Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Archaeocyathids (ancient cups)

These were sessile marine organisms that lived in the Early Cambrian. Before the evolution of the coral animals they were the first reef builders. Appearing in the Early Cambrian about 530 million years ago, they diversified widely with more than 100 families being known. Their reign was to be short lived, by 520 million years ago they were in steep decline, and by the Middle Cambrian most were extinct, and the last known species, Antarcticocyathus webberi disappeared from the fossil record almost at the end of the Cambrian.

It has been suggested that their extinction may have been related to the appearance and diversification of the demosponges.

They inhabited shallow seas around the coasts of the continents, their widespread distribution suggesting they may have dispersed by planktonic larvae. Their relationships are uncertain, though many believe they were probably a type of sponge. The name pleosponge has been used for them by some scientists.

Cambrian Explosion - Archaeocyatha

The occurrence of Archaeocyathans, that are sponge-like, an assemblage of organisms that were heavily calcified, that are now recognised to have developed independently among a number of different groups of sponges, underlines the idea of a radiation during the Cambrian of sponge-like forms. Archaeocyathans built skeletons of microgranular calcite that was high in magnesium but had no spicules (Rowland, 2001), that was conical to cup-shaped. There were also 2 other fossil groups that are believed to have possibly evolved from different sponge groups, the radiocyaths and the cribricyaths, radiocyaths originally being aragonitic (Zhuravlev & Wood, 2008). Pores penetrate the skeletons of Archaeocyathans, and it has been shown by experiments using models in flowing water, that flow patterns through the pores are to some degree similar to those through living sponges, which suggests their food was small particles suspended in the water currents pumped through the pores (Savarese, 1992 and refs within). Reef-like accumulations of Archaeocyathans on the seafloor are commonly formed by the cementing together of the individual skeletons. Deposition by epithelia is suggested by their solid skeleton, but this is generally not well-developed in sponges, though calcareous skeletons with similar textures are built by demosponges. The first known appearance of Archaeocyathans was in Stage 2 following which they diversified so rapidly that they seem to have created a diversity peak that spoils interpretations of benthic diversification in the Cambrian as a logistic process. The authors1 suggest the somewhat artificial taxonomy of the group may have exaggerated this diversity peak.

Sources & Further reading

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


Author: M. H. Monroe
Last Updated 10/05/2014
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