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Australia: The Land Where Time Began |
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Cambrian Explosion - Scalidophora There are 2 crown pseudocoelomate ecdysozoans
phyla, Priapulida from the
Chengjiang Fauna and the
Burgess Shale
(Wills, 1998), and Loricifera (Peel, 2010) from the Sirius Passet Fauna,
northern Greenland, are identified from stem lineages in the Cambrian.
There is also a third extant phyla, Kinorhyncha, that is not known from
the fossil record. Together this extant phyla and 2 extinct phyla are
grouped into the clade Scalidophora. There are also stem groups from the
Cambrian that are often also identified as Scalidophora. In both the
Chengjiang and Burgess Shale assemblages there is an array of forms that
have been included in the Priapulida, that range up to 15 cm in length.
Priapulids of the present are found in very soft substrates where they
are shallow burrowers, and it has been suggested that these larger forms
may also have shared this mode of life. The point has been made in a
review of Chengjiang pseudocoelomates from the Chengjiang and associated
faunas that many features that have been used to identify Cambrian forms
may represent plesiomorphies, and if this is the case it may be
premature to allocate them to crown taxa. The genomes of extant
priapulids are nevertheless highly conserved, and this suggests a low,
possibly basal position within the Ecdysozoa, and it has been suggested
that their extinct forms from the Cambrian may represent their stem
group(s) (Webster et al.,
2006). There are embryos from the faunas of Early Cambrian
to the Early Ordovician preserved in phosphatic sediments that have been
assigned to the Scalidophora. The genus
Markuella contains the
best known species which is believed to represent a stem form of either
Scalidophora or Priapulida (Dong et
al., 2011). It is more common
to find late stage embryos preserved, though it has been reported (X.G.
Zhang, Pratt & Shen, 2011) that there are some cleavage-stage form that
indicates the presence of radial cleavage, as occurs in priapulids. It
has been suggested that radial cleavage is the basal cleavage state of
ecdysozoans on the basis of molecular phylogenetic relationships
(Valentine, 1997).
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |