Early Cambrian eodiscoid trilobite Hupeidiscus orientalis from South China: ontogeny and implications for affinities of Mongolitubulus-like sclerites

Delicately phosphatized specimens of the eodiscoid Hupeidiscus orientalis were documented from the lower Cambrian Shuijingtuo Formation through acetic acid maceration of limestone samples. The material enables us to better understand the meraspid to holaspid ontogenetic development of H. orientalis, and to evaluate its relations with other eodiscoid taxa occurring in South China. The development of its pygidium and thoracic segments is very similar to that of “Shizhudiscus” longquanensis, suggesting that Shizhudiscus is a junior synonym of Hupeidiscus. However, the synonymization of Hupeidiscus with Tsunyidiscus remains open as little is known about the ontogeny of the latter genus. The spines of H. orientalis are of particular importance. Like the enigmatic sclerite taxon Mongolitubulus, its spines are ornamented with a distinctive scaly sculpture, shedding new light on the origin and affinities of Mongolitubulus-like sclerites. The squamose ornament was gradually transformed from granules on the exoskeletal surface at the base of the spines. Other strands of evidence, e.g., the outline, exuviating growth, and the spatio-temporal distribution, also suggest that some isolated sclerites previously assigned to Mongolitubulus are possibly related to the spines of eodiscoid trilobites. This finding implies that such morphologically similar spines may be essentially of multiple pan-arthropod origins (bradoriids, eodiscoids, etc.), and they may be of morphologic analogues rather than homologues, suggesting that Mongolitubulus shall be cautiously used as a generic name unless the circumstance of its type specimens from Mongolia gets clarified.