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Australia: The Land Where Time Began |
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Cambrian Explosion - Annelida In the
Chengjiang Fauna these are poorly
represented by body fossils, though in the
Burgess Shale, there are
several distinctive annelid body types that have been recovered (Conway
Morris, 1979b). These have gross morphology that appears quite as
complex as crown clades, though none have been assigned to extant
groups. There are a variety of phosphatic tubes, which taper more or
less and are open at both ends, among the SSFs. In north Greenland assemblages such
tubes have been found in the living position, embedded vertically in the
substrate, with the larger end upward (Skovsted & Peel, 2011). It is
believed that in life an animal living in the tube was probably a
suspension feeder, and it has been suggested that the tubes were formed
by stem annelids (Skovsted & Peel, 2011). There were also 2 genera of
the unsegmented Sipuncula, which nested within marine annelids, and have
been found in the Chengjiang Fauna (Huang et
al., 2004). Both of these
genera have morphological features that are typical of extant forms,
especially in the family Golfingidae, and have no striking
synapomorphies. Therefore this group has maintained an extremely
conservative morphological mode for 520 My at least, though the sample
is still quite small and it cannot be known for certain that disparate
sipunculid morphologies have never existed. The morphological
conservatism that is displayed by many clades over hundreds of millions
of years is emphasised by the early establishment of this unusual
annelid architecture.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |