Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Deuterostomes: Chordata

Cephalochordates, Uorchordates and Vertebrates, the 3 living subphyla of chordates, all appear in Stage 3. Cathaymyrus, the earliest known cephalochordate, is an elongate animal, about 5 cm long, which has been poorly preserved, from the Chengjiang Fauna (Shu, Conway Morris & Zhang, 1996), whereas Pikaia, a form of Stage 5, about 4 cm long, from the Burgess Shale, is more complete (Briggs, Erwin & Collier, 1994; Conway Morris, 1982: Conway Morris and Caron, 2012). The seriated myotomes (muscle blocks) are shown by these fossils, that are characteristic of extant cephalochordates, and are somewhat similar to those of vertebrates). Uorchordates are clearly morphologically much reduced from their ancestral forms, are represented in the Chengjiang deposits by a benthic seasquirt (J.Y. Chen, 2003).

Rocks from Stage 3 have been found near the Chengjiang localities (Shu et al., 1999). Myllokunmingia is the earliest known putative vertebrate. It appears to have a cartilaginous skull as well as other elements that indicate the presence of tissues derived from a neural crest (though see the discussion on taphonomy below). Tooth-like fossil elements, paraconodonts, first appearing in late Stage 4, are believed to be allied to Conodonta (Bengtson, 1983), a primitive craniate group (Aldridge, 1986); they would thereby be fish-like vertebrates as well. Vertebrates have been estimated by molecular clocks and the evidence from fossils to have arisen at the time of the round of evolutionary innovation that resulted in the body plans of Arthropoda, Mollusca, Brachiopoda, etc., the body plans of the major metazoans.

Sources & Further reading

1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado