BIOL 112 Lecture 35
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Amphibians
3 Extant orders
- Urodela (tailed; salamanders)
- Anura (tailless; frogs)
- Apoda (legless; caecilians)
Dependent on water for reproduction (external fertilization) and gas exchange
- They have lungs, but rely on their skin (pulmocutaneous)
3-chambered heart, double-circulation
- advantage over single-circulation since blood pressure drops after leaving capillaries
- Returning to heart restores high blood pressure
Dual lives
Fish-like aquatic tadpole → complete metamorphosis → frog with limbs and no gills
Amniotes
Evolution to be less dependent on water (similar to how gymnosperms evolved)
- Reptiles (including birds)
- Mammals
Gas exchange exclusively in lungs
Most reptiles have 3 chambered heart
- birds, alligators, and mammals have 4-chambered heart
Amniotic Egg
Derived characteristic for all amniotes
Reproduction is no longer water-dependent.
- Migration to internal fertilization.
- Embryo develops inside amnion membrane (cushion)
- Yolk for nutrient supply (replaced by placenta in most mammals)
- Shell or uterus for protection
Reptiles
- Lizards - most abundant and diverse
- Snakes - evolved from legged ancestor (vestigial pelvic bone)
- Tuatara - living dinosaur; global distribution during pre-cretaceous, but now limited
Theropods
Evolved into Avian Reptiles
- 3-toed
- carnivorous
- endothermic? (compared to most exothermic reptiles)
Birds
Probably evolved from theropods: evidence of intermediate fossils with bird and reptilian characteristics (e.g. Archaeopteryx):
- feathers (B)
- beak (B)
- wings (B)
- teeth (R)
- vertebrae in tail (R)
- claws on wings (== forelimb)
Adapted features to aid flight:
- fewer bones
- honey-comb internal bone structure (hollow)
- Females have one ovary
- No teeth, use beak and gizzard instead
- feathers
- made of same stuff as scales
- probably an exaptation of insulation and/or (sexuall dimorphic) courtship traits
- Keel of sternum for pectoral muscles to attach