raindropple

If it does could someone give me more info on the Pre-Cambrian fossils.

If not will this mean evolution does not explain the Cambrian Explosion?

some info

<a href="http://news.nationalgeographic.com/news/2001/07/0719_crustacean.html" target="_blank">http://news.nationalgeographic.com/news/2001/07/0719_crustacean.html</a>

Thanks

[ May 17, 2002: Message edited by: raindropple ]</p>

hezekiah jones

As your own news article relates, the existence of various fossils does not pose a challenge to evolution. Once again, the debate is not over whether animals evolved but over the "rate" of evolution at various time frames in the earth's history, which, as I understand it, is dependent on all sorts of factors.

There are all kinds of "sudden" appearances of novel organisms in the fossil record. There are also all sorts of creatures living today that have barely changed at all over hundreds of millions of years.

The Cambrian "Explosion" is something of a misnomer since the Cambrian period lasted for several million years. Unfortunately the fossil record will never be "complete." At least one of the questions that arises from the Cambrian fossils, I would imagine, is what were the environmental conditions that precipitated such a wholesale preservation of organisms.

However the morphological aspects of the organisms found among the Cambrian fossils fit the evolutionary history quite well, as far as I know. If there were rabbits, geese, and chimpanzees in the Ediacara, that would indeed be a problem for biologists.

Anyway I'm sure you'll get much better answers than this one.

[ May 17, 2002: Message edited by: hezekiahjones ]</p>

Late_Cretaceous

The cambrian explosion does not contradict nor challenge evolutuionary theory. The only thing is that the fossil record is so sketchy that it does not provide earlier examples of lineages.

There are, however, a few examples of metazoan fossil beds that predate those snapshots of the cambrian explosion.

vendian / ediacaran fossils:
<a href="http://www.ucmp.berkeley.edu/vendian/vendian.html" target="_blank">http://www.ucmp.berkeley.edu/vendian/vendian.html</a>
<a href="http://geol.queensu.ca/museum/exhibits/ediac/ediac.html" target="_blank">http://geol.queensu.ca/museum/exhibits/ediac/ediac.html</a>

and a "ahem" dissenting opinion on ediacaran fauna from John Woodmorappe (comic relief):
<a href="http://www.rae.org/cambrian.html" target="_blank">http://www.rae.org/cambrian.html</a>

and of course, those darlings of the cambrian explosion from the burgess shale
<a href="http://www.burgess-shale.bc.ca/" target="_blank">http://www.burgess-shale.bc.ca/</a>

Nic Tamzek

That article is weird, as the scientists are announcing more "advanced" arthropod 511 mya, whereas the article puts the start of the cambrian at 545 mya. In other words, there are 34 million years to play with there.

Now what *would* be interesting is to apply this technique to the appropriate rocks in the earliest cambrian and edicaran, and see if we can get fossils of those little millimeter-scale critters that were making those tracks and burrows. IMO the most likely ancestors of the bilaterans were (1) soft bodied and (2) near-microscopic and (3) basically very small worms and/or planktonic like many larva today. Such things are known to fossilize rather poorly.

nic

nic

lpetrich

Here's a nice link:

<a href="http://cas.bellarmine.edu/tietjen/Ecology/early_animal_evolution.htm" target="_blank">http://cas.bellarmine.edu/tietjen/Ecology/early_animal_evolution.htm</a>

The author calls the Ediacaran fossils a paleontological Rorschach test; interpretation of them has been very controversial, and it has been difficult to link most of the Ediacaran forms to later fauna and flora.

However, that is much easier for Cambrian fauna, though some Ediacaran fauna does survive into the Cambrian.

For example, some Ediacaran disk-shaped forms seem like jellyfish to some, but seem like holdfasts for seaweed-shaped organisms to others.

In the Cambrian itself, the first examples of familiar taxa did not appear all at once at the base, but instead over several million years -- the "Cambrian explosion" was not the instantaneous event that many creationists present it as being.

That paper also discussed attempts to reconstruct early animal evolution by looking at development-control genes such as Hox genes. From these, one can reconstruct several features of the common ancestor of arthropods and vertebrates, where most of the development-gene research has been done.

However, this common ancestor was likely the common ancestor of all the bilaterally-symmetric animals (Bilateria, of course), which fall into three big groups:

Ecdysozoa (they strip of their old outer skins as they grow): arthropods and nematodes

Lophotrochozoa: mollusks, annelids, lophophorates, flatworms, etc.

Deuterostomia: echinoderms, chordates

This common ancestor was rather sophisticated in some ways. It had:

Three germ layers in the embryo

Nose-to-tail patterning controlled by 8 Hox genes, whose order in the genome is highly conserved

A central nervous system residing in a strip on one side of the body

A simple heart

Belly-to-back patterning, though the direction is reversed between arthropods and vertebrates:

Arthropods: belly, CNS, gut, heart, back
Vertebrates: belly, heart, gut, CNS, back

Eyespots (Pax-6 gene widely shared)

Some sort of limbs (antennae? feeding tentacles?)

Possible segmentation

This is a big jump over the fanciest non-bilaterians, the cnidarians and ctenophores, which have only two embryonic germ layers, only a few Hox genes, a diffuse nervous system, etc.

Which makes the early evolution of bilaterians a big puzzle -- there seem to be no surviving halfway ones like those with a heart and no CNS or vice versa.

Also, early bilaterians do not show up very well in the Precambrian fossil record; there are a few candidates like Kimberella, and there are some phosphatized embryos that have arthropod-ish features. The record for early bilaterians, and pre-Ediacaran animals in general, is simply not very good compared to the record of algae, which extends back at least 1-1.2 billion years.

As to what triggered the Cambrian explosion, there is evidence for a mass extinction at the base of the Cambrian -- there is a big negative excursion in the C-13 concentration that is much like that in the Permo-Triassic mass extinction 300 million years later. This may have cleared away much of the Ediacaran fauna, allowing some very different-looking survivors to proliferate.