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9. The Cambrian Explosion.docx

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Jessica Hawthorn

THE CAMBRIAN EXPLOSION st th Your next exam is November 1 . Today is October 18 , which means that including today, we have four lectures. That is what will be on the exams and the relevant chapters of the book. We are now going to take a joyride through history. We will talk about the major events, the major extinctions, the major geologic events, the major biologic events, all the way up to today. We‟ll start in the Palaeozoic era of what is called the Phanerozoic. We talked about the Cambrian explosion. In the next couple weeks, we‟ll talk about the Palaeozoic. The Palaeozoic runs from 543 million years ago to 245 million years ago and includes a series of periods. Today, we‟ll talk about the Cambrian period which went from 543-490 million years ago, followed by the Ordovician, the Silurian, and the Devonian. We saw this last time (see power-point). The Cambrian is the interval where we have the initial diversification of all the groups of animals that are alive today. Remember, earth is a recycling and dynamic planet. It is continuously cycling its surface. Tectonics is a continuous process. The surface of the earth changes over time. During the Cambrian, you have a large equatorial landmass called Laurentia (what we‟re standing on). It is next to Gondwana, and then you have other landmasses, including Baltica, Siberia, Kazkhstania, China...these are all landmasses. Nothing lives on them in the Cambrian. All the major living organisms at this time are marine organisms. We also saw this last time (see power-point). Canada is sitting on the cretaceous. The red line is where the equator was during the Cambrian. Where is the equator going to be on Earth, at any time? Is it going to be at the poles? The equator is the latitudinal midpoint of the planet, or zero degrees north and zero degrees south. In this perspective (see picture in the power-point), instead of having Canada rotated 90 degrees, the perspective has been shifted to keep it in line. Toronto is in a shallow ocean environment at this time. The Cambrian age rocks that are exposed through the GTA are marine rocks. To the west, in Alberta, you have the sandy bottom. This becomes important. Now, we‟ve got these charts which show geologic events and biologic events. In the Cambrian, you have the radiations of major groups or the predecessors of major groups of living organisms today. These include molluscs (like clams), trilobites (related to sea horses), echinoderms (which are starfish), and the first chordates (the group that includes vertebrates – us). Then, we have a bunch of stuff that is poorly understood. During the Cambrian, you have diversification of modern animal life. When you think about animals, you think of cats, dogs, and so on. When you think about animals, there is a tremendous diversity of stuff that we don‟t think about. The main divisions of animals can be broken down into series and categories. This figure (see power-point) shows the evolutionary relationships of all the major groups of animals. It‟s a branching diagram, and each point where two branches meet is the point where organisms have a common ancestor. Among animals, we have the most primitive animals (jelly fishes), and the more derived animals from them is a group called bilateria. This group includes the majority of organisms you would recognize. The homologous character of all bilateral animals is that we‟re symmetrical down the midline. We have an axis of symmetry. Spiders, worms, bugs, giraffes, starfish, and many animals which we recognize are bilaterally symmetrical. This chart also mentions protostomes and deuderostomes, which are all bilaterally symmetrical. That is a character that unites us and spiders to the exclusion of jellyfish. We‟re more closely related to a spider than we are to a jellyfish. Within the bilateral animals, there are two major divisions: the protostomes and the deuderostomes. These are differentiated by character. This one character happens in early development. After animals have sexual reproduction, after fertilizing the egg, what happens during the early development of the fertilized embryo is you have rapid cell division. Then, the embryo becomes a hollow ball of cells. It goes from being a hollow ball of cells to us. The way it does this is that there is an infolding in by the ball of cells which moves in, contacts the far wall, and breaks the far wall, and develops from being a hollow ball of cells to a hollow donut. It becomes a tube. You are a tube. I am a tube. Our digestive tract is basically a tube. We have an opening at one end and another one at the other. That infolding moving in is called the blastopore. It is creating a hollow tube. The inner lining of the tube is your gut cavity. You have a mouth at one end, and an anus at the other. In the group called protostomes (insects, molluscs), when the blastopore forms and forms the tube, the point where it forms becomes the mouth (first hole). That opening forms the mouth. In deuderostomes (us, echinoderms, starfishes, and so on), when our ball of cells develops, folds in, and becomes a tube, it forms the anus. What that means is that we‟re more closely related to a starfish than either of is to a spider. In other words, we share a common ancestor to the exclusion of spiders. We are deuderostomes. This big differentiation between deuderostomes and protosomes occurs in the Cambrian. Where‟s the record for this? Canada. It‟s in the Canadian Rockies. The „Burgess Shale‟ is a middle Cambrian shallow marine rock. It is world famous. This is what the burgess shale looked like 520 million years ago...a shallow marine environment in a warm ocean near the equator. It is filled with animals many of whom make no sense. It has diversification. What happened was that this guy named Walcott started splitting open shale (which is like slate) from the Burgess Shale. Now, shale is a mudstone, a sedimentary rock formed in shallow marine environments. He starts finding evidence of animals 520 million years old. This is what the depositional environment of the burgess shale looked like in the middle of the Cambrian (see power-point). You had big reefs of algae. At the base of these reefs, you have shallow marine environments represented by those rocks. You have animals living at the base of the reefs. Mud comes off the top of the reefs, buries the animals, and preserves
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