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Australia: The Land Where Time Began |
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Cambrian Explosion - Echinodermata Disarticulated plates from the Cambrian Stage 3 of
well-mineralised echinoderms are the first evidence of their appearance.
They are represented by a few specimens in the
Burgess Shale and are not
found in the Chengjiang deposits, which suggests that the conditions of
their normal habitat didn’t overlap with the type of conditions that
were conducive to preservation of the Burgess Shale type. It has been
argued (Clausen et al., 2010)
that the presence of echinoderms in carbonate sequences from the
Cambrian Stage 2 through to Stage 5 of preferential divergence in
carbonate environments. The many small plates that comprise the
echinoderm skeleton of tend disarticulate after death, though even a
plate that is isolated can be referred with some confidence to
Echinodermata because of the unique stereo texture of their plates. Many
forms of the early echinoderm groups, that mainly appeared between
Cambrian Stage 3 and the Middle Ordovician, lacking the familiar
pentameral arrangement of ambulacral regions present in extant forms,
some of those groups possessing only 1, 2, 3 or 4 ambulacra. Therefore
Echinodermata shows the common metazoan pattern involving achieving a
broad range of different morphologies, at the body sub-plan level, early
in its history. Questions have been raised as to how to handle many of
these early disparate groups taxonomically, as many of them are not
known to be very diverse, have fossil records that are relatively short,
and are relatively rare. The solution, under a Linnaean taxonomic
approach, has been to recognise the disparities by assigning distinctive
morphological groups to taxonomic levels showing morphological
differences similar in magnitude to those seen in extant echinoderm
groups. More than 20 different classes of echinoderm have been
recognised, under this approach, during the Cambrian and
Ordovician
(Sprinkle, 1980; Sprinkle & Kier, 1987), though there are only 5 extant
classes. The systematic practice of many decades ago is reflected in the
large number of echinoderm classes that are relatively short-lived, many
of which had few species. The modern view of early arthropod evolution
contrasts starkly with the earlier approach. According to the present
view just as many distinctive morphologies have been discovered among
arthropods from the Cambrian, though there are only a few Linnaean
classes or orders that have been defined. It seems likely that during
the Palaeozoic there were as many as 30 echinoderm clades that were
morphologically distinct (Sumrall & Wray, 2007). Subsequently, the taxonomy of echinoderms has been
given a cladistic treatment that has the aim of tracing the patterns of
branching of their lineages as novel morphologies appeared and
accumulated instead of identifying their disparate groups (Paul & Smith,
1984; Smith, A.B., 2005; Smith, A.B. & Peterson, 2002). The record of
the early echinoderms is incomplete and the authors1 suggest
their known record may be of not much use for a detailed interpretation
of patterns among body plans. A model that is more general, of the
evolutionary change among the distinctive clades of echinoderms has
resulted from a comparative study of the development of ambulacral
geometry. All except Crinoidea among the extant echinoderm classes have
5 ambulacra that terminate at the central mouth, which is a fully
pentameral symmetry. The crinoids and all fossil echinoderm groups that
have 5 radiating ambulacra, have only 3 that reach the mouth, 2 of which
split during development to produce the 5 of adult specimens (like the
edrioasteroid
Camptostroma, therefore
they can be described as having a 2-1-2 pattern instead of a 1-1-1-1-1
pattern. Fossil echinoderm groups that have fewer than 5 ambulacra in
the adult are not even pentameral and are interpreted (Sumrall & Wray,
2007) as having reduced from the more primitive pattern by various
patterns of loss of ambulacra during early developmental stages. The
losses of ambulacra have been correlated with important body change
differences such as those that are characteristic of many disparate
fossil echinoderm groups. The spotty fossil record makes this
evolutionary model difficult to test, but according to the authors1
the generality of this model make it appealing. It seems clear that
reductions of ambulacra have also occurred within minor branches of some
major clades. And this tends to complicate the phylogeny even further.
The bodyplan sequence of appearance in skeletonised forms does not match
sequential losses of ambulacra, though this is not sufficient to falsify
the hypothesis of Sumrall & Wray when the incompleteness of the early
echinoderm fossil record is taken into account. The earliest Cambrian echinoderms have been
assigned to Helicoplacoidea (Durham & Caster, 1965).
They are known from only 4 small-bodied genera (mostly 4-7 cm)
from stage 3 that were recovered from western North America. The 3
genera that are best known have somewhat fusiform to top-shaped bodies,
with a skeleton of imbricating plates that have a stereom texture that
are arrayed in spiral columns. The columns are mostly interambulacral,
but ambulacral columns, which indicate the presence of a water-vascular
canal system, are shown by a few columns that have plates that are much
smaller. There are 2 ambulacral columns that spiral down from the apex
of the larger end to about the wider parts of the test, at which point
they join; the single ambulacral column that results continues to spiral
down towards the smaller end though it terminates before it reaches the
base. The clearest description has been given of the ambulacral
structures (Durham, 1993), working with a large collection, concluding
that the mouth was at the larger end. It had previously been suggested
that the mouth was at the junction of the 3 ambulacral columns (Paul &
Smith, 1984). An oral framework has not been detected at either site,
though the authors1 suggest that this may be because of the
collapsed, deformed state of the fossils. It would be suggested that the
3-canal system evolved to the 2-1-2, 5 canal system present in the early
echinoderms by the doubling of each of the paired ambulacra, if the
mouth were at the junction. In rocks (mostly in the Kinzers Formation,
Pennsylvania) of early Stage 4 2-1-2 pentaradial groups are the earliest
known. A group which is often treated as a class is the Edrioasteroidea,
which were circular in outline and lay on their dorsal surface, in some
cases being apparently attached to the substrate; they have been assumed
to have been suspension feeders.
Camptostroma, the basal
edrioasteroid, which has been interpreted as a basal form, and has been
recovered from the Kinzers Formation, and it has been hypothesised to
represent the deepest known branch of all the pentaradial echinoderms
(A.B. Smith, 1988). A later branching may have resulted in the
appearance of 2 echinoderm clades that produced the disparate
pentaradial groups of the Palaeozoic, 1 of which was characterised by
attachment stalks. The authors1 suggest that
Stromatocystites,
representing 1 of these major clades may be ancestral to all living
nonstalked echinoderm classes. The model of Sumrall & Wray, if correct,
would imply that the last common ancestor arose following the evolution
of the 1-1-1-1-1 pattern.
Lepidocystis, another
echinoderm from the Kinzers Formation has been hypothesised to represent
a branch that given rise to stalked clades that are extinct such as
Eocrinoidea and the Crinoids. Crinoids are pentaradial in the 2-1-2
fashion, that have ambulacra that extend up their feeding arms (as is
the case in
Lepidocystis); (A.B.
Smith, 1988). They were all stalked and attached to the substrate for
most of their existence, by a stem-like organ that was well-skeletonised,
crinoids without stalks evolving much later. However, it has been
hypothesised that a second lineage also branched off the edrioasteroids,
which later evolved a stalk, diversifying into the Eocrinoidea, an
extinct class. The arm-like structures of crinoids were not invested by
coeloms, and did not bear ambulacra; such arms being called brachioles.
If the suggested phylogenetic hypothesis is correct Edrioasteroidea and
Eocrinoidea are both paraphyletic, though new fossil discovery are
required to test these hypotheses. Another enigmatic group of echinoderms, the
carpoids, that lack pentaradial symmetry, is first known from Stage 5 in
Bohemia. Though 2 of the 3 main groups have been suggested to “exhibit
bilateral tendencies,” in Ken Caster’s phrase (Ubaghs & Caster, 1967,
S583); and according to the authors1 there are no other
echinoids that are remotely like them. The Stylophora, a clade of
carpoids, was at some time in the past proposed to be on the lineage of
vertebrates (Jeffries, 1986). It has subsequently been shown that
carpoids are a branch of echinoderms that are no more closely related to
chordates than any other echinoderm group, though the architecture of
carpoids is clearly outside the borders of all other known echinoderms,
and the time of the first appearance of carpoids is uncertain.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |