Recently, Associate Professor Yun Hao and Professor Zhang Xingliang from the Department of Geology of Northwestern University, in cooperation with the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Macquarie University, Australia, Gottingen University, Germany and other units, published academic papers in the internationally renowned journal Communications Biology, interpreting the cladistic lineage and evolutionary significance of the Cambrian "difficult" class group of cavity opening bone animals, and proving that cavity opening bone is an extinct phylum level basic metazoan.