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Australia: The Land Where Time Began |
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Cambrian Explosion - Other Possible
Deuterostomes Recognisable body plans are maintained by the Crown
phyla as they are tracked back to the Early
Cambrian, though the stem
groups commonly have unusual synapomorphies unknown among crown groups,
or in some cases are mixtures of characters that are not shared by any
known crown groups. One such form from the Burgess Shale is a
soft-bodied fossil with tentacles,
Herpetogaster collinsi,
which has been grouped tentatively with other fossils from the Cambrian
with which they share general morphological features but have uncertain
affinities. This group has been interpreted as possibly being stem
Ambulacraria (Caron, Conway Morris & Shu, 2010), i.e., they branched
following the split between chordates and ambulacararians, though before
the split between hemichordates and echinoderms. According to the
authors1 a segmented region that, if coelomate, is unlike
those of deuterostomes, appears to be included in the trunk of
Herpetogaster. There is a
trace in the tentacles that has been interpreted as either a hydrostatic
canal or a vascular system. Pharyngotremy had been evolved by the
ambulacrarian/chordate LCA, and a possible location for gill pores is
behind the head, though on
Herpetogaster gill pores
are not present equivalently. According to the authors1
interpretation of this animal is very difficult, though as fossils go,
its preservation is reasonably good, which epitomises the problems
encountered when dealing with stem groups. Vetulicolids and yunnanozoans are 2 other possible
deuterostome groups that are often referred tentatively to the chordate
branch, or alternatively as more basic positions as stem deuterostomes,
as well as other phylogenetic positions that have been proposed.
Vetulicolids have bipartite bodies, with an anterior section that is
carapace-like that may be subdivided into 2 or more units, which
sometimes appear to be segmented, and a trunk or posterior section that
in some genera appears to be narrowly segmented in a fashion reminiscent
of arthropods. They have been recovered from both the Chengjiang Shale
Fauna and Burgess Shale Fauna (Aldridge et
al., 2007; Caron, 2006; Shu
et al., 2001). There are
structures on some of the Chengjiang forms that have been interpreted as
gill slits, which suggests an affinity with chordates, whereas the form
from the Burgess Shale Fauna,
Banffia, which is known
from hundreds of specimens, shows structures that can be interpreted as
midgut diverticulae (Caron, 2006), a feature that is not found in crown
chordates but is common in protostomes. The authors1 suggest
that forms assigned to Vetulicolida represent more than 1 group; at the
very least, the morphology of veticulids is sending a mixed phylogenetic
message. Another form present in the
Chengjiang Fauna that
is also known from many hundreds of specimens is Yunnanozoans, though
they are described as being quite enigmatic, and have been reconstructed
as stem hemichordates (Shu, Zhang & Chen, 1996) or stem chordates (J.Y.
Chen et al., 1995). They have
a body that is fish-shaped or lancelet-shaped with a series of
anterolateral structures that make sense as gills, and they have a
remarkable dorsal fin that is seriated or segmented which at first
glance suggests a relationship with the cephalochordate or vertebrate
body plans, though it doesn’t seem likely it functioned in an analogous
manner. Also, there is no notochord structure that has been confirmed,
which weakens an assignment to the chordates. (see Shu, Zhang & Chen,
1996). Important reasons for the uncertainties about the
phylogenetic position of these soft-bodied fossils are taphonomic loss
and blurring of morphological characters. Studied of decay patterns
among modern chordates have revealed a progressive loss of
phylogenetically informative characters, with the result that as decay
progresses, a specimen appears similar to progressively earlier
phylogenetic stages (Sansom, Gabbott, & Purnell, 2010). Therefore decay
is not random with respect to the phylogenetic sequence of chordate
characters. It is suggested by these modern taphonomic experiments that
the interpretation of fossils from the Cambrian may be affected by such
sequential taphonomic biases.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |