Chapter 7: Reptiles: Scales, Ectothermy, and Evolution

Reptiles conquered land far more completely than amphibians by evolving two key innovations: waterproof, keratinized scales that prevent desiccation, and the amniotic egg — a self-contained developmental environment. These adaptations freed them from dependence on standing water for reproduction, opening vast terrestrial and arid habitats. Modern reptiles are the remnants of a group that once included the largest land animals in Earth’s history.

Major Reptile Orders

Order	Common Name	Key Features	Examples
Squamata	Lizards & snakes	Overlapping scales, Jacobson’s organ	Varanus, Python, Iguana
Chelonia	Turtles & tortoises	Bony shell (carapace + plastron)	Chelonia mydas, Testudo
Crocodilia	Crocodiles, alligators	4-chambered heart, archosaur lineage	Crocodylus, Alligator
Rhynchocephalia	Tuatara	Living fossil, parietal eye	Sphenodon punctatus

Ectothermy and Thermoregulation

Unlike endothermic (“warm-blooded”) birds and mammals, reptiles are ectotherms — their body temperature is determined by the external environment. This is not a “primitive” limitation but a successful strategy:

• Lower energetic cost: reptiles need ~10x less food than equivalent-sized mammals
• Behavioral thermoregulation: basking in sun, seeking shade, retreating underground
• Optimal activity range: most reptiles have a preferred body temperature (PBT) of 25–38°C

The Leatherback sea turtle (Dermochelys coriacea) is unusual in maintaining elevated body temperatures through gigantothermy (large body mass retains heat).

Snake Feeding Adaptations

Snakes (Serpentes, within Squamata) are limbless predators with remarkable feeding adaptations:

• Kinetic skull: loosely connected jaw bones allow swallowing prey wider than the snake’s head
• Jacobson’s organ: vomeronasal pit in the roof of mouth detects chemical signals from tongue flicking
• Heat pits: infrared-sensing organs in pit vipers (Crotalinae) and pythons for locating endothermic prey in darkness
• Constriction (pythons, boas): coils prevent breathing; prey dies from circulatory arrest
• Venom (elapids, vipers): neurotoxic or hemotoxic proteins injected via hollow fangs

Crocodilian Parental Care

Crocodilians (Crocodilia) are the most bird-like of living reptiles — they share a common ancestor (Archosauria) with dinosaurs and birds. Remarkably:

• Females guard nests and excavate eggs when they hear hatching calls
• Mothers carry hatchlings in their mouths to water
• Young are protected for up to 2 years — exceptional parental investment for reptiles
• Possess a 4-chambered heart (shared with birds and mammals, independently evolved)

Dinosaurs and the Bird Connection

The Avian hypothesis — that birds are living theropod dinosaurs — is now universally accepted:

• Archaeopteryx (150 Ma) had both feathers and reptilian features (teeth, clawed wings)
• Coelurosaurian theropods (Velociraptor, Tyrannosaurus) share feather impressions in fossils
• Modern birds retain the wishbone (fused clavicles), scales on feet, and air sac respiratory system from dinosaur ancestors

Key Checklist

• I can list the four orders of living reptiles and their key distinguishing features
• I can explain ectothermy and describe behavioral thermoregulation strategies in reptiles
• I can describe the evolutionary connection between dinosaurs and modern birds, citing fossil evidence