Isoxys carbonelli and the palaeoenvironmental disparity of Isoxys during Cambrian Stage 3

In 1927, Rudolf and Emma Richter found a unique specimen of bivalved arthropod, which they named Isoxys carbonelli (Richter & Richter, 1927), and accessioned at the Senckenberg Museum of Natural History of Frankfurt, Germany. Since then, the specimen has been neither redescribed nor re-illustrated. A century later, we redescribe and re-illustrate the holotype and only specimen of I. carbonelli and describe the geological setting with reference to a modern biostratigraphical framework. In addition, we compare I. carbonelli with other Isoxys species and sunnelids (another group of small Cambrian bivalved arthropods) by means of elliptical Fourier analysis and hierarchical cluster analysis. The analysis concludes that I. carbonelli is a valid species, one of the oldest representatives (together with I. zhurensis Ivantsov, 1990 from Siberia). If the specimen is an adult, I. carbonelli was the smallest member of the genus. Isoxys was widespread during Cambrian Stage 3, showing high morphological disparity and size variation. The oldest species lived in warm, shallow marine, carbonate environments alongside archaeocyaths, in the bigotonid trilobite province.