Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Chaetognatha

The phylum Chaetognatha, small-bodied predators that are largely pelagic, is likely to be represented by hook-shaped spines in functional sets closely resembling grasping apparatuses of chaetognaths in the Cambrian (Szaniawski, 1982) as well as body fossils from the Chengjiang Fauna (J.Y. Chen & Huang, 2002) and the Burgess Shale (Szaniawski, 2005). Being neither spiralian nor lophophorate, the Chaetognatha are suggested by molecular evidence to be protostomes allied to Lophotrochozoa, and are possibly basal to those 2 groups (Matus, Copley et al., 2006). The clade, including Chaetognatha may include the LCA of extant lophotrochozoans, if the topology is correct. About 20 % of extant species are benthic and have been studied (Casenove, Goto & Vannier, 2011) for the functional and morphological differences between benthic and pelagic forms. It is suggested by their analysis that species from the Cambrian had a pelagic lifestyle. Chaetognaths have radial cleavage and are literally deuterostomes; therefore it may be that these features were inherited by the lophophorates, though in the branch leading to Spiralia it was replaced by spiral cleavage and protostomy.

Sources & Further reading

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