The Cambrian Explosion

[otseng]

The Cambrian Period

In the sediments of Cambrian age, fossils "suddenly" become common for the first time. This effect, which has come to be known as the "Cambrian Explosion."

Adam Sedgwick named the Cambrian in 1835. He derived the name from ‘Cambria,’ the Roman name for Wales, site of the type area, in which shales and sandstones make up a section about two miles [~3 km] thick. Though these rocks are strongly folded and faulted, some are ossiliferous.

The International Subcommission on Cambrian Stratigraphy has settled on an age of 543 Ma for the Precambrian-Cambrian boundary, and approximately 490 Ma for the Cambrian-Ordovician boundary.

Life on earth has a long and rich history – predating the beginning of the Cambrian by a factor of six or seven. However, much of this early life was microscopic and, microscopic or not, virtually unknown until the 1960s. Prior to then, as far as was widely known even among the scientific community, the fossil record sprang into existence in the Cambrian, already exhibiting a high degree of development and marvellous diversity: the so-called “Cambrian Explosion.”

Today, although the fossil record now extends back 3,465 Ma (± 5 Ma, to the Apex Chert, see Schopf 1999, p. 100) and diverse precursors to the Cambrian biotas are gradually becoming understood, it still appears as if a genuinely rapid diversification of form, particularly among the Metazoa (“animals”), did occur in the Cambrian. During this time, most extant body plans are suddenly found in the fossil record. “Definitive representatives of all readily fossilizable animal phyla (with the exception of bryozoans) have been found in Cambrian rocks, as have representatives of several soft-bodied phyla (Valentine et al. 1991)” (Wray et al. 1996). By way of contrast, “it appears that no [new] phylum-level body plans have arisen in the animal kingdom in the last 500 million years” (Arthur 1997, p. 7).

Extinction

"Three significant paleoceanographic events are juxtaposed in Upper Cambrian (Steptoean Stage) sequences of Laurentia:

1. a mass extinction of trilobites that marks the base of the Steptoean (Marjumiid-Pteroceaphliid biomere);
2. a large positive excursion in carbon isotope values that spans much of the Steptoean (SPICE event); and
3. an imprecisely dated craton-wide drop in sea level (Sauk II – Sauk III event) that was terminated by widespread flooding in the Late Steptoean (mid-late Elvinia Zone time).

The first two events are clearly global in scope, but the scale and timing of the sea level drop is not known in detail outside Laurentia.

Life during the Cambrian period

Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. The only modern phylum with an adequate fossil record to appear after the Cambrian was the phylum Bryozoa, which is not known before the early Ordovician. A few mineralized animal fossils, including sponge spicules and probable worm tubes, are known from the Vendian period immediately preceding the Cambrian. Some of the odd fossils of the "Ediacara biota" from the Vendian may also have been animals in or near living phyla, although this remains a somewhat controversial topic. However, the Cambrian was nonetheless a time of great evolutionary innovation, with many major groups of organisms appearing within a span of only forty million years.

MSN Encarta

At the beginning of the Cambrian period (570 million to 500 million years ago) animal life was entirely confined to the seas. By the end of the period, all the phyla of the animal kingdom existed, except for vertebrates.

For debate:

How does the data from the Cambrian Explosion argue for/against the evolutionary model(s)?
How does the data from the Cambrian Explosion argue for/against the creation model(s)?

[youngborean]

It turns out that much of the ocean floor is quite new.

I guess this was the crux of my point. I thought your explanation was saying that Burgess was the Ocean floor pushed upwards. This is how I understand it. That Burgess was a reef that got push up. Not recycled volcanic material that wen't down then shot up though a volcano like the mountains down in Mexico.

Usually, fossils are in rock that can't be dated directly. One must rely on dating the "datable" strata above and below, which gives an age-range for the fossil. There are creationists who say this isn't valid either, because it doesn't date the fossil...but it's the same principle as finding a house on a street when you know the addresses two blocks north and two blocks south. Interpolation is valid methodology.

Weren't the Burgess fossils found on the surface of the Rockies? How can we interpolate the surface? That was the point I was attempting to reach in my earlier post.

Having said this, the Cambrian Explosion is often (erroneously) thought of as a very sudden appearance, all at once, of many species. At the level of resolution that we are talking about, "all at once" is probably at least 10 million years. If we include the rapidly-increasing knowledge of Precambrian fossils, then we must increase the length of time for this "explosion" considerably. Maybe we're talking 50 or 100 million years. I dunno...that seems like a long time to me.

A long time to me as well.

[Jose]

I guess this was the crux of my point. I thought your explanation was saying that Burgess was the Ocean floor pushed upwards. This is how I understand it. That Burgess was a reef that got push up. Not recycled volcanic material that wen't down then shot up though a volcano like the mountains down in Mexico.

Aha! I see now. The Rockies are partly volcanic, and partly sedimentary. As "granitic intrusions" pushed upward toward the surface, the sedimentary rock on the surface was pushed upward as well. In this case, the surface was the continental plate that had previously been pushed under water, long enough for sediment to form on it. If the granitic intrusions reach the surface, we get volcanic eruptions. If they don't, we get big granite "roots" (if you will) of the mountains.

On the east side of the Rockies near Denver, it is easy to see the sedimentary strata becoming tilted more and more as the mountains become taller. On the west side, say around Grand Junction or Dotsero, the whole sedimentary "layer cake" was lifted up without much tilt to it. The cliffs that formed as the rivers carved through these "layer cakes" show the various strata quite clearly.

Weren't the Burgess fossils found on the surface of the Rockies? How can we interpolate the surface? That was the point I was attempting to reach in my earlier post.

Again , aha! I haven't been to the Burgess Shale, so I can't describe it firsthand. But, my understanding is that it is like sedimentary outcrops that I have seen, which are like these "sliced layer cakes." The "surface" is the side of a cliff, or a steep mountainside. Upper layers are at higher elevation, up the cliff, and lower layers are down the cliff. It actually makes sense. (!)

[otseng]

How does the data from the Cambrian Explosion argue for/against the creation model(s)?

The evidence of the Cambrian layer (Cambrian explosion) shows that almost all the modern phyla are found there. This is consistent with creationism since it says that all the phyla were created at the beginning.

In creationism, it is possible for phyla to become extinct over time, but no new phyla can be introduced over time. This is also evidenced by the fact that many phyla in the Cambrian layer existed but has since become extinct.

Creationism postulates that a global flood was the mechanism to cause the formation of the Cambrian layer (as well as all the geologic layers). If a global flood occurred, one would expect that the lowest layers would have a lot of different types of life buried, which is what we see in the Cambrian layer.

Thus, by looking at the arguments against the evolutionary models and arguments for the creation model, creationism offers a more viable explanation of the Cambrian explosion than does evolutionism.

[Jose]

The evidence of the Cambrian layer (Cambrian explosion) shows that almost all the modern phyla are found there. This is consistent with creationism since it says that all the phyla were created at the beginning.

In creationism, it is possible for phyla to become extinct over time, but no new phyla can be introduced over time. This is also evidenced by the fact that many phyla in the Cambrian layer existed but has since become extinct.

Hmmm...it is interesting to note that the "Cambrian explosion" argument refers to the "major phyla." If we include the minor phyla, we see that roughly half of the animal phyla showed up later. Now, we can argue that the later-evolving phyla were actually present for millions of years before the first identifiable fossils showed up, and thereby claim that they, too, were part of the Cambrian "explosion." If we do, however, we have to use the same logic with the other fossils of the Cambrian, and recognize that their ancestors must have been around for millions of years before they developed hard parts that could be readily fossilized. So, either the Cambrian "explosion" actually occurred over many years before the development of shells, or other phyla appeared later.

By "many years," I refer to the current estimate of 600 million, rather than the measley 10 million that is usually meant by the Cambrian "explosion." Even a 10 million year explosion is pretty slow compared to the predictions of creationism.

Another interesting point: the Cambrian explosion doesn't apply to plants at all. They have a different history, with new phyla (called divisions by botanists) appearing much later.

Creationism postulates that a global flood was the mechanism to cause the formation of the Cambrian layer (as well as all the geologic layers). If a global flood occurred, one would expect that the lowest layers would have a lot of different types of life buried, which is what we see in the Cambrian layer.

One would also expect that this layer would have representatives of the things that are buried in upper layers, which it does not. Virtually everything that existed in the Cambrian has gone extinct, and has been replaced with entirely different things. This is most easily explained by Wen The Eternally Surprised, as God re-creating the world each instant. that way, God can put in new species as needed. Another way to explain it, of course, is by evolutionary mechanisms, which would give the result that we see.

Hmmm...your mention of the global flood reminds me that part way throughthe global flood thread, we had made some predictions based on the model, so that we could evaluate it and see how well it would fit in science classes. How's the research going, to assess whether those predictions are borne out?

[otseng]

How's the research going, to assess whether those predictions are borne out?

It has not slipped my mind. I have placed on order a couple of the books that rjw mentioned - Oceanography: An Introduction to the Planet Oceanus and Understanding Earth. My understanding of the earth sciences is very limited, so I hope to have the chance to read them when I get them and do further learning/research.

[keltzkroz]

I was redirected to this thread, so I'm reposting my question in this one.

Hi people! My reference/source for what I have written is ‘Rare Earth’ by Peter Ward and Donald Brownlee. (Its a good book to read!)

It is interesting that virtually all animal phyla that we know of appeared no later that the Cambrian and none (none that we know of) appeared since that time. Though the fossil record shows abundant evidence for the enormous diversification of species, it is puzzling that no new animal phyla that we know of appeared since then (500 to 600 million years ago). Why is this? Take note that the advent of metazoan evolution was about 1 billion to 600 million years ago, with evolution from the advent of life (3.8 to 3.5 billion years ago) to that period (1 billion to 600 million years ago) featuring very little morphological change. Theories range from environmental causes like the ‘Snowball Earth’ events or the ‘Inertial Interchange’ event, to biological causes, like the advent of predation. I'm curious about what particular circumstances/conditions must be present for new animal phyla to come about. (I like the snowball earth and inertial interchange particularly; they are fun to read)

[gluadys]

I was redirected to this thread, so I'm reposting my question in this one.

Hi people! My reference/source for what I have written is ‘Rare Earth’ by Peter Ward and Donald Brownlee. (Its a good book to read!)

It is interesting that virtually all animal phyla that we know of appeared no later that the Cambrian and none (none that we know of) appeared since that time. Though the fossil record shows abundant evidence for the enormous diversification of species, it is puzzling that no new animal phyla that we know of appeared since then (500 to 600 million years ago).

The first thing to remember is that all taxonomic terms above species are strictly classificatory terms for groups of species. Phyla as such do not exist in nature. Only species do (and we are even pretty fuzzy on where nature divides up species---except where two sexually reproducing species won't or can't mate with each other and produce viable offspring.)

So when it is said that many animal phyla appeared in the Cambrian, what it really means is that species which we classify in those phyla appeared then.

Now how do we decide that a species belongs in a certain phylum? We look for particular characteristics when we have decided are phylum markers. Any species with the requisite phylum characteristics is slotted into that phylum. e.g. a species with a segmented body, a chitinous exoskeleton, and jointed appendages is an arthropod. Who decided this? We did.

So it is not, strictly speaking, phyla which appear, but species bearing phylum markers.

Now the question is: are the species which we find in the Cambrian the same species we find today, even if they are in the same phylum? And here the answer is a resounding "NO". Very, very few (if any) modern arthropods date back to the Cambrian. And very, very few (if any) Cambrian arthropods survive today. The most numerous arthropod of the Cambrian was the trilobite. But non exist today. And the insects which dominate the arthropod phylum today did not show up until well after the end of the Cambrian.

So even though the phylum may be old, that doesn't mean it was the same then as it is now.

Why don't we see new phyla today? Easy. Evolution is a change in species. When we get speciation, the new species will always be in the same phylum as its parental group. Not only that: it will be in the same class, order and family as its parental group, and almost always in the same genus. Darwin was right: Nature does not make leaps. It certainly will never produce a new species so different from its ancestors as to be n a different phylum.

Another way to look at it is this. When Linnaeus classified plants and animals, he was comparing contemporary species which had already been diversifying for hundreds of millions of years. Now suppose we could put a Linnaeus in a time machine and send him back to the Cambrian age and ask him to classify the species then.

Right off the bat you would have no terrestrial life whatsoever, so there go all the major plant classifications. All there is to be see are various algae. You can also skip all insects and spiders and most fresh-water arthropods and molluscs. There are no reef-building corals yet. And while there are a few chordates, there are none yet that have jaws or vertebrae, and none that live on land. So we have no classes of fish, amphibians, reptiles, birds or mammals.

It is entirely possible, that our time-travelling Linnaeus would not have felt it necessary to use seven ranks to classify life. He might have classified everything he found using no rank higher than a family, or an order at most.

We can see the truth of that if we go in the other direction from Linnaeus' time to the present. We do not still use only the seven ranks Linnaeus used. They aren't enough to deal with all the species we have catalogued today. Linnaeus, for one thing, catalogued no microbes. But even among more complex life, there are many many many species being discovered every year. So we have added prefixes to all his ranks: super-this, sub-that, infra-whatever. And some taxonomists have incorporated new ranks as well: tribe (between genus and family), cohort (between order and class) division (between class and phylum) and domain above kingdom. In addition there are many named groups even between all these that have no official ranking at all.

This fits with Darwin's perception that "varieties are incipient species" and that the differences between genera were once no greater than the differences between species.

Diversity is built from the species up. So it is built up within genera. And therefore within families, orders, classes, phyla. What we call phyla were once no more than genera. And the reason no new phyla appear now is because all new species come from existing species and all existing species have already been assigned to a phylum.

I'm curious about what particular circumstances/conditions must be present for new animal phyla to come about. (I like the snowball earth and inertial interchange particularly; they are fun to read)

The circumstances for new phyla to come about are those which develop multicellular organisms from unicellular organisms. And they may indeed be those of snowball earth or a similar event in geological and climatological history. It is in the initial development from unicellular to multicellular forms that all sorts of basic body plans can be tried out, with the most successful ones emerging as enduring phyla. There are actually many phyla in the Cambrian that do not exist at all today, because many of them have no living descendants. The pre-Cambrian Ediacaran fossils also show an amazing diversity of body plans, even different from those found in the Cambrian.

We don't expect to see new phyla developing today, because there is no real advantage to a unicellular creature which has survived successfully to the present to become multicellular and compete with multicellular creatures that have been around for half a billion years already.

That could change if some future disaster destroyed the bulk of multicellular life.

[Jose]

We don't expect to see new phyla developing today, because there is no real advantage to a unicellular creature which has survived successfully to the present to become multicellular and compete with multicellular creatures that have been around for half a billion years already.

That could change if some future disaster destroyed the bulk of multicellular life.

...in other words, any incipient new phyla that might begin to emerge would be out-competed by the existing species that are already highy efficient at making a living in their particular niches. Newcomers tend to be less-well adapted, and cannot easily displace those who are already there. Hence, major evolutionary shifts often follow mass extinctions--like the diversification of mammals only after the extinction of the dinosaurs. Or, they follow adaptive innovations that open up new possibilities--like the colonization of land.

At present, the world is pretty full of advanced microbes and advanced multicellular plants and animals. There's a lot of competition for the new guy on the block.