Minnemooseus
Member Posts: 3945 From: Duluth, Minnesota, U.S. (West end of Lake Superior) Joined: 11-11-2001 Member Rating: 10.0
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Message 29 of 199 (27481)
12-20-2002 11:24 AM
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A nice posting has happened in Yahooland, at the "Evolution versus Creationism" group. The message is titled "The Cambrian "Explosion" in the real world. It is from Ray T. Perreault, responding to a JimG. The following is it, in it's entirety.
quote:
JimG--1) The Cambrian explosion, when most of the phyla in existence today sprang into existence in a just a few million years, with no sign of any likely preceding, ancestral forms (as the theory requires). This is not entirely accurate. It is a very popular simplification, and
among the various cults of Creationists, the last phrase has become added
as a mantra. For a full picture, the Vendian must be taken into account,
as well as the Ordovician Period following the Cambrian. Only thus, can
the actual context of the event be bracketed properly. THE VENDIAN General Fauna--The Vendian is the latest Precambrian, with a fauna
dominated by flat, large, organisms of poorly known biology. They have a
quilted body plan, as opposed to the tubular organization of standard
animals. There is little evidence of internal organs. Seilacher has
proposed the Kingdom Vendobionta for these oddities. In addition to the
Vendobionta, there are a number of organisms which are referrable to
Animalia, and seem to represent early representatives of modern phyla.
Sponges, jellyfish and sea pen octocorals (Charnia) are fairly certainly
documented. Spriggina bears a very close resemblance to trilobites, but
lacks eyes (not a problem, many later trilobites do, too) and the
post-cephalic region is not trilobed. In the earliest Cambrian
trilobites, the Olenellids, the trilobed post-cephalic region is followed
by a long "tail" in some genera, which is reminiscent of the Spriggina
condition. The later portion is only occasionally preserved.
Tribrachidium is held by some to be an ancestral echinoderm, I personally
have doubts. It does resemble an edrioaster, but those do not appear
until the Ordovician. Collectively, these are termed the Ediacarian
Fauna. There are Precambrian trails of a type made by worms, but no body
fossils are known. Thus, it is definite that the Cambrian "Explosion"
has antecedants. Charnia was first described in 1958, so the information
is not new. There have been a very few recent discoveries of Vendian animals which
secreted tubes for habitation, but little is known of them as yet. Preservation--There is a major preservational difference in Vendian vs.
Paleozoic fossil assemblages. This is due to the microbial mats endemic
in the Vendian. These mats existed because no efficient grazing animals
existed yet. The mats likely consisted of a framework of filamentous
plants with single celled micro-organisms and infiltrated mud. Vendian
faunas are preserved as impressions into these mats, and have a more
three-dimensional structure. Preservation of soft-bodied creatures after
the Vendian is in the form of very thin films, almost two dimensional. Vendian/Cambrian transition--The Vendian ended with a major biotic
change, which involved the disappearance of the microbial mats, and near
extinction of the Vendobionta. Recent recognition of preservation modes
of the Vendian vs. Cambrian led to recognition of Vendobionts as late as
middle Cambrian. Efficient mulloscoid grazers with radulas are
considered to be the cause of destruction of the microbial mats. THE CAMBRIAN Basal Cambrian (Tommotian)--As research on Cambrian paleontology
increased in the 70s and 80s, it became clear that a good definition of
the beginning of the Cambrian was necessary. Formerly, it was the first
trilobites, but it became clear that there was a considerable undefined
pre-trilobite section. By international convention, in 1990, the base of
the Cambrian was defined biostratigraphically as the first appearance of
the "small shelly fauna." This is a mostly microscopic array of
extremely varied shapes of skeletal elements, most of which defy any
attempt at identification. It has a nearly worldwide distribution, and
is an easily identified marker zone. This is the point where hard parts
became very common in the fossil record. The few which have been
identified from the somewhat later lagerstatten show that basically
soft-bodied creatures were studded with numerous plates of several
shapes. In some sections, a zone is recognized below the base of the small shelly
fauna with animal trackways of legged and legless types, but almost no
hard parts. Identifiable animals in these beds include sponge spicules, grazing
molluscs, and brachiopods, a phylum of filter feeding lophophorates.
Lophophorate phyla also include acorn worms, bryozoa, echinoderms, and
the chordate phyla. The base of the Tommotian is dated at app. 540 MA. LATER CAMBRIAN FAUNAS The best known soft-bodied faunas cluster around the middle Cambrian, in
various parts of the world. The first studied, Burgess Shale, is about
the youngest in age. The Chinese fauna is somewhat earlier, and is still
under study. These lagerstatten are augmented by well-preserved hard
part faunas of widespread occurrence. General aspects--Trilobites dominate, everywhere. Most phylum level
invertebrates have made their first appearance by now, but at the class
level, not many would be recognizable today. Molluscs are represented by
hyolithids, monoplacophores, and several rare species which cannot be
assigned to known classes without difficulty. Echinoderms are present,
the only living class is sea cucumbers. Others belong to the extinct
subphyla Homalozoa and Blastozoa. No echinoderm shows the characteristic
five fold symmetry clearly until the latest Cambrian. Crustacean and
chelicerate arthropods have appeared, but not Uniramia. There are no vertebrates. Chordates are represented by genera similar to
Amphioxus. Terrestrial life is completely absent, with the possible
exception of lichens. Faunas show a high disparity, i.e., much diversification at the phylum
and class level, even more than today, with little diversification at
lower taxonomic levels. Differences between phyla and classes are
blurry, since the lineages had not yet retrenched from the original
adaptive radiations. The Chinese are at the forefront of evolutionary
studies on these lines. Even at this time, there is evidence of reliction and extinction at high
taxonomic levels. Most of the Tommotian shellys are already gone. The
original armor of small sclerites and spines studded into the skin has
been replaced by larger shells. This has been theorized as a fusion of
the smaller elements into larger compound plates, but such a development
shows its derivation. More likely, one or a few plates expanded, the
others were suppressed. The phylum Archaeocyatha, considered more primitive than sponges, were
the first animal reef builders from the lower Cambrian. They died out
totally in the middle Cambrian. Entrenchment of phylum level taxa--The lagerstatten show a large number
of organisms whose affinities fit no phylum as recognized in
post-Cambrian times. Most show characters that combine phyla, or
characters of one phylum combined with unique alien characters.
Entrenchment of phyla was accomplished by extinction of lineages as much
as by further evolution of surviving lineages. This extinction appears
to have happened shortly after the Burgess, as late Cambrian show
entrenchment in high progress. LATE CAMBRIAN/ORDOVICIAN DEVELOPMENTS From the late Cambrian through the end of the Ordovician, almost all
marine invertebrate classes made their first appearance. Many archaic
Cambrian lineages disappeared or relicted during this time, as well.
After the Ordovician, only terrestrial life, and a very few marine
classes remained to appear. There was still considerable extinction to
follow, and many organisms of the time would be alien to us. But after
the Ordovician, the seas did look recognizable. General aspects--The first true corals appeared in the Ordovician, as the
extinct Rugosa and Tabulata. Echinoderms expanded and diversified explosively in the latest Cambrian
and Ordovician, and all classes appeared by the end. Many went extinct,
as well. Pentameral forms quickly took dominance. Brachiopods
diversified rapidly. The phyla Bryozoa, Hemichordata, and Conodonta first appeared in the
latest Cambrian. Gastropods evolved from the Monoplacophora in late Cambrian, bivalves and
nautiloid cehpalopods first appeared in the Ordovician. Arthropods continued to diversify rapidly, with most crustacean and
chelicerate classes appearing. The subphylum Uniramia appeared in the
Silurian. The first well-known vertebrates appeared in the upper Ordovician, as the
jawless ostracoderms. No fish until the Silurian. CONCLUSIONS The phyla Cnidaria, Porifera, Arthropoda, and possibly Echinodermata
predate the Cambrian on the basis of fossil evidence. Trace fossils
indicate worm phyla present as well. The Cambrian "Explosion" was stretched out over a period at least 30
million years, and may be an artifact of visibility and preservation as
much as rapid diversification. The base of the Cambrian is defined as the first appearance of abundant
hard parts, not as the first multicelled life. Disparity of phylum level taxa as defined from the post Cambrian is
partly a result of extinction of early phylum level lineages, eliminating
many connecting forms. Climatic changes about 15 million years after the Vendian extinction
resulted in widespread shallow epicontinental seas offering multitudinous
new niches for adaptive radiation. Ray
Moose [This message has been edited by minnemooseus, 12-20-2002]
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