Lower Cambrian halkieriids and other coeloscleritophorans from Aksu-Wushi, Xinjiang, China

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Abstract
Halkieriids are known largely from isolated sclerites, although it is understood that originally they formed the scleritomes of slug-like animals in Lower Cambrian benthic environments. The Yurtus Formation (?Tommotian–Atdabanian equivalent) of the Aksu-Wushi area, west Xinjiang, China, has yielded abundant sclerites. These are secondarily phosphatized, but show considerable detail. The sclerites can be divided into three classes: cultrates, long palmates, and short palmates; definite siculates have not been recognized. In some samples elongate to semicircular shells co-occur with the sclerites; these may derive from the same halkieriid scleritome. The external surface of the shells may bear a subdued ornamentation. The concave interior, however, is remarkable on account of a unique structure consisting of a group of nodular-like units. These may be arranged in a low cone or dispersed in a small circle. In addition to the sclerites of halkieriids and their possibly associated shells, rarer material is attributable to the related group of siphogonuchitids.