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The Origin and Early Evolution of Amniotes

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 )

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The Origin and Early Evolution of Amniotes

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  1. 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

  2. FIG. 26.3

  3. Fig. 26.1

  4. Fig. 26.2

  5. Fig. 26.4

  6. 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

  7. Fig. 30.16

  8. Fig. 26.6

  9. Fig. 26.9

  10. Fig. 26.12

  11.  FIG. 26.13 FIG. 26.14 

  12. FIG. 26.15

  13.  FIG. 26.16 FIG. 26.18 

  14. Fig. 26.17

  15. Fig. 26.19

  16. Fig. 26.20  FIG. 26.20 FIG. 26.24 

  17. Fig. 26.27

  18. FIG. 26.28

  19. Fig. 26.5

  20. 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

  21. Fig. 26.22

  22. Fig. 26.26

  23. Fig. 26.23

  24. Fig. 26.21

  25. Fig. 26.25

  26. 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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