The embryonic development of the early Cambrian Quadrapyrgites and its phylogenetic implication
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Abstract:
The Cambrian explosion is one of the most important biological innovations in life history. The Chengjiang biota (518 Ma) from South China records the last major episode of the big bang of life, while the Kuanchuanpu biota (535 Ma) from the lower-most Cambrian, which is earlier than the Chengjiang biota, provides a crucial taphonomic window for investigating the first major episode of the Cambrian explosion. The Kuanchuanpu biota discovered from the early Cambrian phosphatic lime-stone in Ningqiang and Xixiang Couties, Shaanxi Province, is well-known as it preserved abundant soft-bodied microscopic animals and associated embryos. Among these animal fossils, Quadrapyrgites is a very common taxon. It is widely thought to be a sister group of coeval Olivooides, because they share similar developmental process and body plan. However, the phylogenetic positions for both taxa remain uncertain. To test all the previous affinity hypothesis for Quadrapyrgites , herein we reconstructed the three-dimensional structures of several embryonic specimens of Quadrapyrgites , using scanning elec-tronic microscopy and high-resolution X-ray tomographic microscopy. For the first time, we revealed that the gastrulation process of the embryos of Quadrapyrgites is typical invagination. The new data presented here suggests that the Quadrapyrgites and their sister groups should be more likely diploblastic cnidarians rather than triploblastic cycloneuralians.
