Abstract:

Well known for its abundant and extraordinary soft-tissue fossils, the Chengjiang fauna has witnessed the main phase of the Cambrian explosion and the first great congress of the ancestors of nearly all major phyla of animals on Earth. The large-scale survey and exploration of the fauna by Chinese paleontologists, partly in collaboration with international scientists, over the past 30 years can be broadly divided into three stages. In the first decade since 1984 (1984-1994), a large number of invertebrates, including basal animals and the protostomes of the early animal tree were discovered, but the subkingdom Deuterostomia was completely unknown. Then in the second decade (1995-2005), the great discoveries of various deuterostomes led to the construction of the basic framework of the Deuterostomia and then to the formation of the tripartite phylogenetic trees of early animals (TPTEA, including basal animals, protostomes and deuterostomes), for the first time. In the third decade (after 2005), the academic community began to think about the internal relationships between the formation of TPTEA and the multi-episode Cambrian explosion, leading to the new hypothesis of the three-episode Cambrian explosion. The Chengjiang fauna is important for deciphering the fauna structure, paleoecological environment, and so on. However, its core academic values mainly rest on two aspects. Firstly, The Chengjiang fauna, as the main phase witness of the Cambrian explosion, has created a nearly complete phylogenetic framework of the TPTEA on Earth for the first time. The three-phase radiation hypothesis reveals the essential connotation of the Cambrian explosion: a step-wise divergent evolution of animals, from basal to highly advanced groups, lasting about 40 million years. In the first phase, it gave birth to a bulk of basal animals (including some now extinct “animal” groups) in the latest Ediacaran, probably including some pioneer protostomes. The second phase took place in the first epoch of the Cambrian period (Terreneuvian), giving rise to the main invertebrate protostomes with a persistent prosperity of basal animals. The third phase proceeded in Cambrian Epoch 2 (represented by the Chengjiang fauna), which not only maintained the prosperity of basal animals and protostomes, but also, more importantly, gave birth to all the main phyla of the subkingdom Deuterostomia. Thus, the rudimental framework of the whole TPTEA has been shaped, with the termination of the major innovation events of the Cambrian explosion. Here, we discussed the evolutionary properties of Ediacaran biota, small shelly fossils and the Chengjiang fauna in the Cambrian explosion with emphasis on the biological properties of several important animal groups. The order Myllokunmingiida is the only known oldest vertebrate, while Yunnanozoon and Haikouella are neither vertebrates nor stem-group chordates but a special group of basal deuterostomes; Cheungkongella is a credible ancestor of the urochordate and it supports the classical hypothesis on the origin of the urochordates; and the gill slits were first invented in the members of the phylum Vetulicolia to provide key information on the origin of the deuterostomes. The second core value of the Chengjiang fauna is of profound humanistic and philosophical significance: the discoveries of the ‘first gill openings’, ‘first brain’, ‘first vertebrae’ and ‘first heart’ provide the pivotal evidence for solving the unsolved mystery of the origin of the main basic human organs as described in Darwin’s “The Descent of Man”. In addition, the morphological and anatomical information of the Chengjiang fauna can provide important clues for a better understanding of most components of Ediacaran and Cambrian metazoans.

Key words: core value of the Chengjiang fauna, essential properties of the Cambrian explosion, tripartite phylogenetic tree of early animals (TPTEA) and three-phase radiation, oldest vertebrates Myllokunmingiida, basal chordates, Vetulicolia, Ecdysozoa