Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 34.5 Amniotes Are Tetrapods That Have a Terrestrially Adapted Egg
Amniotes, which include reptiles (birds included) and mammals, are tetrapods that evolved key adaptations for life on land—most notably the amniotic egg. This innovation allowed vertebrates to reproduce away from water and colonize a variety of dry terrestrial habitats.
The Amniotic Egg
- Named for the amnion, one of four extraembryonic membranes:
- Amnion: cushions embryo in fluid
- Chorion: gas exchange
- Yolk sac: nutrient storage
- Allantois: waste disposal
- Shell (in most reptiles and some mammals): prevents desiccation, allows gas exchange
- Enabled reproduction without needing water—embryo develops in a private "pond"
Other Key Adaptations in Amniotes
- Rib cage ventilation: more efficient than throat-based breathing
- Less permeable skin: helps prevent water loss
- Internal fertilization: occurs before shell forms
- Loss of shell in most mammals: embryo develops inside uterus within the amnion
Reptiles
- Includes turtles, tuataras, lizards, snakes, crocodilians, and birds
- ~20,800 species: largest groups are squamates (lizards/snakes) and birds
General traits:
- Keratin scales: protect and retain moisture
- Internal fertilization and shelled eggs laid on land
- Most are ectothermic (cold-blooded)
- Birds are endothermic—maintain body temperature metabolically
Major Reptile Groups
1. Turtles
- No skull holes (though fossil ancestors had them)
- Shell fused to ribs and vertebrae
- Marine turtles: flipper-like limbs, large bodies
2. Lepidosaurs
- Includes tuataras and squamates (lizards and snakes)
- Tuataras now found only on islands near New Zealand
- Snakes: evolved from legged lizards; adaptations include venom, flexible jaws, heat sensors
3. Archosaurs
- Includes crocodilians and birds
- Crocodilians: adapted for aquatic life; nostrils on top of head
Birds: Living Dinosaurs
- Descended from theropod dinosaurs; Archaeopteryx is key transitional fossil
- Flight adaptations:
- Wings with β-keratin feathers
- Light skeletons: hollow bones, no bladder, reduced organs
- Efficient lungs and circulation
- Endothermic; excellent vision
- Diverse beaks and feet for different ecological niches
- Complex behaviors and parental care are common
- Ratites (ostrich, kiwi): flightless but strong legs
- Penguins: wings adapted for swimming
In a Nutshell
Amniotes are tetrapods that broke free from water-dependent reproduction through the evolution of the amniotic egg. Reptiles and birds diversified across land, sea, and air, with birds representing a lineage of feathered, flying dinosaurs. Their success lies in efficient respiration, diverse ecological roles, and reproductive strategies adapted to life on dry land.