Australia: The Land Where Time Began

A biography of the Australian continent 

Epibolic Gastrulation from the Early Cambrian - the Kuanchuanpu Member, Dengying Formation, Ningqiang, Shaanxi, South China

The apparently sudden appearance of diverse skeletal metazoans, and increased complexity of both body and trace fossils, marks the Cambrian Explosion. According to the authors¹ a unique window for investigation into the ontogeny of metazoans is provided by some of the fossils such as eggs and embryos of Lower Cambrian age that have been recovered from the Kuanchuanpu Member of the Dengying Formation at Ningqiang in Southern China. Gastrulation, the cell migration stage after cleavage, can be seen as the embryonic analog of the transition of the protozoan to metazoan grades of complexity. An example is the embryonic developmental sequences of Punctatus emeiensis, which is well documented, as it has stellate spines covering the body surface, that it shares with the embryonic growth stages represented by Olivooides and their growth stages represented by P. emeiensis. Consideration of the variation of Olivooides was neglected, though Olivooides was considered to be a taxonomic mixture. A specific type of embryo is identified that has zigzag blastopore lips that differs from typical candidate embryos for P. emeiensis, for which the gastrulation process is reconstructed and is comparable to the epibolic gastrulation of extant metazoans. For this 'type' both the embryos and adults display radial symmetry, suggesting an affinity distinct from that of echinoderms. The authors¹ also suggest that the body plan of penta-radial symmetry which is seen widely among the 'small shelly fossils' (SSFs) of the Early Cambrian was probably rooted deeply in the Ediacaran.

Sources and further reading

Yao, Xiaoyong, Jian Han, and Guoxiang Jiao. "Early Cambrian Epibolic Gastrulation: A Perspective from the Kuanchuanpu Member, Dengying Formation, Ningqiang, Shaanxi, South China." Gondwana Research 20, no. 4 (11// 2011): 844-51.