Lecture 18

Phylogeny and Characteristics of Reptiles

Reptiles: first truly successful land colonizers.
Three major innovations that facilitated land adaptation:
- Amniotic Egg: Faster development of larger offspring, embryo carries its own water.
- Scales/Bony Plates: Protection from desiccation and support for complex respiratory mechanisms.
- Strong, Crushing Jaws: Allowed exploitation of new food resources.

Non-Monophyletic Group: Reptiles do not share a unique, unifying characteristic across all members.
Classification based on four primary orders:
- Order Rhyncocephalia: Tuataras, unique evolutionary history, parietal eye.
- Order Crocodylia: Crocodiles and alligators, complex social behaviors, nests.
- Order Chelonia: Turtles and tortoises, characterized by a protective shell.
- Order Squamata: Snakes and lizards, very diverse.

Structure:
- Yolk Sac: Provides nutrition to the embryo during development.
- Chorion: Outer membrane for gas exchange; fuses with allantois for improved oxygen exchange.
- Allantois: Waste disposal sac and gas exchange.
- Amnion: Fluid-filled sac that surrounds and protects the embryo.
Advantages:
- Enables development in dry environments without requiring a water medium.
- Eliminates larval stage, leading to more efficient development.
Essential for Successful Land Colonization.

Scales:
- Protect against desiccation and injury (consist of keratin).
- Some reptiles shed skin periodically; scales grow throughout life in others.
Jaws:
- Strong, capable of crushing, allows diversification of diets.
- Different muscle arrangements lead to diverse jaw structures (anapsids, synapsids, diapsids).

Ectotherms: Body temperature regulated by external factors.
Endotherms: Maintain body temperature through metabolic processes.
Mechanisms of heat exchange include:
- Conduction: Transfer of heat through direct contact.
- Convection: Heat transfer by movement of air or liquid.
- Radiation: Heat emission as electromagnetic waves.
- Evaporation: Cooling effect via moisture loss.
Behavioral adaptations to maintain optimal temperature levels; include basking in sunlight and seeking shade.

Physical Traits:
- Amniotic eggs, scales, strong jaws.
- Efficient lungs with greater surface area; don’t rely on skin respiration.
Circulatory System: 4-chambered heart, efficient systemic and pulmonary circulation.
Reproductive System: Typically dioecious with internal fertilization; most are oviparous with some exceptions (viviparous).
Nervous System: More complex, better vision; evolved mechanisms for thermoregulation and behavioral adaptations.

Reptilian evolution marked by the development of amniotic eggs and adaptive features.
Diversity within reptiles characterized by their classification into four main orders based on anatomical and ecological traits.
Innovations such as stronger jaws and adapted skin structures have enabled various feeding strategies and environmental adaptations.