Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Cnidaria

The most basal cnidarian group are the Anthozoans, burrows attributed to sea anemones being found near, and possibly below the base of the Cambrian. It is possible that calcitic tubes and cones found in the small shelly fauna may represent Cnidarians, though there are other possible relationships. Body fossils that have been described from the Fortunian-age, Stage 1, strata in China have been assigned to sea anemones (Han et al., 2010). A considerable degree of diversification of cnidarians by the time of the basal Cambrian strata is suggested by the interpretation of these exquisitely preserved polyps as stem-group hexacorals. In the Chengjiang Fauna there are additional body fossils (Hou et al., 2005). These fossils have 12 tentacles and have a similar symmetry to extant scleractinian corals, which have 6-fold symmetry. Undoubted pelagic cnidarians (Medusozoa) have been reported from faunas that were exceptionally well preserved in the Stage 4 Marjum Formation, Utah (Cartwright et al., 2008) and also from Wisconsin. A considerable degree of morphological variety is shown by these forms, the features of some specimens suggesting that had affinities with Cubozoa, some to Hydrozoa, and others to Scyphozoa – the 3 major classes of crown medusozoans, though none of these possible assignments are considered to be conclusive. It has been concluded that only Scyphozoa can be recognised from the Cambrian with any certainty (G. A. Young & Hagadorn, 2010), though some of the specimens from Utah may represent hydrozoans. Among these fossils the nature of the disparity suggests that there was a significant early radiation or medusozoans which produced a variety of linages that were morphologically distinctive, a common evolutionary pattern among metazoans from the Cambrian.

Sources & Further reading

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