The Origin and Early Evolution of Amniotes

Amniotes diverged from anthracosaurs in the Carboniferous period (late Paleozoic) The Amniotic Egg (shelled egg) - Enclosing the Pond

Amnion: membrane enclosing the embryo (cushioned in amniotic fluid) Allantois: sac serves as respiratory surface and waste container; highly vascularized Yolk Sac: contains nutrients for embryo; modified into part of placenta in live-bearers Chorion: outer membrane; highly vascularized (gas exchange occurs) Shell: porous, parchment-like or leathery in reptiles; calcium absorbed by growing embryo

Skull Morphology and Amniote Radiation Anapsid: no temporal opening behind orbit (extinct groups, incl. mesosaurs) Diapsid: two pairs of temporal openings, separated by arch (incl. extant reptiles) Synapsid: single pair low on cheeks, bordered by arch (extinct taxa, led to mammals)

Other Derived Characteristics of Amniotes Jaws: bones and muscles allow crushing and gripping forces (vs. suction of fishes) Rib ventilation: draw air into lungs (aspiration); limbs freed rib muscles for new function Thicker, waterproofed skin: more keratin (beta keratin in reptiles) scales and scutes Circulation: atria completely partitioned; two ventricles incompletely separated (excl.

crocodiles, w/ complete separation); functionally separated pulmonary, systemic flows Excretion: excretion of ammonia (amphibians) involves loss of water; bladder in reptiles

absorbs water, nitrogenous waste excreted as uric acid Nervous system: enlarged cerebrum; related to sensory integration and locomotion

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Class Reptilia – Diversity and Taxonomy

Class Reptilia: ~8,000 extant species; “Age of Reptiles” in Mesozoic era (165 my span); most ectothermic; many extinct forms (ex. dinosaurs, ichthyosaurs) Order Testudines (Chelonia) – Turtles

Little change in morphology from appearance in Triassic Shell with carapace (dorsal) and plastron (ventral); outer layer of keratin and inner layer of

bone (fused ribs and vertebrae); limbs and girdles positioned inside ribs, can be withdrawn (and head) for defense

Respiration hindered by shell; lungs ventilated by limb movement and “false diaphragm” Small brain, poor hearing, but acute olfaction and vision Internal fertilization; oviparous, with gender dependent on nest temperature in some (high

temperatures produce females) Diversity includes snapping turtles, giant tortoise, desert tortoise, sea turtles (incl.

leatherback) Order Squamata – Lizards and Snakes (95% of extant reptiles); with kinetic skulls

(movable joints) that allow prey manipulation Suborder Sauria (lizards): include geckos, iguanids, skinks (reduced limbs), monitors (incl.

Komodo dragon), chameleons, and beaded lizards (venomous); ectothermic; eyelids

Suborder Serpentes (snakes): limbless; two halves of lower jaw kinetic (allows ingestion of large prey); opening of trachea anterior; rely mainly on chemoreception; most oviparous

Order Sphenodonta – Tuataras Two species of Sphenodon in New Zealand (protected); “living fossils”; parietal (3rd) eye

Order Crocodilia – Crocodiles and Alligators Elongated skull with massive jaw muscles; secondary palate allows breathing with full

mouth; oviparous, with high nest temperatures producing males (vs. females in turtles)

Traits shared with birds: true four-chambered heart, thecodont teeth (in sockets) Diversity includes American alligator, saltwater crocodile, Nile crocodile, caimans

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Snakes – Locomotion, Prey Capture, and Special Senses

Locomotion Lateral undulation: lateral force exerted against surface irregularities Concertina movement: braces S-shaped loops against sides of channel Rectilinear movement: sections of body rest on ground with intervening

sections pulled forward by muscles Side-winding: throws body forward in loops with body at 60° angle to

direction of travel; little contact with ground; used by sidewinder Prey Capture – most snakes capture prey with mouth and swallow

alive Venom: ~20% of species; vipers with movable, tubular fangs in front of

mouth; cobra family (incl. mambas, coral snakes, and kraits) with short, erect fangs in front of mouth; most sea snakes rear-fanged; Australia: 4/1 ratio venom/non

Constriction: capture prey larger than mouth; most ambush (ex. pythons)Special Senses

Jacobson’s (vomeronasal) organs: pair of pit-like organs in roof of the mouth, lined with chemoreceptors; forked tongue picks up scent molecules of prey (can impart direction to prey)

Pit organs: found only in pit vipers (Subfamily Crotalinae, incl. cottonmouths, copperheads, and rattlesnakes); heat-sensitive pit organs between nostrils and eyes enclose free nerve endings; sense differences of ~0.003 °C; track prey and aim strikes

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The Evolution and Extinction of the Dinosaurs and Other Mesozoic Reptiles

Diversity of Extinct Reptiles: evidence of parental care, herding, and endothermy for certain taxa Order Saurischia (“lizard-hipped”)

Sauropods: herbivorous, massive, quadrupedal (ex. Brachiosaurus) Theropods: carnivorous, bipedal (ex.Tyrannosaurus, Velociraptor);

certain taxa gave rise to birds Order Ornithischia (“bird-hipped”): most armored or with horns; all

herbivorous (ex. Stegasaurus, Triceratops, hadrasaurs) Order Pterosauria: winged, flying archosaurs (ex. Pteranodon) Therapsids and Pelycosaurs: early mammal-like amniotes (ex. Dimetrodon); relatives of early mammals

Ichthyosaurs: dolphin-like marine reptiles (incl. Shonisaurus) Plesiosaurs: large, long-necked marine reptiles

Extinction of the Dinosaurs Evidence suggests that many dinosaurs were already in decline

before meteor impact at end of Cretaceous (K/T boundary); impact crater located off Yucatan Peninsula; evidence for impact included shocked quartz, iridium layer

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