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Australia: The Land Where Time Began |
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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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |