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11. Rise of Fish + Evolution of Amniotes.docx

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Jessica Hawthorn

RISE OF FISH AND THE EVOLUTION OF AMNIOTES We‟ll finish off the Palaeozoic. We‟ll talk about the Devonian, which was from 417-354 million years ago, and the Carboniferous, which lasted from 354-290 million years ago. These terms, Cambrian, Ordovician, Silurian are used by geologists internationally as a tool to communicate with each other. These time periods are defined by actual historical events, primarily extinctions. What happens is that not everybody always agrees as to what a time period should be called. The carboniferous is a stretch of time which, in North America, is divided into two subdivisions, the Mississippian (354-323 million years ago), and the Pennsylvanian (323-290 million years ago). They don‟t have this division in Europe and Asia. When we last talked, we‟re talking about the Devonian. In the Devonian, we still have the same arrangement of continents. We have an equatorial Laurasia, and a southern Gondwana. Canada at this time is a large volcanic island. In fact, most of Canada is under water. We also talked about reefs last time. The Devonian is an interval of massive reef build up. You‟re looking at a cross section of reef (in the picture on the power-point). On one side is what is called the talus (the sloped, angled beds). There are also horizontal beds. So, this is an interval of massive reef build up. It is also the interval when we have the initial radiation of animals and plants onto land. So, what are the things that are different about living on land as opposed to living in water? For one, the actual pressure of the medium (i.e., either water or land) that you‟re moving through is less if you‟re under water. Gaseous atmosphere is simply less dense than the aqueous hydrosphere. The specific gravity is greater on land. The issues/problems evolutionarily that living organisms faced in evolving from living in water to living on land were (1) desiccation (loss of water because of dehydration which results from living on land), (2) gravity (going from a more dense medium to less dense medium), (3) the acquisition of oxygen or carbon dioxide through respiration or photosynthesis (i.e., these two processes are different in water than they are on land), and (4) reproduction (i.e., the way that sexual reproduction occurs in water is different than how it occurs on land). We‟ll see that plants dealt with these problems in a certain way, vertebrates dealt with them in a certain way, arthropods deal with them in a certain way. These organisms all had to overcome these four issues. Let‟s see how plants did it. First off, they dealt with this new ecosystem by developing specialized tissues that collect, store, and transport water throughout the plant: roots. Roots also anchor the plant. How did plants deal with increased specific gravity? They developed support structural tissues, woody tissues, and, specifically, bark (in larger trees). How did plants carry out the central function of photosynthesis after moving onto land? They were forced to concentrate the tissues within which photosynthesis occurs into specific regions. Chlorophyll is concentrated in specific tissues and that is where energy is generated. How did plants reproduce? They evolved spores and seeds. These were impermeable. Now, what does that mean? Water cannot evaporate across the surface of a seed, nor can it flood into the seed if the seed falls into water. Essentially, plants developed impermeable spores and seeds which don‟t lose water. That is the important thing. We have the evolution of vascular plants. Basically, we‟ve got plants with internal tissue systems, vascular systems that allow the transport of water throughout the plant. Because of this, the transportation of water from the roots at one end up to the leaves at the other end – against the pull of gravity – is made possible. If you think about a tree, which you should, most of the moisture is drawn up through the roots and transported against gravity up to where the leaves are. The leaves are the area where energy is generated through photosynthesis. In the Devonian, what we get are small plants, but they are vascular. They have internal tissues. They‟re spore- bearing plants. They don‟t have well developed seeds that later plants develop. They include such things as mosses and ferns. We do not yet have in the Devonian the plants that we see in the world today. The plants that we see now have only recently evolved. In fact, flowering plants are younger than dinosaurs. Grass also evolved very recently. These plants in the Devonian are simple. They include a plant called Sphenophyta, which is considered to be an early representative of most plants. These plants also include horsetails. Now, these early plants are growing right next to water or even directly out of the water. They do not have leaves at this point. This is what the Devonian would‟ve looked like (picture in the power-point). It was a barren surface and plant colonies were concentrated around the edges of landmasses. By the end of the Devonian, there are actually some fairly large plants. These are called Lycopsids. They are about 30 feet tall, but they‟re not trees. Rather, they‟re mosses. They‟ve independently evolved large size, root systems, and radiating branches with photosynthetic tissues. They‟re trees but not the trees that we know today. They have evolved roots and crowns. They‟re basically giant mosses. What else radiates in the Devonian? Insects radiate out onto land. This (picture in the power- point) is a fossil dragon fly. The dragon fly literally had the wingspan of a six feet tall man. The thing about insects is that their respiration is very different from the way we do it. The way they breathe is that they have pores running along the side of their body, and air moves out of these spiracles across respiratory tissues. That‟s how oxygen is pulled in and carbon dioxide is released. How do we breathe? We are actively expanding our rib cage and then contracting muscles and pushing our rib cage back in. We‟re actively pulling air in and pushing air out. Insects only do this passively. They can thus only grow to a certain size. After a certain size, they can only passively draw in so much oxygen. Then, there are vertebrates – us. How do we deal with the four problems mentioned earlier? We have to face desiccation (i.e., loss of water due to dehydration when living on land). Hence, we reduce the permeability of our skin. How many people took a shower this morning? When you take a bath or shower, water falls on the skin and you dry it off. Water does not transport into your skin and into your tissue. Your skin is impermeable to water. What about sweating though? What happens with sweating is that we have specialized pores that concentrate and release water. Your skin is actually impermeable. However, not all of it is impermeable. The membranes in your nose are permeable. You do not lose water across your skin. If your skin was permeable, and you stepped into the shower, you would fill up like a balloon. So, we (vertebrates) have impermeable skin which protects us from desiccation. Now, not all vertebrates do this. There is only a derived group of vertebrates that have truly impermeable skin. How many people have seen a frog or toad in the wild? Frogs are amphibians. Frogs and toads have permeable skin and are tied to water environments. They have specializations. They have ways of getting around this dependence on water environments. Some can be found living in desserts, but that‟s not the point. Amphibians have permeable skin and retain that condition. Organisms with truly permeable skin are reptiles and mammals. Amphibians have reduced permeability relative to reptiles and mammals. The second issue for vertebrates is support against gravity. You have to brace your body. Have you ever gone swimming? Because water is dense, you‟re buoyant. When you move in the water, you‟re not pushing off against a solid (i.e., water is a liquid). What you‟re doing is that you‟re propelling against a fluid. You‟re not supporting your weight fully as the water is dense enough to keep you afloat. On land, we‟re supporting ourselves all the time. In our case (i.e., Homo sapiens), we‟re bipeds (i.e., we have two feet). We‟ve descended from boreal mammals. We‟re actually quite terrible at handling ourselves on land. With birds, they have a body that sits and is perfectly balanced on their legs. Their weight is distributed right on top of their hips. We, on the other hand, have a vertebral column that sits straight up. This column is hooked onto our legs because we move around on our legs. We have to be able to brace our legs against our body. The skeleton is there for support. This is how we deal with incre
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