved soft tissue, most specimens are preserved as laterally compressed carapaces; thus, taxonomic delineation within the genus relies predominantly on carapace morphology. Previous studies have suggested thatB. yunnanensismay be dimorphic, with two morphotypes growing isometrically along a shared ontogenetic trajectory. Newly collectedBranchiocarismaterial from the Hongjingshao Formation (new Malong Biota, Qujing, eastern Yunnan), and the Balang Formation (western Hunan), integrated with previously published records, demonstrates thatBranchiocariswas widely distributed across South China from Cambrian Series 2, Stage 3, to the Miaolingian Wuliuan Stage. This genus occupied a spectrum of depositional environments, ranging from shallow-water deltaic settings and deep-water shelters to deep-water shelf and slope transition zones. These findings lend additional support to the “deep-water refugia” hypothesis the early Cambrian. Preliminary observations suggest thatBranchiocaris specimens from deep-water settings show a notable increase in carapace size, which may be associated with lowered diversity and abundance of apex predators such as Anomalocaris. Geometric morphometric analyses of carapaces from the Chengjiang and new Malong biotas support the recognition of a new species from the new Malong Biota, herein described asBranchiocaris? langzansp. nov. Furthermore, the unassigned specimens from the Balang Formation in western Hunan represents the first reported occurrence of this genus from the region. This work is supported by NNSF under grant 42162001; Yunnan University National-Level Innovation and Entrepreneurship Projects under grant 202510673089