Soft anatomy of sunellid arthropods from the Chengjiang Lagerstutte, Lower Cambrian of southwest China

Abstract

Two bivalved arthropods of the family Sunellidae Huo, 1965 are described from the Lower Cambrian Helinpu Formation of southwestern China: Sunella cf. shensiensis Huo, 1965 and Combinivalvula chengjiangensis Hou, 1987. Both taxa preserve soft anatomies, described here for the first time, including a pair of lateral eyes and a possible median eye, a trunk carrying flaplike appendages, and a simple gut with caeca. The family Sunellidae includes three genera: Sunella Huo, Combinivalvula Hou, and Jinningella Huo and Shu. In contrast to most other Cambrian arthropods with a larger bivalved carapace, especially those from the Burgess Shale–type deposits, sunellids lack a limbless abdomen protruding posteriorly beyond the carapace. This, coupled with a combination of a number of snared features (e.g., the presence of cardinal spines and distinctive anterodorsal sulcus, elongated valves, and median eye), appears to support sunellids as a clade. The distinctive anterodorsal sulcus extending from the anterodorsal corner to the anteromedian part of the carapace is regarded as an autapomorphy for this clade. Sunellids resemble Isoxys Walcott, 1890 to which they may be closely related; both possess a bivalved, elongated carapace with cardinal spines that almost entirely covers the body. However, neither cephalic appendages nor proximal portions of trunk limbs are visible in sunellids, and thus, their systematic position remains uncertain.