Lecture 4.1: Vertebrates - Tetrapods and Amniotes

Introduction

Includes frogs, toads, salamanders, and caecilians
Key Evolutionary Traits:
- First land vertebrates
- Evolved lungs for breathing air
- Cutaneous respiration (gas exchange through skin)
Metamorphosis:
- Aquatic larvae stage with gills, transition to terrestrial adults with lungs
Limitations:
- Need moisture; thin skin and eggs dry out easily
- Ectothermic (rely on environment for heat)
Orders:
- Urodela: Salamanders (some lack lungs, use cutaneous respiration)
- Anura: Frogs and toads (distinguished by skin texture and environment)
- Apoda: Caecilians (limbless, burrowers)

Amnion: Key innovation allowing eggs to be laid in drier environments
Amniotes: Includes mammals, reptiles, and birds
- Embryos surrounded by amniotic fluid for cushioning and hydration
Skull Structures:
- Anapsids: No temporal fenestra (all extinct)
- Synapsids: One temporal fenestra (includes mammals)
- Diapsids: Two temporal fenestrae (includes reptiles and birds)

Class Reptilia: Paraphyletic group excluding birds
Characteristics:
- Scaly skin preventing water loss
- Ectothermic; reduced caloric needs but dependent on environmental temperature
- Improved circulatory system compared to amphibians
Orders:
- Dinosaurs: Includes birds (avian dinosaurs)
- Pterosauria: Flying reptiles, not dinosaurs
- Crocodilia: Crocodiles, alligators, and relatives
- Rhynchocephalia: Tuatara (distinct from lizards, unique dentition)
- Squamata: Lizards and snakes (snakes are limbless tetrapods)
- Testudines: Turtles and tortoises, have anapsid morphology

Evolution:
- Evolved from non-avian dinosaurs
- Flight as a key evolutionary driver
Traits Related to Flight:
- Endothermic (warm-blooded, high caloric needs)
- Asymmetrical feathers for flight
- Lightweight, air-filled bones
- Beaks instead of jaws with teeth
- Specialized airflow with air sacs in lungs
Highly diverse, with over 30 orders

The lecture ends with a brief mention of mammals, to be covered in the next lecture.