Amniotes (especially dinosaurs)
The amnion and amniotic egg
Frog: Lizard:
many eggs laid in water Fewer eggs laid on land
fully aquatic larval stage no larval stage
metamorphosis to develops directly to
juvenile juvenile
sexually mature adult sexually mature adult
mates in water mates on land
external fertilization copulation
amniotic egg - embryo plus extraembryonic layers
(see pp. 171-172;
10th: 117-118)amnion is fluid-filled sac that protects embryo
yolk sac provides food
allantois gathers waste
chorion surrounds all the other layers
egg is enclosed in semi-permeable shell
Leading hypothesis of amniote relationships
(see cladogram Fig. 28-2;
10th: 27-2)Amniota
Synapsida
various extinct lineages
Mammalia
Sauropsida (or Reptilia w/ birds included)
turtles
Diapsida
Lepidosauria
squamates (lizards incl. snakes)
various extinct marine reptiles
Archosauria
crocodiles
pterosaurs, dinosaurs (incl. birds)
Dinomyths
1. Dinosaurs are big lizards
2. Dinosaurs were all big extinct reptiles
3. Dinosaurs had a sprawling stance
4. Dinosaurs were all big
5. The largest dinosaurs lived in swamps
6. T. rex ate Brachiosaurus
7. Dinosaurs dragged their tail
8. Dinosaurs were cold-blooded and sluggish
9. Dinosaurs were stupid
10. Dinosaurs went extinct
More detail:
1. Dinosaurs are big lizards
• actually are closer to crocodiles
• think birds (see cladogram on p. 585;
2. Dinosaurs were all big extinct reptiles
• pterosaurs are sister taxon of dinosaurs
• ichthyosaurs, etc. are nearer to lizards
• pelycosaurs are nearer to mammals
3. Dinosaurs had a sprawling stance
• instead had fully upright stance
• synapomorphy: hip socket has central hole,
hind limbs directly under body, swing fore-aft
(other archosaurs have solid hip socket and
sprawling stance)
• not necessarily faster, but better endurance
because movement doesn't constrict the lungs
4. Dinosaurs were all big
• earliest dinos (240 mya) were small
• more recent dinos as small as hummingbirds
5. The largest dinosaurs could only support their body weight in swamps
• an elephant would sink in the mud
• more likely lived in savanna-like habitat
6. T. rex ate Brachiosaurus
• T. rex was over 70 million years later
• T. rex did not live together with cavemen
7. Dinosaurs dragged their tail
• probably held it upright as in T. rex
8. Dinosaurs were cold-blooded and sluggish
• some were slow (ectothermic)
• others were possibly warm-blooded
(endothermic)
9. Dinosaurs were stupid
• active predators are often cunning
• evidence for nesting and mating behavior
10. Dinosaurs went extinct
• many did, but not birds
• birds are dinosaurs, reptiles, vertebrates
• conventional "dinosaurs" is paraphyletic
because it excludes birds