29.4 Reptiles

Amniote Evolution and Reptile Characteristics

Amniotes revolutionized land colonization with their specialized egg, enabling embryos to develop outside water. This adaptation, along with waterproof skin and efficient respiratory systems, allowed reptiles to thrive in diverse terrestrial habitats. Reptiles also evolved diverse skull structures and unique characteristics like ectothermy and scaly skin that helped them adapt to environments ranging from deserts to forests.

Key Features of Amniotes

The amniotic egg is what made full-time life on land possible. It contains four extraembryonic membranes, each with a specific job:

• Amnion surrounds the embryo in a fluid-filled cavity, cushioning and protecting it
• Chorion lies just inside the shell and facilitates gas exchange between the embryo and the outside environment
• Allantois stores metabolic waste and also helps with gas exchange
• Yolk sac provides nutrition for the developing embryo (think of the yolk in a chicken egg)

Beyond the egg, amniotes share other traits suited for terrestrial life:

• Waterproof skin with scales or scutes prevents water loss through evaporation, which is critical in dry environments
• More efficient respiratory and circulatory systems improve oxygen uptake and delivery to tissues
• These adaptations together allowed reptiles to colonize habitats that amphibians couldn't, including deserts, grasslands, and dense forests

Skull Structures in Reptile Groups

One of the main ways biologists classify amniotes is by the number of temporal fenestrae (openings in the skull behind the eye socket). These openings provide attachment points for jaw muscles, so their arrangement reflects differences in jaw mechanics and feeding.

• Anapsids have no temporal fenestrae. Turtles are the classic living example, though their classification here is debated.
• Synapsids have a single temporal fenestra on each side. This group includes extinct "mammal-like reptiles" such as Dimetrodon and Cynognathus, and ultimately gave rise to modern mammals.
• Diapsids have two temporal fenestrae on each side. This is the largest group and includes most modern reptiles (lizards, snakes, crocodilians) as well as birds and the extinct dinosaurs.

Defining Characteristics of Reptiles

• Ectothermic ("cold-blooded"): reptiles rely on external heat sources to regulate body temperature rather than generating their own metabolic heat
• Scaly, dry skin made of keratin prevents water loss and provides physical protection
• Three-chambered heart with two atria and one partially divided ventricle. This allows some mixing of oxygenated and deoxygenated blood. Crocodilians are the exception: they have a fully four-chambered heart, similar to mammals and birds.
• Metanephric kidneys that excrete nitrogenous waste primarily as uric acid instead of urea. Uric acid is semi-solid (the white, chalky paste in bird and reptile droppings), which conserves water.
• Amniotic eggs with leathery or hard calcified shells, typically laid on land (buried in sand, soil, or leaf litter)
• Cloaca: a single opening that serves the digestive, urinary, and reproductive tracts
• Many reptiles have heterodont dentition (different tooth types), though this varies widely across groups

Thermoregulation and Reptile Diversity

Because reptiles are ectotherms, thermoregulation is central to their daily behavior. They can't shiver or sweat the way mammals do, so they use other strategies:

• Behavioral: basking in sunlight to warm up, retreating to shade or burrows to cool down, orienting the body to maximize or minimize sun exposure
• Physiological: some species can alter blood flow to the skin or change skin color to absorb more or less heat

Reptilia includes several major living groups:

• Squamata (lizards and snakes) is by far the largest and most diverse order of modern reptiles
• Crocodilia (crocodiles, alligators, gharials)
• Testudines (turtles and tortoises)
• Rhynchocephalia (tuataras, with only one surviving species, Sphenodon punctatus, found in New Zealand)

Reptile Evolution and Diversity

Evolutionary Milestones of Reptiles

1. Carboniferous Period (359–299 mya)

   • The earliest known amniotes appeared, such as Hylonomus and Paleothyris. These were small, lizard-like animals that represent the starting point for all later reptiles, birds, and mammals.

2. Permian Period (299–252 mya)

   • Synapsids diversified and became the dominant land animals for a time. Early diapsids also appeared, including forms like Protorosaurus.

3. Triassic Period (252–201 mya)

   • After the massive Permian-Triassic extinction wiped out roughly 90% of species, reptiles radiated into the empty niches. Major lineages emerged:
     • Archosaurs (leading to crocodilians, pterosaurs, and dinosaurs)
     • Lepidosaurs (leading to lizards, snakes, and tuataras)
     • Testudines (turtles)

4. Jurassic and Cretaceous Periods (201–66 mya)

   • Dinosaurs dominated terrestrial ecosystems (Tyrannosaurus rex, Stegosaurus). Marine reptiles like ichthyosaurs and plesiosaurs filled ocean niches. Lizards and snakes continued to diversify.

5. Cretaceous-Paleogene Extinction (66 mya)

   • A mass extinction event eliminated all non-avian dinosaurs along with many other reptilian lineages. Avian dinosaurs (birds) survived.

6. Cenozoic Era (66 mya to present)

   • With many ecological niches now open, surviving reptile lineages underwent adaptive radiation. Modern diversity includes Komodo dragons, chameleons, sea turtles, and thousands of lizard and snake species.