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Lecture 18

BIO153 Lecture 18.pdf

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Christoph Richter

2009 BIO153: Lecture 18 The Tetrapods Mar 25 th, 2009 The tetrapods are the vertebrate lineage that successfully colonized the terrestrial environment. The transition from an aquatic to a terrestrial lifestyle involves: 1. Changes in skeletal system 2. Changes in gas exchange system 3. Changes in circulatory system 4. Changes in reproduction 1. Changes in skeletal system: ▯ ribs to protect internal organs ▯ interlocking vertebrae (to prevent rotation of the body during movement) ▯ pectoral & pelvic girdles (for articulation of the limbs; separation of the axial and appendicular skeleton) ▯ mobile neck (for improved feeding, use of sense organs) ▯ limbs ▯ changes in the skull (discussed further in the amniotes) 2. Changes in gas exchange system: there are fundamental differences between breathing O 2 dissolved in water vs. breathing O 2dissolved in air: ▯ air: O = 210 ml/l vs. water: O = 3-9 ml/l. 2 2 The amount of oxygen in water is temperature dependent – amount of O 2 decreases as temp increases – while the amount of O 2 in air is constant with temperature, but varies with altitude (atmospheric pressure) ▯ water is denser, more viscous & diffusion slower ▯ harder to extract O2from H O2than from air Breathing water: ▯ gill filaments are oriented to the direction of water flow ▯ unidirectional flow of water over respiratory surface ▯ highly efficient: because water is dense and viscous, flow-through system is better than a tidal system 1 The transition to air breathing: a respiratory exchange surface must be moist and highly vascularized. a. vascularized pharyngeal sac ▯ found in lungfish ▯ amphibian lung b. skin respiration ▯ salamanders (skin, oral mucosa) Air breathing in tetrapods: “tidal” breathing: because air is easier to move than water, tidal (i.e. in and out) breathing is possible. 2 types of tidal breathing: a. positive pressure: “swallowing air” (amphibians) ▯ inspired air is gulped, creating increased pressure in the internal cavity (like filling a balloon) b. negative pressure: ▯ ribs & diaphragm create vacuum that draws air in passively ▯ reptiles, birds & mammals 3. Changes in the circulatory system: ▯ in fish, the 2-chambered heart receives only deoxygenated blood from the body ▯ blood is pumped to the gills to pick up oxygen before continuing to the rest of the body ▯ single circuit In tetrapods: ▯ double circuit: pulmonary and systemic ▯ different blood pressures in the 2 systems; separate controls ▯ pulmonary circuit at low pressure (to protect delicate lung capillaries); systemic circuit at high pressure (to quickly deliver oxygenated blood to working tissues) ▯ 3 or 4 chambered heart (3 chambered in amphibians; 4 chambered in reptiles, birds and mammals) 2 4. Changes in reproduction: ▯ external fertilization (common in fish) to internal fertilization: less sperm is needed; ensures paternity; protects gametes and zygotes ▯ protected egg: shell; membranes; internal development ▯ parental care is common in many tetrapod groups (also common in many fish!) Major lineages in the tetrapods: 1. Amphibians ▯ most have smooth, moist skin; often have chromatophores, poison glands ▯ some (e.g. toads) have dry skin ▯ respiration through lungs, skin, gills ▯ 3-chambered heart; double circuit ▯ metamorphosis (egg and larval stages in water (or moist environments; adult stage may be completely terrestrial) Amphibians, as their name implies (amphi = both; bios = life) are incompletely transitioned to the terrestrial environment; their reproduction is still tied to water. ▯ prone to desiccation (most frogs can survive after losing 50% of their body water – this is an adaptation that can permit some frogs to survive complete freezing of body tissues) ▯ aquatic reproduction (metamorphosis) Some adaptations that allow amphibians to reproduce outside of standing water: ▯ eggs may be laid in moist terrestrial environments (e.g. leaf litter) ▯ eggs and/or larvae may be retained by parent: in skin, in stomach… 2. Amniotes: ▯ Carboniferous period (360-285 mya) dominated by amphibians, but a new form appearing… ▯ cotylosaurs: early reptiles ▯ Amniota: all vertebrates with extra- embryonic membranes surrounding the egg ▯ reptiles, birds & mammals 3 Amniote characteristics: ▯ lungs: complex; used for CO 2umping ▯ no more positive pressure respiration; head can be smal
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