Chapter 18: Amniote Origins and Nonavian Reptiles

Derived characters * Amniotic egg * Rib ventilation * Desiccation-resistant skin * Head, shoulder, and ankle skeletal characteristics

\ * Allowed amniotes to be more energetic * Allowed amniotes to exploit dry, terrestrial habitats

^^Amniotic Egg^^

Eggs with four extraembryonic membranes * Amnion * Encloses embryo in fluid * Cushions and provides an aqueous medium for growth * Allantois * Functions in respiration and excretion * Important role in placental development in mammals * Chorion * Outer layer of double membrane that surrounds embryo * Contributes to placenta in mammals * Along with allantois, forms efficient respiratory organ * Yolk Sac * Membranous sac * Filled with nutrients that support growth of embryo * Shell * May be mineralized * Tough * Leathery and flexible in some species * Absent in most mammals * Mechanical support * Semipermeable barrier * Allows gas exchange * Limits water loss
All amniotes: * Lack gilled larvae * Internal fertilization

^^Thicker and More Waterproof Skin^^

Amphibians: * Required for cutaneous respiration * Vulnerable to dehydration and physical damage
Most amniotes: * Result: changes in skin morphology * In general * Much thicker * More keratinized * Keratin = tough protein * Also found in scales, hair, feathers, claws * Less permeable to water

^^Rib Ventilation of the Lungs^^

* reflect increased metabolic demands and reduced ability of skin to serve as gas-exchange organ

Amniote ventilation * Air drawn into lungs by expanding thoracic cavity * Some aquatic species’ adaptations * Aquatic turtles supplement pulmonary respiration with pharyngeal or cloacal respiration * Sea snakes: most gas exchange is cutaneous

^^Stronger Jaws^^

Bone and muscles of early tetrapod jaws became adapted to seize prey
Tongue is also muscular and mobile * Moves food in mouth for mastication and swallowing

^^Water-Conserving Nitrogen Excretion^^

Most amphibians excrete metabolic waste as ammonia or urea * Ammonia: toxic, even at low concentrations * Must be removed in dilute solution → water required * Not adaptive for vertebrates in dry, terrestrial habitats
Mammals * Urea * Less toxic than ammonia * Concentrated in kidneys * Reduces water loss through excretion
Birds and Nonavian reptiles * Concentrated uric acid * Water reabsorbed to conserve it

^^Expanded Brain and Sensory Organs^^

Turtles * Unique among tetrapods * Shell * Most distinctive feature * Dorsal carapace * Ventral plastron * Nearly all species must breathe inside a rigid box

Reproduction * Oviparity * Eggs buried in nests in the ground * Environmental sex determination * Widespread among turtles * Most species: No parental care * Exception: South American River Turtle * Females stay near nests * Call hatchlings to them * Migrate together to flooded forests
Shells provide significant protection * Adults of most species have shells that few predators can crush * Some have flexible hinges that allow front or rear of shell to close
Shells provide significant protection, but they aren’t impenetrable * Hatchlings * “bite-sized” * Shells not rigid enough to resist crushing * Humans * Annual automobile mortality * 1 – 25% * Especially high in eastern United States and Great Lakes region * Dense networks of roads criss-cross turtle habitats * Serious issue * Estimates of long-term population viability * 2 – 3% added mortality is more than many populations can withstand * Habitat destruction, overexploitation for food, and introduced species that prey on eggs are also serious issues for turtles

Shared characteristics * Skin with overlapping, keratinous scales * Successive generations of epidermal cells are shed at intervals * Leglessness * Reduction or complete loss of limbs widespread among some groups of lizards * All snakes are limbless * Presence of hemipenes in males * Forked * Only one used during copulation * Individuals are either right- or left-penised * Preferentially employ one or the other

Determinate growth * Allows adults of many species to remain small enough to eat insects * ~80% of extant species weigh less than 20g. as adults * Less than a mouse * Can readily capture insects * vs. large insect-eating vertebrates * Require morphological specializations to capture tiny prey
Body Forms * Smallest: 3 cm long * some geckos and chameleons * Largest: 3 meters long * Komodo Monitor (Komodo Dragon) * Enormous variety of body forms and prey preferences

Limbless * Usually lack both pectoral and pelvic girdles * Vestigial pelvic girdles in pythons and boas * Do not “unhinge” their jaws * Do have extremely kinetic skulls * Allow extensive movement of jaws * Paired * Each side of head acts independently
Most employ chemical senses to hunt prey * Jacobson’s Organs * Located in roof of mouth * Lined with richly innervated olfactory epithelium * Forked Tongue * Collects scent molecules * Drawn past Jacobson’s organs * Info transmitted to brain
Pythons, boas, and pit vipers also have pit organs * Located between nostrils and eyes * Heat-sensitive * Can distinguish temp. differences smaller than 0.003º C * Used to track warm prey and aim strikes * Don’t have to rely on vision * Effective in total darkness
Other snakes kill prey by injecting them with venom * Less than 20% of species are venemous * Vipers * Large, moveable, tubular fangs at front of mouth * Lie in sheath when mouth is closed * Venom injected through canals in fangs * Releases prey after bite * Follows it and waits for it to die * Best-known North American species are pit vipers * Possess pit organs * Rattlesnakes, cottonmouths, copperheads * Family Elapidae * Short, permanently erect fangs in front of mouth * Cobras, mambas, coral snakes, and kraits

Currently * 32 natural populations * Small islands off the coast of New Zealand * 9 translocated populations on those small islands * 5 translocated populations in sanctuaries on the North and South Islands of New Zealand
~60 cm long (~23.6”)
Nocturnal
2 rows of teeth in upper jaw * Lower jaw teeth fit between them
Prey * Mostly invertebrates * Opportunists * Occasionally will eat frogs, lizards, or sea birds

Living fossils * Mostly unchanged for nearly 200 million years * Face an uncertain future, thanks to humans
Adaptations * Well-reinforced skull * Massive jaw musculature * Wide gape * Rapid, powerful closure * Teeth set in sockets * Complete secondary palate * Only found in mammals * Can breathe with a full mouth
Reproduction * Oviparity * 20-90 eggs in clutch * Buried in dead vegetation or sand * Sex determined by nest temperature * Low temp: female * High temp: male * Extensive parental care by the mother * Guards nest * Responds to hatchlings calls * Opens nest to help them escape * Guards young for 2+ years after hatching