TWO STRANGE SHELLY FOSSILS FROM THE LOWER CAMBRIAN OF SOUTHEASTERN SHAANXI PROVINCE, CHINA

Phosphatized and phosphatic small shelly fossils are one of our major sources of information about the evolution of animals during the Early Cambrian. Although progress has been made in understanding some of these fossils, many remain enigmatic, both with regard to their phylogenetic affinities and often even the overall morphology of the animal from which isolated sclerites come. We describe two particularly unusual fossils from the later Early Cambrian (Qiongzhusian or Atdabanian) Xihaoping Member of the Dengying Formation from Xiaowan, Xixiang County, southeastern Shaanxi Province, China, which was deposited in shallow marine waters along the present northern margin of the Yangtze Platform. The first of these is a cap-shaped fossil, which bears a spine on its concave surface; the area around the base of the spine is covered with numerous tiny bumps, similar to structures seen on some chancelloriid sclerites and other problematic shelly fossils. Although many Cambrian cap-shaped fossils have been interpreted as the shells of univalved molluscs, the presence of a long spine in the fossils from Xiaowan is inconsistent with such an interpretation, and instead they are probably sclerites from a larger animal. Similar fossils have been hypothesized to be halkieriid valves, but the rarity of halkieriid sclerites in the present samples argues against this view. Another strange fossil, known only from the Xihaoping Member, was described as Acidocharacus Qin & Ding 1988 and consists of a tall spine attached to a rounded conical base. The base is covered with tiny round bumps similar to those found on the cap-shaped fossil, while the spine frequently bears smaller lateral spines. The function of these spines, and the nature of the animal that produced them, remain entirely obscure. Although both of these fossils are still enigmatic, they help emphasize the variety of bizarre sclerites present in early skeletal faunas.