Coquina: Cambrian explosion, check it out!

[Martin]

http://www.edge.org/q2005/q05_3.html

 

scroll down to where Donald Williamson offers a theory of

how the Cambrian revolution might have happened

it has to do with larvae

and with barriers between species not being completely effective

 

I am impressed by Donald Williamson.

 

His essay is the next to last one on that page, which is page 3 of the whole set. It is nearly at the bottom. only one short one came after it.

 

 

 

there was something also by someone named John McWhorter that impressed me but I cannot remember what it was or why.

 

that was on this other page.

 

http://www.edge.org/q2005/q05_9.html

 

oh yes it was GREAT it was about the Indonesian little people and he is a linguist and he identified possible traces in some languages of the existence of the different species of people

 

McWhorter essay is the THIRD essay on that page, page 9 of the set.

so you dont have to scroll down very far

 

Coquina I hope you can find the bit by Donald Williamson and are willing to read it because i would like to know if you think he is on to something

(or did you already know about these ideas)

[coquina]

Many well preserved Cambrian fossils occur in the Burgess shale, in the Canadian Rockies. These fossils include small and soft-bodied animals, several of which were planktonic but none were larvae.

 

How does he know whether they were larvae or not - is there some dead give away that would be obvious in a soft-tissue fossil, which are poorly preserved at best?

 

Compared with modern animals, some of them seem to have the front end of one animal and rear end of another. Modern larvae present a comparable set-up: larvae seem to be derived from animals in different groups from their corresponding adults. I have amassed a bookful of evidence that the basic forms of larvae did indeed originate as animals in other groups and that such forms were transferred by hybridization. Animals with larvae are "sequential chimeras", in which one body-form—the larva—is followed by another, distantly related form—the adult. I believe there were no Cambrian larvae, and Cambrian hybridizations produced "concurrent chimeras", in which two distantly related body-forms appeared together.

 

I don't exactly get why the question of whether there were larvae or not enters into the picture.

 

It would seem to me that the development of larvae would be in response to the changing climactic conditions throughout the year. (Not correct syntax but taking a shortcut here...) The species had to have a way of encapsulating themselves to survive winter or drought conditions. If the climate didn't change very much, there would be no advantage to a pupae or larval stage.

 

Have you ever looked at the Paleomap Project? It is here:

http://www.scotese.com/Default.htm

 

From that section you can go to "earth history" or "climate history".

Here's the page for "climate history" during the Cambrian.

http://www.scotese.com/mlcambcl.htm

The climate of the Cambrian is not well known. It was probably not very hot, nor very cold. There is no evidence of ice at the poles.

 

I guess there is another advantage to a larval stage - it's distribution to other areas. It would become an advantage for finding new niches if the one in which the organism lived became overcrowded, because they could be carried to new areas by currents without having to expend energy.

 

About 600 million years ago, shortly before the Cambrian, animals with tissues (metazoans) made their first appearance. I agree with Darwin that there were several different forms (Darwin suggested four or five), and I believe they resulted from hybridizations between different colonial protists. Protists are mostly single-celled, but colonial forms consist of many similar cells.All cambrian animals were marine, and, like most modern marine animals, they shed their eggs and sperm into the water, where fertilization took place. Eggs of one species frequently encountered sperm of another, and there were only poorly developed mechanisms to prevent hybridization.

 

I'd like to know how he made that quantum leap. Only a tiny fraction of fossils from the Cambrian has been preserved - the land that existed then has been subducted, melted, and extruded several times over, along with whatever fossils might have been there.

 

At any rate - I think he is putting the cart before the horse, because he starts with the theory that there were two distinctly different animals that hybridized to become a third.

 

I think that if life evolved into two separate types, the split is at the bilaterally symmetrical critters and those that have radial symmetry. Regardless - in Cambrian times, there existed a multitude of niches, with very few organisms to fill them. As mutations occurred, more of them were able to find a way to fill a separate niche without competing with others.

 

You must remember, the fossils found in the Burgess shale are probably only a millionth of the species that were alive at the time.

 

As a comparison - lets take the movie "Titanic" - A thousand years has gone by an someone finds a tape in an old blockbuster store. Out of the entire tape they are only able to retrieve ten frames, and none of them happened to contain the iceberg.

 

The finders might figure out that a great ship had sunk and that a great number of people had been drowned, but the entire reason for the disaster would remain unknown. The only way to know for sure what happened would be to find more tapes, with other undamaged sections and splice them together until you got enough to tell the whole story.

 

At this time - people can speculate, but they don't have enough information to come to a valid conclusion.

 

I'll have to check out the other link later.

 

Ophiolite - what's your take?

[Ophiolite]

It's interesting. I like off-the-wall ideas, because that's where tomorrow's accepted wisdom comes from. [in my heart of hearts I don't believe in the Big Bang, but in Fred Hoyle's Steady State Universe - and don't these recently discovered 'new' young galaxies give pause for thought? Sorry, red shifting off topic.]

 

That said I share some of Coquina's reservations. How can he be sure there were no larvae? I really disliked "Compared with modern animals, some of them seem to have the front end of one animal and rear end of another" - that read like the sort of argument I expect to hear from a creationist.

 

I think it is interesting speculation, but no more. For me there are three factors responsible for the Cambrian explosion: the 'rebound' from snowball earth conditions; the evolution of exoskeletons; something as yet unidentified. Perhaps this hybridisation is the third factor.

 

Coquina, I would challenge you on the land life issue. I believe he is essentially correct here. Land masses are continental and sialic. It is largely the simatic oceans which get recycled. There is enough pre-Cambrian terrestrial rock that if there were significant land life we would have detected it.

 

All in all an idea that is worth further consideration, but........

[coquina]

Coquina, I would challenge you on the land life issue. I believe he is essentially correct here. Land masses are continental and sialic. It is largely the simatic oceans which get recycled. There is enough pre-Cambrian terrestrial rock that if there were significant land life we would have detected it.

 

I don't know the word "sialic" - do you mean that they are composed mostly of granite as opposed to basalt, the therefore "float" on the mantle? I thought that the "cratons" or "continenal shields" were more or less permanent, but that they are only the interior parts of the continents, and that the balance was lost not only to subduction, but erosion.

[Ophiolite]

Sial and sima are, as you suspected, two short hand terms to cover the general composition of continents and oceans respectively. SiAl - is short for silicon and alluminium, major elements in granites and their derived sediments. SiMa - is short for silicon and magnesium (simg is unpronounceable) major elements in basalts, which pretty much make up the ocean crust.

Your understanding is, I think, broadly correct, but the scale of the factors involved have important consequences. Let me expand on that.

The cratons/continental shields are pretty much permanent (well, reasonably so), as you say. We tend to think of the Baltic shield and the Canadian shield as type examples, but they don't stop at their exposed edges. Much (most?) of Siberia is underlain by rocks of comparable type. Unlike Finland these are covered by thick sequences oof younger rocks. The same story in Poland and Germany and on across the North Sea to the UK.

So you are right that it is the edges of continents that are prone to depletion by subduction, but it is the cores that make up the greater part of the continental masses. Also, when a continental plate collides with an oceanic plate it is the latter that subducts. The continental edge is in fact much more likely to grow by accretion as sediments lying on the continental slope and the ocean floor are welded to the continent.

Erosion will gradually remove material, but there are very deep roots on large mountains, and isostasy will keep raising them - the remains of the Caledonian mountains formed in the Palaeozoic would have been at least as large as the Alps. Now we can barely muster 4,400'.

I'll try to find some relevant links, but a first pass was not successful. I am not up too date with the thinking of the current rate of continental growth.

[coquina]

Thanks for the imput - I hadn't read those terms before. I am familiar and understand the terms "felsic" and "mafic", which seem very similar, but not quite the same.

 

Look at the map from the Paleo Map project of where the oceans and land masses were placed during the Cambrian:

 

http://www.scotese.com/newpage12.htm

 

If it is reasonably accurate, it is easy to see how life would have rapidly dispersed - one it had got a foothold - lots of contiguous continental shelves that would have been a great breeding ground for life.

 

The area that formed the "Burgess Shale" would have been on the east coast of Laurentia:

http://www.paleoportal.org/time_space/period.php?period_id=16