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Australia: The Land Where Time Began |
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Cambrian Explosion - Small Shelly Fossils Small shelly fossils (small shellies or SSFs) are
mostly less than 2 mm in their greatest dimension (see Bengtson, 2005;
Bengtson & Conway Morris, 2002). There are some whole shells among the
SSFs, such as those of small brachiopods, molluscs and a variety of
cones for which their affinity is uncertain, and there are also
individual plates, spines and other skeletal elements (sclerites) from
larger animals that had been reserved in the same phosphatisation
processes. In the living animal the complete skeletal assemblage is its
scleritome which is sometimes orders of magnitude larger than the
constituent sclerites. It has been shown by a new compilation of
origination patterns of the small shellies that they are apparently at
the base of the Cambrian, their diversity increasing steadily through
Stage 1 (Maloof, Porter et al.,
2010). The authors1 suggest that it was probably during Stage
2 that they attained their greatest diversities, and during Stage 3 they
were still fairly diverse, which is when the record of the Cambrian
Explosion is at its height (Porter, 2004). The presence of many different clades is suggested
by morphological diversity small shelly sclerites from the Early
Cambrian, the most common of which are lophotrochozoans. Deuterostomes
are present in the explosion faunas of Stage 3 and Stage 5, which are
exceptionally preserved, though they are largely absent. Some
scleritomes are comprised of more than 1 type of sclerite, thereby
complicating the interpretation; however there are now a variety of
scleritomes that are well preserved, in which the sclerites are in the
living positions, which preserves the body plan of a number of
sclerite-bearing animals. An example is
Microdictyon, a type of
sclerite that was found and described in small shelly assemblages a
number of years before the discovery of the complete animal.
Microdictyon is now known
to have had the plates embedded in the body wall above the limbs of a
branch of animals that is now extinct that is related to onychophorans
and arthropods, the lobopodians (Liu, J. et
al., 2008; Ramskold & Chen,
1998), therefore they belong to the superphylum Ecdysozoa. Ecdysozoans
tend to be relatively rare among the classic small shelly assemblages,
except for the sclerites of lobopodians, though Ecdysozoans have the
richest, most disparate fossil record of any superphylum from the
Cambrian. For other scleritomes discoveries of a few
associated sclerites can only hint at the existence of a scleritome. An
example is a group, cambroclaves, for which the full scleritome is not
known, all that is known of their scleritome is small groups of their
sclerites that were found associated together at a location in
Kazakhstan, which permitted a partial reconstruction of their geometry.
It is suggested by these sclerites, which are mostly less than 10 mm
long, that at least some surface of the animal was covered by a mosaic
of many sclerites. Interpretation of growth and possible life habits of
this form is allowed by these small insights (see Conway-Morris &
Bengtson, 1990). At the inferred posterior end of each sclerite the
spines bend forwards (or possibly backwards), rather than outward, which
suggests the animal crawled or burrowed, possibly using the spines to
gain purchase during movement, and possibly not for protection. It is
suggested the sclerites appear to have been composed originally of
low-magnesium calcite and interlocked so it seems growth might have
needed to have been accommodated by the secretion of new files of
sclerites in the zone of growth. The authors1 suggest it is
plausible cambroclaves may have been benthic bilaterians, though before
they can be included with certainty in the metazoan tree will require
the finding of more fossils that provide more information about them. It is clear the SSFs are a large, phylogenetically
disparate assemblage, though many SSFs are still not well known,
representing a thriving fauna, the preservation of which was restricted
to a narrow taphonomic window (Porter, 2004). The appearance of
different SSF groups generally correlates with abundance changes in the
sedimentary record of phosphatic deposits. From Stage 4 and onward such
deposits in the sedimentary record become relatively rare which
presumably reflects a change in the chemistry of seawater. The diversity
of small shelly fossils declined as the phosphatic deposits became
increasingly rare, and by Stage 5 SSFs are essentially gone from the
fossil record. As SSFs are not found in many phosphatic deposits from
the Ediacaran, the authors1 suggest it is likely that during
most of the Ediacaran small shelly faunas were rare of completely
absent, evolving mainly early in the Cambrian. The disappearance of the
SSFs is associated, at least partially, with an episodic extinction
(Zhuravlev & Wood, 1996), though the clades some of them belonged to are
still represented in some later deposits.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |