Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Early Metazoan Grades and Clades – Fossil Representatives

In this section the authors¹ review the morphologies of the major groups that appear as fossils during the Cambrian Explosion, and concentrate on features that suggest an affinity with extant groups. Architectural grades do not appear in a regular sequence of complexity over time, instead body plans ranging across levels of complexity, from sponges to chordates, appear geologically abruptly. These organisms must have had a rich history of divergence and expansion for which there are few if any known records, though their appearance is the earliest known in the fossil record in most of these groups. The fauna of the explosion reflects the exploration of many adaptive pathways in the marine biosphere that was evolving. Marine ecosystems with characteristics that can be recognised in the oceans of the present, a legacy that extends back about 500 My, as the burgeoning fauna interacted with physical and biotic aspects of the environment.

Basal Metazoan Clades

Within the metazoan tree a suite of sponges and sponge-like organisms have been found to be at the base of the tree, and are important species in the Cambrian Faunas. There are a variety of problematic groups, as occurs in many other groups, the phylogenetic affinities for which are still unclear.

Sources & Further reading

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