Birth and early evolution of metazoans

• Overview of latest information on Cambrian explosion.
• The diversity of early animals increased in 3 distinct phases.
• The third phase in Cambrian Epoch 3 marked the apex and end of explosion.

The reconstruction of the phylogenetic tree of animals (TOA) has long been one of the central interests in biological and paleobiological sciences. We review the latest results of paleontological and stratigraphical studies on the Ediacaran–Cambrian sequences mainly from South China for revising the TOA in accordance with modern genome biology. A particular focus is given to the pattern of animal diversification based on the fossil first appearances of high-rank clades chiefly in phylum-level. The results show an abrupt divergence of lineages during the Ediacaran–Cambrian transition; however, the appearances of metazoan phyla were obviously diachronous, with three major phases recognized herein. The first phase is marked by the appearances of basal metazoan phyla in the latest Ediacaran. Very few unequivocal bilaterian clades were present at this phase. The second phase occurred in the Terreneuvian (Cambrian Stages 1–2), represented by the occurrences of many lophotrochozoan lineages. This phase also involves the appearances of calcified basal metazoan lineages, and possibly, those of contentious ecdysozoans in the latest Terreneuvian, but no deuterostome has been known from this age. The third and also the largest phase occurred in the Cambrian Stage 3, which involve all the three supraphylogenetic clades of the Eubilateria. A number of lophotrochozoan lineages, the bulk of ecdysozoans, and all deuterostome phyla, appeared for the first time in this phase. Since there is no unambiguous evidence for bilaterians in the Ediacaran, the Cambrian explosion sensu stricto was an abrupt diversification of bilateral lineages in a short time of ca. 25 million years across the Ediacaran–Cambrian boundary. Next critical issues in research include high-resolution chrono- and chemostratigraphic analyses, correlations between biotic events and environmental perturbations, physiological approach to the biological connotation of biomineralization, and exploration for the lost mid-oceanic biota and environments, which are crucial in understanding the entire picture of the Cambrian explosion.

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