Cambrian explosion

[TeleMad]

Might be a good topic for all of us to learn more about through discussion.

 

My first point is that although the the extant and some extinct phyla (apparently) appeared during the Cambrian explosion, it's not like all the major types of animals we are familiar with arose then. A few examples (the info is from a 1999 general biology text I have).

 

1) Fish

The first fishes were jawless. They did not appear until sometime in the Ordovician period, which followed the Cambrian. This means the first fish didn't appear until about 65 million years after the beginning of the Cambrian period. The fish most of us are familiar with are not jawless, but jawed. These fish didn't appear until the Devonian period, which didn't start until about 162 million years after the beginning of the Cambrian period. So although the phyla fish belong to arose sometime during the Cambrian explosion, jawed fish didn't arise until 162 million years after the beginning of the Cambrian period.

 

2) Amphibians

The first amphibians also didn't appear until the Devonian period: again, about 162 million years after the beginning of the Cambrian period.

 

3) Reptiles

The first reptiles appeared during the Caroniferous period: that's about 210 million years after the beginning of the Cambrian period.

 

4) Insects

The first insects also appeared during the Caroniferous period: again, that's about 210 million years after the beginning of the Cambrian period.

 

5) Mammals

The first mammals appeared sometime during the Triassic period, which started about 222 million years after the beginning of the Cambrian period.

 

6) Birds

The book doesn't mention the first birds in the chart I am looking at, and I haven't followed the recent findings of ancient bird fossils. But I believe they couldn't have arisen before the Triassic period. If so, that also would be at least 222 million years after the Cambrian period began.

[Fishteacher73]

One issue of the examples brought up are that they are all vertebrates. Only the precursers of verts were showing up in the fossil record at the begenning of the Cambrian. Pikia, if I recall, was from the Burgess Shale and is considered one of the possible ancestors of the verts. and this was from the middle Cambrian.

 

Most of the Cambrian Explosion (which is arguable to a degree of there was really an explosion or not) was in the form of invertebrates such as trilobites and gastropods. It is argured that many of these lines of organisms existed prior to the Cambrian, but did not fossilize very well for the lack of hard parts and only really "appeared" because of the evolutiopn of hard parts such as beaks and pens(gastropods) and exoskeletons(Perhaps one of the reasons that trilobite fossils are so common).

 

In Gould's Wonderful Life he pushes the envelope on what is currently considered the diversity of the Cambrian. While early on the estimates for all these new phyla seem almost inexhaustable. A famous quote, that I cannot recall whom actually said it, concerning the examination of the Burgess Shale was, "Oh, F***! Not another phyla." Many of these "new" phyla have since thenm been re-organized and placed in already existing phyla.

[TeleMad]

One issue of the examples brought up are that they are all vertebrates.

 

Insects aren't vertebrates. And I could have created a similar list for plants.

 

Fishteacher73: Only the precursers of verts were showing up in the fossil record at the begenning of the Cambrian.

 

That was part of the point, though not stated. In another thread Lolic posted Wells' statement that Darwin's tree of life was upside down because the phyla all appeared at basically the same time, at the Cambrain explosion. Even if they did, that doesn't mean Darwin's tree of life is upside down or that it is destroyed. Darwin's tree of life would be upside down if all the representatives of each phylum appeared during the Cambrian explosion and then they died off over time, clipping branches off of the tree leave to fewer and fewer until arriving at just one organism. That's not what happened. Each of the individual phyla appears as a root of its own tree, and over the course of dozens of millions, and hundreds of millions, of years after the beginning of the Cambrian period these trees branched out over and over again. Sure, there was clipping going on, but one can't deny the expansion of each tree. And these individuals trees of life starting at the Cambrian period are themselves branches of a more unified tree that arose farther back in time.

[Fishteacher73]

Insects aren't vertebrates. And I could have created a similar list for plants.

 

True, missed that. Sorry

 

Well as with many trees, a bit of pruning is good. :)

 

There have been a number of other models drawn up to represent the various inter-relations of life and their relative heritage. One model that i favor is one that displays the relative small amount of muli-cellular life ther is as compared to protists, monerans, and archaea.